long-term effects of organic amendments on soil fertility. a review

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Long-term effects of organic amendments on soilfertility. A review

Mariangela Diacono, Francesco Montemurro

To cite this version:Mariangela Diacono, Francesco Montemurro. Long-term effects of organic amendments on soil fertility.A review. Agronomy for Sustainable Development, Springer Verlag/EDP Sciences/INRA, 2010, 30(2), .

https://hal.archives-ouvertes.fr/hal-00886539https://hal.archives-ouvertes.fr

Agron. Sustain. Dev. 30 (2010) 401422c INRA, EDP Sciences, 2009DOI: 10.1051/agro/2009040

Review article

Available online at:www.agronomy-journal.org

for Sustainable Development

Long-term effects of organic amendments on soil fertility. A review

Mariangela Diacono1, Francesco Montemurro2*

1 CRA - Research Unit for Cropping Systems in Dry Environments Bari, Italy2 CRA - Research Unit for the Study of Cropping Systems Metaponto (MT), Italy

(Accepted 24 September 2009)

Abstract Common agricultural practices such as excessive use of agro-chemicals, deep tillage and luxury irrigation have degraded soils,polluted water resources and contaminated the atmosphere. There is increasing concern about interrelated environmental problems such assoil degradation, desertification, erosion, and accelerated greenhouse effects and climate change. The decline in organic matter content ofmany soils is becoming a major process of soil degradation, particularly in European semi-arid Mediterranean regions. Degraded soils are notfertile and thus cannot maintain sustainable production. At the same time, the production of urban and industrial organic waste materials iswidespread. Therefore, strategies for recycling such organic waste in agriculture must be developed. Here, we review long-term experiments(360 years) on the effects of organic amendments used both for organic matter replenishment and to avoid the application of high levelsof chemical fertilizers. The major points of our analysis are: (1) many effects, e.g. carbon sequestration in the soil and possible build-up oftoxic elements, evolve slowly, so it is necessary to refer to long-term trials. (2) Repeated application of exogenous organic matter to croplandled to an improvement in soil biological functions. For instance, microbial biomass carbon increased by up to 100% using high-rate composttreatments, and enzymatic activity increased by 30% with sludge addition. (3) Long-lasting application of organic amendments increasedorganic carbon by up to 90% versus unfertilized soil, and up to 100% versus chemical fertilizer treatments. (4) Regular addition of organicresidues, particularly the composted ones, increased soil physical fertility, mainly by improving aggregate stability and decreasing soil bulkdensity. (5) The best agronomic performance of compost is often obtained with the highest rates and frequency of applications. Furthermore,applying these strategies, there were additional beneficial effects such as the slow release of nitrogen fertilizer. (6) Crop yield increased byup to 250% by long-term applications of high rates of municipal solid waste compost. Stabilized organic amendments do not reduce the cropyield quality, but improve it. (7) Organic amendments play a positive role in climate change mitigation by soil carbon sequestration, the size ofwhich is dependent on their type, the rates and the frequency of application. (8) There is no tangible evidence demonstrating negative impactsof heavy metals applied to soil, particularly when high-quality compost was used for long periods. (9) Repeated application of compostedmaterials enhances soil organic nitrogen content by up to 90%, storing it for mineralization in future cropping seasons, often without inducingnitrate leaching to groundwater.

soil fertility / soil carbon and organic matter / organic amendments / long-term experiments

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4021.1 Soil management strategies for a sustainable agriculture . . . . . . . . . 4021.2 Purpose of this review article . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404

2 Effects of organic amendments on the soil-plant system . . . . . . . . . . . . . . . 4072.1 Effects on soil biological, chemical and physical fertility . . . . . . . . 407

2.1.1 Biological fertility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4072.1.2 Chemical fertility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4092.1.3 Physical fertility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410

2.2 Effects of organic amendments on plant nutrition and yieldingresponses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4132.2.1 Yield response. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4132.2.2 Quality response. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414

* Corresponding author: [email protected]

Article published by EDP Sciences

http://dx.doi.org/10.1051/agro/2009040http://www.agronomy-journal.orghttp://www.edpsciences.org

402 M. Diacono, F. Montemurro

3 Environmental impact and sustainability of organic amendments . . . 4143.1 Organic matter evolution and soil carbon sequestration . . . . . 4143.2 Heavy metal concentration in the agro-ecosystem . . . . . . . . 4163.3 N pool fate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417

4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418

1. INTRODUCTION

Indiscriminate use of agro-chemicals, excessive and deeptillage, and luxury irrigation have degraded soils, particularlyin semi-arid Mediterranean areas, as well as polluted surfaceand groundwaters, and contaminated air (Lal, 2008). Soil is anessential non-renewable resource with potentially rapid degra-dation rates and extremely slow formation and regenerationprocesses (Van-Camp et al., 2004). Soil degradation is as oldas agriculture itself, its impact on human food production andthe environment becoming more serious than ever before, be-cause of its extent and intensity (Durn Zuazo and RodrguezPleguezuelo, 2008). There is a strong link between soil degra-dation and desertification on the one hand, and risks of ac-celerated greenhouse effects and climate change on the other(Komatsuzaki and Ohta, 2007). Durn Zuazo and RodrguezPleguezuelo (2008), confirmed that reduced precipitation orincreased temperature accelerates land degradation throughthe loss of plant cover, biomass turnover, nutrient cycling andsoil organic carbon storage, accompanied by higher green-house emissions. The reason for the earths increased temper-ature along with change in rainfall amount and distribution,must be sought in the use of fossil fuel, that has drasticallydisturbed the global carbon cycle with its attendant impact onclimate change (Lal, 2008). According to Komatsuzaki andOhta (2007), there is also evidence that continuous croppingand inadequate replacement of nutrients, removed in harvestedmaterials or lost through erosion, leaching, and gaseous emis-sions, degrade soil physical, chemical and biological proper-ties, intensifying global warming. Moreover, the production ofurban and industrial organic wastes is increasing worldwide,and strategies for disposal in such a way that these do not fur-ther degrade soil, contaminate water or pollute air must be de-veloped and optimized (Dring and Gth, 2002; Lal, 2008).

Soil fertility can be defined as the capacity of soil to pro-vide physical, chemical and biological needs for the growthof plants for productivity, reproduction and quality, relevant toplant and soil type, land use and climatic conditions (Abbottand Murphy, 2007). It is becoming understandable that theproper agricultural use of soil resources requires equal consid-eration for biological, chemical and physical components ofsoil fertility, thus attaining a sustainable agricultural system.

The term sustainable agriculture is used in this reviewwith the meaning given by Tilman et al. (2002), as referring topractices that meet current and future society needs for foodand feed, ecosystem services and human health, maximizingthe net benefit for people. Namely, sustainability implies bothhigh yields, that can be maintained, and acceptable environ-mental impact of agricultural management.

It is relevant to note that organic farming is the only sus-tainable form of agriculture legally defined. According to thecurrent legislation (EC Council Regulation N. 834, 2007), soil

fertility management relies on a complex long-term integratedapproach rather than the more short-term one of conventionalagriculture. One of the possible main tools for the mainte-nance and the improvement of soil fertility in organic farmingis to adopt crop rotations, including a mixture of leguminousfertility-building crops and plants with different rooting depths(Watson et al., 2002). Furthermore, organic wastes such asanimal manures, by-products of several kinds and compostedresidues can be used as amendments to increase soil fertility,since they are important sources of nutrients for growing cropsand means for enhancing the overall soil quality (Davies andLennartsson, 2005).

1.1. Soil management strategies for a sustainableagriculture

Soil organic matter plays an important role in long-term soilconservation and/or restoration by sustaining its fertility, andhence in sustainable agricultural production, due to the im-provement of physical, chemical and biological properties ofsoils (Sequi, 1989). The organic matter content is the resultof the inputs by plant, animal and microbial residues, and therate of decomposition through mineralization of both addedand existing organic matter. More specifically, the generic termorganic matter refers to the sum of all organic substancespresent in the soil. This sum comes from residues at vari-ous stages of decomposition, substances synthesized throughmicrobial-chemical reactions and biomass of soil microorgan-isms as well as other fauna, along with their metabolic prod-ucts (Lal, 2007). Decomposition of organic matter is chieflycarried out by heterotrophic microorganisms. This process isunder the influence of temperature, moisture and ambient soilconditions and leads to the release and cycling of plant nutri-ents, especially nitrogen (N), sulfur and phosphorus (Murphyet al., 2007).

The different fractions of organic matter undergo the hu-mification process, which is the changing from recognizableparts and pieces of plants or animals into an amorphous, rot-ted dark mass. The products of humification are the humicsubstances that in soil are dark brown and fully decomposed,i.e. humified (Fig. 1). The humic substances are one of themost chemically active compounds in soils, with cation andanion exchange capacities far exceeding those of clays. Theyare long-lasting critical components of natural soil systems,persisting for hundreds or even thousands of years (Mayhew,2004). In fact, the turnover rate of organic materials varies con-siderably, from less than 1 year, as for microbial biomass, tomore than 1 thousand years of stable humus (Van-Camp et al.,2004). The organic matter is being progressively depleted, par-ticularly in the Mediterranean area, where the warm climate

Long-term effects of organic amendments on soil fertility. A review 403

Figure 1. The diagnostic horizons of a Vertisol (Sparacia experimen-tal farm - Department of Agronomy, University of Palermo) show-ing the presence of well-transformed, dark-colored humified organicmatter in the topsoil.

and the intensity of cultivation increase the rate of decomposi-tion (Montemurro et al., 2007). By contrast, the build-up oforganic matter in soils is a process much slower and morecomplex than its decline (Van-Camp et al., 2004). Since or-ganic matter contents are difficult to measure directly, a greatnumber of methods measure the soil organic carbon level, mul-tiplying it by conversion factors ranging from 1.7 to 2.0 to ob-tain organic matter values (Baldock and Nelson, 2000).

Several investigations have demonstrated that soil organicmatter is a very reactive and ubiquitous soil quality indica-tor that influences the productivity and physical well-being ofsoils (Lal, 2006; Komatsuzaki and Ohta, 2007). As a conse-quence, agricultural management practices that enhance soilorganic matter content are used for preserving farming outputand environmental quality; thereby they can be considered assustainable activities (Lal, 2004).

Conservative soil tillage systems, e.g. no-till, which leavesmore residues on the surface because the soil is not turnedover, can maintain or improve the organic carbon content andthe related soil fertility properties (Ismail et al., 1994; Johnsonet al., 2005). Crop rotations usually increase organic matterand prompt changes in N sources, affecting their availabilityfor plants and, as a consequence, the N efficiency is greaterwhen a crop rotation is adopted (Montemurro and Maiorana,2008).

The inclusion in a rotation of cover crops or green manurescan also enhance the efficient use of nutrients by plants, mainlyowing to the increase in soil microbial population and activity(Watson et al., 2002). Cover crops are generally grown to pro-vide soil cover during the winter months, thus preventing soilerosion by wind and rainwater strength, which reduces organicmatter content in the long run. Moreover, Komatsuzaki (2004)indicated that cover crop utilization is a technique that limitsnutrient leaching, scavenging the soil residual N and makingit available for subsequent cultivation.

In addition, the summer green manures are field crops orforage ones, such as leguminous and non-leguminous plants,usually incorporated into the soil soon after flowering, to im-prove soil fertility. In particular, leguminous green manurescan fix large quantities of atmospheric N2. They also provideuseful amounts of organic matter, as well as non-leguminouscrops which, nevertheless, cannot fix atmospheric N2 (Daviesand Lennartsson, 2005). During 32 years of winter wheat crop-ping, Prochzkov et al. (2003) found higher average yieldswith green manuring compared with straw incorporation intothe soil, probably due to a slower decomposition and followingrelease of nutrients by the latter.

There is increasing interest in the alternative fertility build-ing strategies previously described, because conventional in-puts, such as synthetic fertilizers, should be excluded or re-duced in sustainable agricultural management. Within thiscontext, soil fertility could also be improved with organicwaste application such as by-products of farming, or municipalactivities including animal manures, food processing wastesand municipal biosolids, wastes from some industries, such assewage sludges, wastewaters, husks and vinasse. Both groupsof wastes present generally notable contents of organic matterand substantial quantities of nutrients and their use in agri-culture can contribute to closing the natural ecological cy-cles (Montemurro et al., 2004; Montemurro and Maiorana,2008). Increased recycling of organic residues as fertilizersand soil amendments on cropland avoids both utilization ofnon-renewable resources, e.g., fossil fuel and peat, and ex-cess of energy expenses, i.e., production of chemical fertilizersand pesticides, treatment and landfill disposal of such organicwastes (Mondini and Sequi, 2008). Biodegradable wastes canalso be considered valuable resources to promote soil fertil-ity. However, this benefit occurred only if they were appliedaccording to good practices, taking into account the needs ofthe soil, its use and the climatic conditions (Van-Camp et al.,2004). Moreover, it is necessary to adopt waste managementstrategies, such as controlled biodegradation processes, able toboth minimize their potentially negative environmental impactand increase agricultural utilization (Montemurro et al., 2009).

Despite the fact that manure characteristics are influencedby many factors such as species and age of the animal, ra-tion fed, and collection and storage method, it can be gen-erally assumed that their ratio of nutrients are different fromthose removed by common crops (Edmeades, 2003). As a con-sequence, soil manure application, often in excess of crop re-quirements, can cause a significant build-up of phosphorus (P),N and salt. Nowadays, the industrialization of livestock enter-prises has led to other problems linked to the land distribution


(a)

(b)

Figure 2. Aerobic biodegradation can enhance the quality of wasteswhich will be applied to soils. Figure 2a shows a compostingwindrow stirred mechanically by a turning machine, while Figure 2bshows an experimental field with compost application.

of solid and liquid animal manures, enhancing the interest inpretreatment technologies, which can convert manure into amore valuable and easily usable resource called compost(Figs. 2a and 2b).

Compost is a stabilized and sanitized product of com-posting, which is the biodegradation process of a mixtureof organic substrates carried out by a microbial communitycomposed of various populations, both in aerobic conditionsand solid state (Insam and de Bertoldi, 2007). During thecomposting process, the simple carbonaceous and nitrogenouscompounds are transformed through the activity of microor-ganisms into more stable complex organic forms, which chem-ically resemble soil humic substances (Epstein, 1997).

The soil application of co-composted manure has severaladvantages over fresh manure, such as reduced numbers ofviable weed seeds, reduced volume and particle size, whichfacilitates land distribution, a better balanced nutrient compo-sition, stabilized organic matter and a slower release of nu-trients. This topic has recently been reviewed by Moral et al.(2009).

From all the above, it can be summarized that the environ-mental impact of conventional farming practices and globalconcerns about soil degradation have increased the interest insustainable agricultural strategies such as land application ofwaste materials. This is a way to avoid disposal costs and re-cycle nutrients into soil, unlike commercial fertilizers (Millerand Miller, 2000; Van-Camp et al., 2004). However, it is neces-sary to point out that the utilization of various organic residuesin agriculture depends on several factors, including the charac-teristics of the waste, such as nutrient and toxic element con-tent, availability, the transportation costs and the environmen-tal regulations, as reviewed in detail by Westerman and Bicudo(2005).

1.2. Purpose of this review article

The Rothamsted experiments, lasting for more than100 years, are the oldest and most continuous agronomic tri-als in the world, that measure the effects on crop yields of in-organic fertilizers in comparison with farmyard manure andother organic materials. Since their results are well summa-rized in the Guide to the classical and other long-term experi-ments, datasets and sample archive, they will not be discussedfurther in this review (Rothamsted Research, 2006).

This paper, focusing on recently published data, gives em-phasis to long-term field trials, particularly regarding raw andcomposted agro-industrial and municipal waste application.Although there is a large amount of literature relating to theinfluence of raw and composted organic materials on soilfertility, only a few published studies have focused on stud-ies about long-term effects of these amendments on the soil-plant system, for a sustainable crop production. The presentwork attempts to address this issue by using the results fromlong-lasting fertility trials. Long-term research on organicamendment use on cropland is particularly relevant becausemany effects, e.g. organic matter enrichment and possible soiltoxic element accumulation, evolve slowly and are difficult topredict.

We have collected published literature, investigating abroad array of organic residues, composts and experimen-tal conditions, especially analyzing their impact on soils andcrops in relation to modern sustainable agriculture. Experi-mental data of field trials were selected lasting for at least3 years up to 60 years, stressing longer-term research. There-fore, experimental designs considered in this review rangedfrom those usually known as mid- to long-term ones. Sum-maries of the essential data for the longest duration trials aregiven in Table I.

The organic amendments covered in this paper are definedin Section 2 along with their effects on the soil-plant system,


Table I. Summary of the main data of long-term trials (selected data from experiments of 10 years).

Site Organic materials Application rate Crop Trial period Reference

Punjab, India Rice straw compost 8 t ha1 Ricewheat rotation 10 years Sodhi et al. (2009)

Obere Lobau Biowaste compost 9, 16 and 23 t ha1 Cereals and potatoes 10 years Erhart et al. (2005);near Vienna, Hartl and Erhart (2005);

Austria Erhart et al. (2008)

Turin, Italy (1) Cattle slurry; (1) 100 t ha1; Maize for silage 11 years Monaco et al. (2008)(2) Composted (2) 40 t ha1

farmyard manure

Linz, Austria Composts of: urban 175 kg N ha1 Maize, summer- 12 years Ros et al. (2006a)organic waste; green wheat and winter-

waste; cattle manure barley rotation

or sewage sludge

Bennett, CO, Anaerobically 2.2, 4.5, 6.7, 8.9 Two-year wheat 12 years Barbarick and

USA digested and 11.2 t ha1 fallow rotation Ippolito (2007)biosolids

Ravenna, Municipal-industrial (1)(3) 5 and Winter wheat 12 years Mantovi et al. (2005)

Italy wastewater sludge: 10 t ha1 maizesugarbeet(1) Anaerobically rotation

digested (liquid slurry);

(2) belt filtered

(dewatered sludge);

(3) composted with

wheat straw

Toledo, (1) Barley straw and (1) 3 and 2.5 t ha1; Barley, wheat and 16 years Dorado et al. (2003)Spain crop waste; (2) 30 t ha1 sorghum

(2) two-year-old cattle

manure

Murcia, Municipal solid waste 65, 130, 195 and none 17 years Bastida et al. (2008)

Spain 260 t ha1

Orange, VA, Aerobically digested 42, 84, 126,168 Barley, radish and 19 years Sukkariyah et al.

USA sewage sludge and 210 t ha1 romaine lettuce (2005)

Tanashi City, (1) Dried sewage (1) 259.8 t ha1; Maize, barley and 19 years Kunito et al. (2001)Tokyo, Japan sludge; (2) 82.5 t ha1; rye

2) rice husk (3) 77.2 t ha1

compost;

(3) sawdust compost


Table I. Continued.


Tokyo, Japan (1) Sewage sludge (1)(2) 240 kg N ha1 Maize and barley 23 years Zaman et al. (2004)composted with ricehusk;(2) sewage sludgecomposted withsawdust

Alberta, Solid beef cattle 30, 60 and 90 t ha1 Barley, canola, 25 years Whalen and ChangCanada manure in dryland soils and triticale and maize (2002)

60, 120 and 180 t ha1

in irrigatedsoils

Yamaguchi, Rice strawcow dung 15 t ha1 Double cropping 25 years Shindo et al. (2006)Japan compost (paddy rice

and barley)

Czech (1) Straw harvest; (6) 10 t ha1 Winter wheat 32 years Prochzkov et al.Republic (2) straw harvest + continuous cropping (2003)

green manuring;(3) strawincorporation;(4) strawincorporation + greenmanuring;(5) straw burning;(6) farmyard manure

Punjab,India Farmyard manure 20 t ha1 Ricewheat and 34 years Kukal et al. (2009)maizewheatsystems

Gumpenstein, (1) Cattle (1) 120 kg N ha1+ Cereals; rape; pea; 38 years Antil et al. (2005)Austria slurry+straw; 0.2 kg m2; flax

(2) animal manure (2)(4) 240 kg N ha1

(solid);(3) animal manure(liquid);(4) cattle slurry(semi-liquid)

Fidenza, Italy (1) Farmyard manure; (1)(2) 5 t ha1 Two-year rotation (1) 40 years; Saviozzi et al. (1999)(2) aerobically digested sewage consisting mainly of (2) 12 yearsdigested sewage maize and wheatsludge

Legnaro, (1) Farmyard manure; (1) 20 and 60 t ha1; Maize 40 years Nardi et al. (2004)Italy (2) liquid manure (2) 120 t ha1


Table I. Continued.


Bologna, Cattle manure, cattle 6 t ha1 after wheat Two-year maize- 34 years Triberti et al. (2008)Italy slurry, wheat or corn and 7.5 t ha1 after winter wheat

residues maize crops rotation

Central Farmyard manure 4 t carbon ha1 Maize 47 years Elfstrand et al.Sweden (2007)

Martonvsr Farmyard manure 35, 40, 80 and 104 t ha1 Martonvsr: Martonvsr: Sleutel et al. (2006)and wheatwheat 54 years;Keszthely maizemaize; Keszthely:Hungary Keszthely:wheat 41 years

wheatsugar beetmaizemaize

Towada, Rice straw compost 11 t ha1 Potato, maize and 60 years Takeda et al. (2005)Japan soybean in rotation

whereas the environmental impact and sustainability are out-lined in Section 3.

2. EFFECTS OF ORGANIC AMENDMENTSON THE SOIL-PLANT SYSTEM

Miller and Miller (2000) highlighted that organic mate-rial application to cropland could affect soil properties, butthe effects generally may not be apparent over a short timeperiod. More specifically, Tittarelli et al. (2007) pointed outthat the simplest method of examining the agronomic value ofstabilized organic materials is the calculation both of organicmatter supply and plant nutrients. The slow release of thesenutrients is responsible for the increase in crop yields in thesubsequent years, thus determining the difficulty of quicklyevaluating the true agronomic value of these organic materialsas amendments. However, there is a considerable variabilitybetween experimental techniques, climate, soil type and or-ganic material characteristics, and therefore attention must bepaid to generalizing the effects of composts and raw waste ap-plication on the soil-plant system.

2.1. Effects on soil biological, chemical and physicalfertility

Van-Camp et al. (2004) found that organic amendmentsinfluence soil characteristics by the interdependent modifica-tion of biological, chemical and physical properties (Fig. 3).Also, fertility improvement through an effective managementof these properties has the capability of optimizing crop pro-duction. In this subsection, we selected papers whose primarypurpose was to examine the effect of different organic amend-ments on overall soil fertility. However, it is necessary to take

into account that sometimes it was difficult to compare the re-sults, because of the different assessment methods used in thestudies.

2.1.1. Biological fertility

Microbiological and biochemical soil properties are veryreactive to small changes occurring in management practices.Therefore, it is possible to use them in a basic analysis forevaluating the effects of the application of different sourcesand amount of organic matter on soil characteristics duringexperimental trials. Microorganisms, e.g. bacteria, fungi, acti-nomycetes and microalgae, play a key role in organic matterdecomposition, nutrient cycling and other chemical transfor-mations in soil (Murphy et al., 2007). Since organic carbon(C) is utilized for energy by decomposer microorganisms, itsfate is to be either assimilated into their tissues, released asmetabolic products, or respired as carbon dioxide (CO2). Themacronutrients N, P and sulfur (S), present in the organicchemical structures, are converted into inorganic forms. Sub-sequently, they are either immobilized and used in the synthe-sis of new microbial tissues or mineralized and released intothe soil mineral nutrient pool (Baldock and Nelson, 2000).For assimilation by microorganisms of decomposing organicresidues, the N has to be assimilated in an amount determinedby the C/N ratio of the microbial biomass. More specifically,the amount of N required by the microorganisms is 20 timessmaller than that of C. If there are both a low concentrationof easily decomposable C compounds and a larger N quan-tity in respect to that required by the microbial biomass, therewill be net N mineralization with release of inorganic N. Onthe contrary, Corbeels et al. (1999) found that if the amount ofN present in the residues is smaller than that required by the


Soil organic carbon improvement

Enhancement of water holding capacity

Increase in nutrient retention capacity

Improvement of structure and soil biological quality

Resistance to drought

Increase in water use efficiency

Decrease in loss of nutrients

Increase in N supply over time

Reduction in soil loss by erosion

Increase in microbial and

enzymatic activities

Increase in soil organic matter content and overall soil fertility

Org

anic

am

endm

ents

app

licat

ion

Figure 3. Effects of increasing soil organic matter content and overall soil fertility by soil organic carbon improvement (adapted from Lal,2006).

microbial biomass, further inorganic N will need to be immo-bilized from the soil to complete the decomposition process.

It is difficult to distinguish between the direct and the in-direct effects of an amendment on the behavior of soil mi-croorganisms. In soils amended with compost or other raworganic materials, even in association with mineral fertilizerN, autochthonous microbiological activity and growth can bestimulated. However, different authors (Ros et al., 2006a; Kauret al., 2008) suggest that a direct effect from microorganismsintroduced with the compost is detectable. Several long-lastingexperiments have demonstrated that soil biological properties,such as microbial biomass C, basal respiration and some enzy-matic activities, are significantly improved by compost treat-ments. This is particularly evident in the upper layers of thesoil because of the added labile fraction of organic matter,which is the most degradable one (Zaman et al., 2004; Roset al., 2006a, b; Tejada et al., 2006, 2009). Since generallythe composts are slowly decomposed in the soil, the contin-uous release of nutrients can sustain the microbial biomasspopulation for longer periods of time, compared with min-eral fertilizers (Murphy et al., 2007). In fact, an interestingresidual effect of composts on the microbial activity has oftenbeen observed in many experimental seasons after their ap-plication, which also results in a longer availability of plantnutrients. Ginting et al. (2003), for example, found 4 yearsafter the last application of compost and manure that theresidual effects resulted in 20 to 40% higher soil microbialbiomass C compared with the N fertilizer treatment. Research

on the effect of different doses of raw municipal solid waste inMediterranean semi-arid conditions, ranging between 65 and260 t ha1, demonstrated 17 years after a single application ofthis organic amendment an average increase of 70% in organicmatter content. The water-soluble C fractions also increasedby up to 195 t ha1 application rate, above which they leveledoff (Bastida et al., 2008). The authors also found that the enzy-matic activities of urease, -glucosidase, alkaline phosphataseand o-diphenyloxidase associated with humic substances in-creased significantly in all the amended plots, thus improvingsoil biochemical quality.

Sixteen years after 1 and 3 t ha1 year1 sludge application,in a field situated close to Lund in the southern part of Swe-den, an increasing trend for substrate induced respiration wasfound, ranging from 20 to 22%, due to the organic materialinputs, and enhanced by from 11 up to 33% acid phosphataseactivity involved in the mineralization of organically-bound P(Mats and Lennart, 1999). This behavior follows the increas-ing rates of sludge addition.

As a general rule, the quantity and quality of organic ma-terial added to soils are the major factors in controlling theabundance of different microbial groups and the activity ofmicroorganisms involved in nutrient cycling. Enzyme activityand microbial biomass analysis indicated that microbial prop-erties were stimulated, e.g. microbial biomass C increased toabout 100%, more by high rates and composted than by lowrates and fresh paper mill residuals (78 t ha1 and 22 t ha1,respectively) (Leon et al., 2006).


0

20

40

60

80

100

MSW

comp

ost

NPK+

FYM

FYM

NPK+

FYM/

comp

ost

Orga

nic m

anure

comp

osted

FYM

Cattle

manu

re

Cattle

slurr

y

Organic amendments

Soi

l org

anic

car

bon

incr

ease

(%)

Figure 4. Soil organic carbon increases after different long-term organic amendment applications.Note: data taken from: Montemurro et al. (2006) (means of crops); Kaur et al. (2008) (at 015 cm depth); Kukal et al. (2009) (means of crops;at 015 cm depth); Mandal et al. (2007) (means of 5 trials data); Meng et al. (2005); Monaco et al. (2008); Triberti et al. (2008). SOC = soilorganic carbon; MSW = municipal solid waste; FYM = farmyard manure.

As regards the effects of the quality of amendment, Monacoet al. (2008) pointed out that microbial activity, measuredby the potential soil respiration parameter, gave a reliableand useful indication of the amount of easily decomposableorganic C, and this parameter was lower when the C was par-tially humified before soil input. In fact, after 11 years ofrepeated applications of different organic materials, soil res-piration was in the order of 640.9 < 682.1 < 755.1 mg C-CO2 kg1 for farmyard manure, cattle slurry and straw, respec-tively. The effect of green manure amendments was studied ina 47-year field experiment (Elfstrand et al., 2007). The authorsfound a higher abundance of bacteria and fungi in the greenmanure treatment, i.e. 34.3 and 1.8 nmol g soil1, respectively,in respect to the unfertilized one (20.3 and 0.9 nmol g soil1),measured before maize sowing. Furthermore, a higher fun-gal/bacterial ratio was noticed, equal to 0.054 compared withfarmyard manure (0.036) and sawdust (0.046) application.This behavior might be attributed to differences in the qual-ity of the organic matter added.

The reviewed results suggest that exogenous organic matterapplications to cropland lead to an improvement in soil biolog-ical functions, depending on the quantity and type of materialsapplied.

2.1.2. Chemical fertility

A considerable number of studies, concerning long-termfertility trials, pointed out that soil organic material applica-tions increased the organic carbon stock and, therefore, in-creased the cation exchange capacity. This effect was due tothe high negative charge of organic matter. This is importantfor retaining nutrients and making them available to plants(Garca-Gil et al., 2004; Ros et al., 2006b; Weber et al., 2007;Kaur et al., 2008). Figure 4 shows the organic carbon increase,

as a consequence of several organic amendments long-lastingapplication, ranging from 24 to 92%.

Habteselassie et al. (2006a) found that, over a 5-year period,the C pool was enhanced by 115% in dairy-waste compost-treated soil. Moreover, the dairy-waste compost increased or-ganic carbon by 143 and 54% as compared with ammoniumsulfate and liquid dairy-waste treatments, respectively, appliedat the same available N level (200 kg Nha1). This C stored inthe soil organic matter accounts for approximately 11% of thetotal amount of C applied.

After 3 years of municipal solid waste compost and olivepomace compost application, the total organic carbon signifi-cantly increased by 24.0 and 43.2% for cocksfoot and alfalfaplots, respectively, in respect to the unfertilized control, indi-cating that these amendments positively affected the organicmatter (Montemurro et al., 2006). Other authors also showedthat municipal-industrial wastes stimulate plant growth, indi-rectly and with a long-term effect, by improving organic matter(Mantovi et al., 2005; Cherif et al., 2009).

Under 736-year fertility experiments in five different rice-based cropping systems, the application of organic amend-ments at 510 t ha1year1, through farmyard manure or com-post combined with balanced mineral NPK, increased organiccarbon by 10.7% (Mandal et al., 2007).

In a ricewheat system, farmyard manure application at20 t ha1 showed, after a period of 32 years, higher organiccarbon concentration of 17% compared with NPK fertilizersin the 015 cm soil layer (Kukal et al., 2009). Nevertheless,the results of repeated applications of either digested sewagesludge over 12 years or farmyard manure for 40 years in-dicated that such organic amendments were inadequate forrestoration of organic matter lost as a consequence of cultiva-tion (Saviozzi et al., 1999). In fact, the amount of organic car-bon in the undisturbed site was 120 and 156% higher than thatin farmyard and sludge cultivated soils, respectively. On theother hand, contrary to common belief, over a 25-year period


of intensive rice-wheat cropping, a depletion of organic carbondid not occur, but rather an improved organic carbon concen-tration of 38% (Benbi and Brar, 2009).

In terms of sustainability, only farmyard manure fertiliza-tion maintained the total organic carbon level of 40 t C ha1,measured in the top soil layers at the start of a 40-year ex-periment, while the average total organic C depletion was23% with liquid manure and mixed fertilization treatments,43% with mineral fertilizers alone and 51% in the control(Nardi et al., 2004). Furthermore, the presence of weaklyacidic chemical functional groups on organic molecules makesorganic matter an effective buffer, as supported by the findingsof Garca-Gil et al. (2004). These authors observed a long-(9 years) and short-term improvement in the soil humic acidbuffering capacity in municipal solid waste compost-amendedsoils, derived from a residual effect of a single application andcumulative effects from repeated ones. These distributions willfavor the general soil fertility status and crop production.

There are reports in the literature of long-term compost andmanure application both increasing (Eghball, 2002; Garca-Gilet al., 2004; Butler and Muir, 2006) and decreasing (Menget al., 2005; Bastida et al., 2008; Bi et al., 2008) the pH of soils,depending on their initial pH and organic residues. Butler andMuir (2006) observed that soil pH increased on average by 0.5units as the dairy manure compost rate doubled in magnitudefrom 11.2 to 179.2 t ha1.

As previously explained, with long-term compost use theresidual effects on crop production and soil properties can lastfor several years, since only a fraction of the N and other nutri-ents becomes available for plants in the first year after spread-ing (Hartl et al., 2003; Eghball et al., 2004). As an estimationof the the available N from compost treatment in the first yearof application, Tittarelli et al. (2007) mentioned a release ofonly 3035% of the total N content. The N release from com-post will mostly occur in the first two years after application,suggesting that a distribution frequency of once in every sec-ond year may be better than other application strategies, espe-cially with higher rates (Zhang et al., 2006).

More specifically, it is well known that many microorgan-isms convert organic N into inorganic N forms by mineraliza-tion. A large number of authors confirmed that N mineraliza-tion from compost is very limited in the short term. However,there is a significant residual effect from the cumulative appli-cations which becomes visible later after 45 years, resultingin deferred higher N availability and yields (Eghball, 2002;Blackshaw et al., 2005; Barbarick and Ippolito, 2007; Leroyet al., 2007). Regular addition of organic material to soil formore than 10 years, through compost or manures, enhancedboth soil C and N stocks and resulted in build-up of N, indicat-ing a physical protection of this nutrient within macroaggre-gates (Whalen and Chang, 2002; Meng et al., 2005; Malloryand Griffin, 2007; Sodhi et al., 2009). According to Hartl andErhart (2005), the organic N content increases by about 10%compared with the control, in the upper 30 cm of soil, after 10years of compost treatments. This result was complemented bysignificant increases in organic C, of 22%, indicating that theorganic N was tied up in organic matter. After 4 years of veg-etable compost applications, significantly higher soil total N

concentration was observed on compost plots compared withplots without it (Nevens and Reheul, 2003).

The C/N ratio of organic material can be used as a good in-dicator of nutrient supply. Tejada et al. (2009) showed an opti-mum balanced C/N ratio (1012) for soils amended with com-posts originating from leguminous residues, due to organicmatter mineralization overcoming immobilization. These find-ings were not in accordance with those of Weber et al. (2007),who found the C/N ratio clearly increased, from 10.7 up to22.2, in the third year after municipal solid waste compost ap-plication. This behavior can be explained by a depletion of Nreserve, probably because of plant N uptake. However, thereare often other explanations for such an increase in the C/Nratio. In fact, it is well known that when a compost that has ahigh C/N ratio is added to soil, the microbial population com-petes with plants for soil N, thus immobilizing it (Amlingeret al., 2003).

Soil available potassium (K) content increased on averageby 26%, as compared with control, in 5-year compost treat-ments derived from organic household wastes and yard trim-mings (Hartl et al., 2003). These treatments are a rich sourceof K, probably due to the large proportion of woody plant ma-terial and kitchen refuse in the raw material.

As regards the P from organic amendments, He et al.(2001) reported that compost applications can increase plant-available P in the soil. The biosolids-municipal solid wasteco-compost, applied once in 4 years, has also been found toeffectively supply P to soil at 015 cm depth. The soil ex-tractable P concentration increased on average from 7.2 to86 mg kg1 soil with enhanced application rates from 0 to200 t ha1 (Zhang et al., 2006). Furthermore, Eghball (2002)suggested that 4-year beef cattle manure and composted ma-nure application based on N needs of corn could eventuallyresult in soil accumulation of P, since the manure or compostN/P ratio is usually smaller than the corn N/P uptake ratio.

Our overall literature analysis demonstrates that several or-ganic amendments long-lasting applications enhanced soilavailable potassium, extractable phosphorous and organic car-bon content, and resulted in deferred N availability.

2.1.3. Physical fertility

Aggregate stability is a keystone factor in questions of soilphysical fertility and can be enhanced by means of an appro-priate management of organic amendments, which can main-tain an appropriate soil structure. This agronomic procedurecould improve pore space suitable for gas exchange, water re-tention, root growth and microbial activity (Van-Camp et al.,2004). Soils rich in organic matter are less prone to erosionprocesses than soils with low content, such as those whichpredominate in arid and semi-arid areas (Durn Zuazo andRodrguez Pleguezuelo, 2008).

The topic of soil structural stability has been reviewedrecently by Abiven et al. (2008). Their literature analysisvalidated the conceptual model proposed by Monnier (1965).This author considers different patterns of temporal effects on


aggregate stability, depending on the nature of the organic in-puts. Easily decomposable products have an intense and tran-sient effect on aggregate stability, while more recalcitrant ones,such as lignin and cellulose, have a lower but longer-lastingeffect.

Results of Albiach et al. (2001) pointed out that organicmatter and carbohydrates appeared to be the parameters mostclosely related to structural stability of soil aggregates ob-tained with five applications of a municipal solid waste com-post. Another 5-year field trial confirmed these findings, sug-gesting that municipal solid waste compost, applied every2 years, can be used to increase soil aggregate stability by29.3% in respect to the control, thus improving soil resistanceto water erosion (Annabi et al., 2006).

The organic matter stabilizes soil structure by at least twodifferent mechanisms: by increasing the inter-particle cohe-sion within aggregates and by enhancing their hydrophobicity,thus decreasing their breakdown, e.g. by slaking. More specif-ically, the increase in soil microbial activity, especially dueto the addition of composted residues, could be responsiblefor the increase in soil structural stability (Van-Camp et al.,2004). The relationships between soil biological activity andthe functioning of soil are very complex. According to Abivenet al. (2008), several biological binding agents have been rec-ognized as accountable for aggregation and aggregate stability.Polysaccharides synthesized by microorganisms, particularlyat the beginning of the organic matter decomposition, tend toadsorb the mineral particles and increase their inter-cohesion.Conversely, products rich in humic compounds, such as ma-nures or composts, would also be expected to increase aggre-gate hydrophobicity of clays. This has mainly been proved inlong-term trials. In fact, after 16 years of either farmyard ma-nure or crop waste applications, substantial improvements insoil physical properties have been noticed, markedly aggregatestability and water retention, due to the increased concentra-tion of humic colloids in soil (Dorado et al., 2003). In par-ticular, the authors found that the structural instability indexdecreased by 2.5 units with respect to control plots.

Tejada et al. (2009) observed that three different composts,consisting of leguminous plants, non-leguminous plants andthe combination of both plant residues, applied at rates of 7.2and 14.4 t organic matter ha1 during a period of 4 years, had apositive effect on soil physical properties. More specifically, atthe end of the experimental period and at the highest rate, thesoil structural stability was the highest in the non-leguminousplant compost treatment (28.3%), followed by the combinedone (22.4%). This result was due to greater amounts of humicacids provided to the soil, 63.6 and 59.5 g kg1, respectively,which are then directly involved in clayorganic complex for-mation.

Another recent study showed that, after 10 cycles of ricewheat cropping, the amount of water-stable aggregates wassignificantly higher in plots amended with rice straw compostat 8 t ha1 to both rice and wheat, as compared with inorganicfertilizers (Sodhi et al., 2009). The authors also suggested thatthis higher amount of water-stable aggregates can be ascribedto the regular addition of organic matter to soil, resulting inenhanced microbial activity and production of microbial de-

composition products, which helps with binding of aggregates.Leon et al. (2006) found that the application of medium andhigh rates, equal to 38.1 and 78.4 t ha1, of composted papermill residuals over 4 years, caused an increase in amounts ofwater-stable aggregates on average of 25% compared with thecontrol. There was no significant difference (P 0.05) be-tween the two compost rates tested, suggesting that the lowestamount of amendment was enough to maximize water-stableaggregation.

These positive results of long-term compost applicationagreed with the findings reported by Tejada et al. (2008). Theauthors observed that composted leguminous plants, alone ormixed with beet vinasse, an agro-industrial by-product, hada positive impact on soil structural stability, which increasedby 5.9 and 10.5% compared with the unfertilized treatment,respectively. Conversely, fresh beet vinasse application de-creased soil structural stability by 16.5% in respect to the con-trol, probably because high quantities of destabilizing mono-valent cations were introduced into the soil by this organicmaterial.

Whalen and Chang (2002) noted that 25 years of an-nual beef cattle manure applications, at rates of morethan 30 t ha1 year1 to dry land soils and at more than60 t ha1 year1 to irrigated soils, can shift the aggregate sizedistribution from larger (> 12.1 mm) to smaller (< 2.0 mm)dry-sieved aggregates, because of dispersive agents in the ma-nure. Consequently, many years of continuous manure appli-cations could raise the risk of wind erosion because of a greaterproportion of the soil small aggregates. Moreover, different au-thors have shown typical decreases in soil bulk density, on av-erage of 15%, after long-term compost, farmyard manure ordigested sewage sludge agricultural use, suggesting that theaddition of composted and fresh organic matter facilitated, asa consequence, the soil porosity connected to soil bulk density(Saviozzi et al. 1999; Meng et al., 2005; Tejada et al., 2008;2009). Porosity is a measure of the size and system of voids inthe soil matrix, affecting both aeration and water movement.On the other hand, Eghball (2002) reported that soil bulk den-sity was unaffected by 4-year manure or compost application.

Leon et al. (2006) found a strong correlation (r = 0.65,P 0.001) of total C with soil moisture content after appli-cation of paper mill residual by-products, corroborating thathigh C level in soils increases the water-holding capacity be-cause of the effect of organic matter on soil aggregation. Thisincrease provides more available water to plants, also helpingwith resistance to drought.

On the basis of the information presented in this subsection,it can be summarized that repeated applications of organicamendments can increase soil physical fertility, mainly by im-proving aggregate stability. In general, a rise in the above in-vestigated overall soil fertility influences crop yield response,as framed in the following subsection. Selected studies aresummarized in Table II, focusing on the effects of organicamendments on yields.


Table II. Summary of the effects of organic amendments on crop yields (selected data).

Crop and Amendments and application Sub-treatment or Highest yield Yield Referencetrial period rate comment (t ha1) increase ()

(% t1 of totalC applied)

Wheat MSW composts C1 (40 t ha1) Higher HM C2(): 60.2 t ha1; 17 Cherif et al.(5 years) and C2 (80 t ha1); FYM content in MSW C2Cf: 61.9 t ha1 (2009)

(40 t ha1); MSW+Cf; FYM+Cf compost treatments

Wheat Municipal-industrial wastewater Mineral fertilizer I (at 10 t ha1year1) 10; 11 Mantovi et al.maize sludge: dressings: four for sugarbeet: (2005)sugarbeet (I) anaerobically digested, increasing rates of 60.5 t ha1

rotation (II) dewatered and urea, plus II (at 5 t ha1 year1)(12 years) (III) composted (two rates: superphosphate for wheat: 6.23 t ha1;

5 and 10 t ha1 year1)

Cereals and Biowaste compost: C1 Mineral fertilizer: N3+C1: 5.2 t ha1(); 15 Erhart (2005)potatoes (9 t ha1 year1), C2 three increasing N2+C1: 4.9 t ha1

(10 years) (16 t ha1 year1) and C3 rates (N1; N2; N3)(23 t ha1 year1)plus 5 treatments withcombined fertilization

Maize VFG compost: C1 (22.5 t ha1) Mineral fertilizer C2 plus cattle 5 Leroy (2007)(7 years) and C2 (45 t ha1); slurry: (from compost

cattle slurry 22.4 t ha1 C applied)

Tall fescue Three food waste compost with: Two composting (1) 79 t ha1 0.2() Sullivan et al.(7 years) (1) yard trimmings; methods: (I) aerated (2003)

(2) yard trimmings + mixed static pile;paper waste; 3) Wood waste + (II) aerated turnedsawdust (each at 155 t ha1rate) windrow

Silage (1) Low dairy-waste compost Other treatments: (1) 24.6 t ha1 9.7 Habteselassiecornfield rate (to provide 100 kg ha1 of ammonium sulfate (in the last year et al. (2006a)(5 years) available N); at 100 kg N ha1 of the experiment)

(2) high dairy-waste compost and 200 kg N ha1

rate (200 kg N ha1);(3) low liquid dairy-waste rate(100 kg N ha1);(4) high liquid dairy-waste rate(200 kg N ha1)

Note: MSW = municipal solid waste; HMs = heavy metals; Cx = compost (x = 1, 2, etc.); Cf = chemical fertilizer; (*) in respect to the controland regarding the yield indicated as (**); VFG = vegetable-fruit-garden waste; (***) in respect to the lowest yield, found in wood waste +sawdust, and for the second method.


2.2. Effects of organic amendments on plant nutritionand yielding responses

The N dynamics in compost-amended soils could be af-fected by different site-specific factors, e.g. compost matrices,composting conditions, climate, soil properties and manage-ment practices. It can be generally assumed that the promptavailability of N to plants is low, as already explained in thisreview, since the majority of total compost N is bound to theorganic N-pool (Amlinger et al., 2003). In fact, the greatest to-tal N content in compost is not readily available, but it can bemineralized and subsequently taken up by plants or immobi-lized, denitrified and/or leached.

It is important to take into account that the slow releaseof nutrients from compost or green manures should be ade-quately controlled to match temporal crop demand with nu-trient supply. The increase in the N-use efficiency decreasesloss to leaching when there is wide drainage, e.g. during thefall-winter period, or volatilization (Tilman et al., 2002). Theplant-available N in the application year was expected to behigher for fresh manure than for stable composted manure. Infact, in a 4-year experiment, it ranged from 26 to 67 and from12 to 18 kg ha1 for fresh and stabilized manure, respectively(Blackshaw et al., 2005). Furthermore, Zhang et al. (2006) re-ported that the amount of N used by crops from municipalsolid waste compost for barley, wheat and canola, at two dif-ferent sites, was 11, 3, 1 and 2% for the first and subsequent3 years. These results indicate the complexity of estimating Nrelease from different composts and its relationship with plantN uptake. Barbarick and Ippolito (2007) showed that six ap-plications once every second year of anaerobically digestedbiosolids, in a 2-year wheatfallow rotation, provided about9 kg N t1 biosolids. Sullivan et al. (2003) noted that 7 yearsof one-time food waste compost application consistently in-creased tall fescue N uptake by a total of 294 to 527 kg ha1.Therefore, the increase in grass uptake due to amendment was15 to 20% of compost N applied. However, compost N immo-bilization/mobilization is predominantly linked to the degrad-ability and balance of soil C pools, as will be better explainedin Subsection 3.3 of this review.

Moreover, Eghball et al. (2004) found that the lowest rateof P (125 kg ha1) was applied for the annual P-based beefcattle manure whilst the greatest amount (594 kg P ha1) wasapplied for the biennial N-based cattle manure compost treat-ment. The latter resulted in P leaching to a soil depth of 45 to60 cm. On the contrary, the P-based application was environ-mentally sound, since it provided nutrients for the crop, whilemaintaining the amended soil P at a level similar to the un-treated control.

2.2.1. Yield response

There are three possible scenarios relating crop yield oragronomic productivity to organic C content of soil: (i) in-crease in crop yield as a consequence of organic carbon poolenhancement; (ii) no or little decrease in crop yield with re-duction in the organic carbon pool, and (iii) increase in crop

yield with decrease in the organic carbon pool (Lal, 2006).These apparently conflicting responses depend on several fac-tors such as the previous organic carbon pool, soil manage-ment, and use of chemical fertilizers and organic amendments.

As demonstrated by several long-term experiments on cropnutrition and yielding responses, the benefits of increasedorganic matter content will differ on the basis of the ratesupplied. In a 5-year trial, Hartl et al. (2003) found thatevery second year spreading of 40 t ha1 biowaste com-post, from source-separated organic household waste andyard trimmings, resulted in slightly higher (9%) rye yieldsthan other rates. This result suggested that beneficial use de-pends on choosing the best amount and frequency of compostapplication.

Frequently, the best rate is the highest, as supported byButler and Muir (2006), who observed the greatest tall wheat-grass dry matter yield with the highest composted dairy ma-nure rate of about 180 t ha1. More specifically, forage yieldincreased from 32 to 96% with 11.2 and 179.2 t compostha1, respectively, in the first of two growing seasons. Sullivanet al. (2003) highlighted the long-lasting effect of a one-timehigh rate (155 t ha1) food waste compost application in pro-viding slow-release N for crop growth, over a 7-year period.These results agreed with the findings of Habteselassie et al.(2006a), who found that soils with about 100 t ha1 dairy-waste compost maintained N supply to the plants throughcontinuous mineralization, as shown by available inorganic Npools, silage corn yield and plant N content analysis.

Wheat grain yield was enhanced on average by 246%, inrespect to the control, with a high rate (80 t ha1) of mu-nicipal solid waste compost applied annually over 5 years,alone or combined with mineral fertilizer, reaching 60.2and 61.9 t ha1, respectively (Cherif et al., 2009). Similarly,Erhart et al. (2005) investigated the agronomic performanceof biowaste compost, on cereals and potatoes, showing that thehighest application rate equal to 23 t ha1year1 increased cropyields by 10% compared with the unfertilized control, on theaverage of 10 years. These findings are in contrast with thoseof Mantovi et al. (2005). In repeated sewage sludge applica-tions for 12 years on a winter wheatmaizesugarbeet rotation,the composted sludge gave a significantly lower yield, equalto 51.8 t ha1, for sugarbeet with higher dose spreading com-pared with non-composted biosolid treatment yield (60 t ha1,on average). Moreover, residual effects of amendments on bothsoil properties and crop production can last for several years,as referred to in the preceding subsection of this review. Forinstance, in a 7-year trial Eghball et al. (2004) confirmed thatavailable P concentration in the soil surface can contribute tocorn crop P uptake for more than 4 years after the last bien-nial N-based compost application, being 241% higher than thecontrol.

The examined literature pointed out that the use of com-posts or of other organic amendments in combination withmineral fertilizers enhanced crop yield in many cropping sys-tems over more than 10 years, compared with compost andamendments alone (Ros et al., 2006a; Bi et al., 2008). The sub-ject has recently been discussed by Montemurro (2009), whofound that the multiple application of municipal solid waste


compost associated with mineral fertilizer increased wheatyield by 8% compared with mineral N alone and induced amore productive stability and N uptake throughout the years.Furthermore, maize grain yield was the highest where farm-yard manure at 10 t ha1 was applied along with recommendedNPK fertilizer for 34 years, under a maizewheat croppingsystem (Kaur et al., 2008). In a study by Sleutel et al. (2006),41 years of application of 35 t farmyard manure ha1, plus anequivalent amount of NPK in mineral fertilizer, every 5 yearsin a wheatwheatsugarbeetmaizemaize crop rotation, in-creased grain yield by 124 and 55% for maize and wheatin respect to the control. Moreover, in a 7-year study, Leroyet al. (2007) investigated vegetable-fruit-garden-waste com-post combined with cattle slurry applied at both 22.5 t ha1yearly and 45 t ha1 every other year. Both the applicationstrategies resulted in 25 to 43% higher maize yields in respectto the two organic amendments provided alone. On the con-trary, Edmeades (2003), after reviewing 14 long-term soil fer-tility trials, concluded that, despite the positive effects of or-ganic manures on soil biological and physical properties, noadvantage to crop yields with respect to the application of thesame amounts of nutrients as mineral fertilizers was found.However, it is necessary to note that in organic farming sys-tems the use of mineral fertilizers is not a possible alternative.After all, the analyzed soil management practices used in sus-tainable farming systems have potential for producing compa-rable yields to conventional farming ones. Poudel et al. (2002)reported that the average tomato fruit and corn grain yields, fora 5-year trial period, were 71.0 and 11.6 t ha1, respectively,both not significantly different among organic, low-input andconventional farming systems.

From all the above discussion, there is clear evidence thatthe best agronomic performance of compost, particularly ifcombined with mineral fertilizers, is often obtained with bothhigh rates and frequency of applications, leading to residualeffects as a slow-release nitrogen fertilizer.

2.2.2. Quality response

From the yield quality viewpoint, the reviewed long-termresearch showed, for example, that source-separated organicwaste compost as well as mixtures of sugarbeet vinasse com-posted with other agro-industrial solid wastes did not ad-versely affect the quality of products (Madejn et al., 2001;Erhart et al., 2005). In particular, winter rye protein concentra-tion was similar in compost and mineral fertilized treatments,and the nitrate concentration of potato tubers in compost treat-ments was not significantly higher than in the unfertilizedcontrol (Erhart et al., 2005). The crop quality in some caseswas even improved by compost fertilization. In one above-mentioned study, the partial substitution of mineral with or-ganic N not only did not reduce the quality of wheat, withrespect to mineral fertilizer, but also increased the proteincontent by 6% in comparison with the unfertilized control(Montemurro, 2009). In another study the wheat grain qual-ity, expressed by its apparent specific weight as means of 12cropping years, was worsened by excessive N supply, so high

rates (10 t ha1 year1) of liquid and dewatered sludge wereparticularly detrimental, while compost was safer (Mantoviet al., 2005). In fact, a downward trend was observed as74.7 > 73.5 > 72.8 kg hL1 for composted, dewatered andliquid sludge, respectively.

Therefore, considering that stabilized organic amendmentapplication does not reduce the crop yield quality, as reviewedhere, but can even enhance it, their use can appear moreprofitable.

3. ENVIRONMENTAL IMPACTAND SUSTAINABILITYOF ORGANIC AMENDMENTS

As recently highlighted by Lichtfouse et al. (2009), whileconventional agriculture is driven almost solely by productiv-ity and profit, sustainable agricultural systems aim at develop-ing new farming practices that are also safe and do not degradethe environment. By taking this into account, and from theprevious discussions, it is possible to suggest that appropriateorganic matter management is a fully sustainable pattern, be-cause it fulfils three requisites: sustainability of resources, hu-man health and economic sustainability. Conversely, organicamendments that can improve organic matter and the linkedsoil fertility can also be a source of environmental pollution,especially when they are improperly used. In fact, when theapplication rates of manure are calculated on the N crop re-quirement, the amount of P added often exceeds the plant P re-quirement, resulting in soil P accumulation (Miller and Miller,2000). According to Komatsuzaki and Ohta (2007), soil man-agement strategies for sustainable agriculture should focus notonly on increasing organic matter in the soil, but also on theuptake or stocking of soil residual nutrients in such a way toprevent excess nutrient leaching into the groundwater.

The cropland application of immature compost can pro-duce environmental and agronomic problems. In fact, if theorganic material has not been sufficiently stabilized, its ap-plication increases ammonia volatilization, decreases the soiloxygen concentration, produces some phytotoxic compoundsand immobilizes soil mineral N. Therefore, organic productsof high quality must be produced and their stability must beaccurately assessed.

With regard to toxic elements, e.g. heavy metals, their ac-cumulation in soils is the most often cited potential risk, par-ticularly for biosolid waste compost use. On the other hand,there is increasing positive evidence of the impact that com-posts and wastes can have on soil C sequestration, as we willdiscuss in the following pages.

3.1. Organic matter evolution and soil carbonsequestration

In recent years, global concern over increased atmosphericCO2 and methane (CH4) emissions has raised interest aboutthe potential role of soils as a source or sink of C and in study-ing organic matter dynamics and related C sequestration ca-pacity. In fact, organic matter varies in both decomposition


rate and turnover time. and the soil C pool can be a sourceor sink for the atmospheric pool, depending on land use andmanagement (Van-Camp et al., 2004). When organic materi-als, such as composted wastes, are added to soil, at least a shareof their organic C is decomposed producing CO2, which is agood indicator of the decomposition rate, while another part issequestered in the soil.

For the purpose of this review, the term carbon seques-tration is used according to Lal (2004, 2007), implying thetransfer of atmospheric CO2 into the soil C pool through: (i)humification of crop residues and other wastes added to thesoil, (ii) formation, in arid and semi-arid regions, of secondarycarbonates or leaching of bicarbonates into the groundwater,so CO2 thus adsorbed is not immediately re-emitted, (iii) for-mation of organo-mineral complexes which encapsulate C andprotect it against microbial activities, and (iv) translocation oforganic carbon into the sub-soil, that can move it away fromthe zone of disturbance by plowing and other agronomic prac-tices, minimizing the risks of being removed by erosional pro-cesses.

Article 3.4 of the Kyoto Protocol emphasizes the agricul-tural role in CO2 sequestration in tilled soils and advocatessustainable cropping techniques for this purpose. These prac-tices primarily include the reduction of soil disturbance andthe optimization of water-use efficiency. However, soil incor-poration of organic materials and N fertilization can also influ-ence C dynamics (Triberti et al., 2008). Any attempt to enrichthe organic carbon reservoir through sequestration of atmo-spheric C will help to manage global warming.

According to Feller and Bernouxs (2008) findings, it is es-sential to consider that when an organic waste contains a highpercentage of fossil carbon, the C in soils originating from thisfraction should not be referred to as sequestered C, i.e. origi-nating from the atmosphere, but as stored C.

As explained by Mondini and Sequi (2008), it is also im-portant to consider that the reduction in the soil of the C poolcontributes both to an atmospheric enrichment in CO2 concen-tration and an involution in soil fertility. This reduction alsoinduced the onset of degradative processes such as erosion,salinization, desertification, compaction, nutrient deficiency,etc. However, whereas the exact magnitude of the historic lossof organic carbon may be debatable, Lal (2004) suggested thatthe troubling process of organic carbon depletion can be re-versed.

The effects of organic amendments on organic carbon in-crease should be studied through long-term field experiments,as already seen in Subsection 2.1.2. of this review, because ofthe long time needed to attain a new equilibrium after environ-mental change in the organic matter.

Triberti et al. (2008) reported that 29 years after the start ofa trial comparing cattle manure, cattle slurry and crop residueswith mineral fertilization, the cattle manure gave the quick-est organic carbon stock build-up, 0.26 t organic carbon ha1year1. Each t dry matter ha1 of applied cattle manure, con-taining 33.1% of organic C, increased the organic carbon stockby 27 kg C ha1 in the 00.4 m soil layer. This increase corre-sponded to the highest sequestration efficiency, equal to 8.1%of the C added, due to its low degradability, as compared with

3.8 and 3.7% in the instances of cattle slurry and cereal cropresidues. On the whole, the recycling of cattle farming by-products for CO2 sequestration purposes poses environmentalrisks, such as pollution by nitrate N (NO3N), that can limitits sustainability in fertile cropland of developed countries.

About 25 and 36% of applied manure and compost C re-mained in the soil after 4 years of application, indicatinggreater C sequestration with composted than non-compostedmanure (Eghball, 2002). According to Sodhi et al. (2009), a10-year application of rice straw compost, either alone or incombination with inorganic fertilizers, results in C sequestra-tion in macroaggregates. In fact, with the application of 8 tcompost ha1, the C concentration in the 12 mm size fractionincreased by 180 to 191%, respectively, over unfertilized con-trol. From trials still in progress for more than 10 years, there isevidence that the organic carbon sequestration rate increasedmore due to farmyard manure and composted farmyard ma-nure in comparison with mineral fertilizers or other organicmaterials (Monaco et al., 2008; Kukal et al., 2009).

Mandal et al. (2007) observed that the total quantity of soilC sequestered over a long period was linearly related to the cu-mulative crop residue C inputs, and the rate of the conversionto organic carbon was higher in the presence of added organicmaterials, i.e. 6.9% of each additional t C input ha1, than intheir absence (4.2%). Indeed, it has been calculated that therate of organic carbon sequestration is on average 0.3 to 0.5 t Cha1 year1 under intensive agricultural practices (Lal, 2007).As reported by Mondini and Sequi (2008) organic matter isthe largest C stock of the continental biosphere, with 1550 bil-lion tons. On the time scale of several decades, in arable soilswhich receive high organic materials, relatively less organicmatter is stabilized either by association with the silt plus claymineral fractions, or by its inherent biochemical recalcitrance(Sleutel et al., 2006).

Shindo et al. (2006) reported that continuous compost ap-plication in a field subjected to 25 years of double croppingcould increase both the amounts of fulvic and humic acids,and the total humus content. From the organic carbon se-questered quality standpoint, Nardi et al. (2004) found that,over 40 years, farmyard manure fertilization improved by116% the production of humus with a high degree of polycon-densation, a high-quality fraction usually linked to soil fertil-ity. Conversely, the absence of organic fertilizer inputs deter-mined the opposite, with a higher percentage of non-complexand lightweight humus.

Besides, the soil C sequestration should not be restricted toa mere quantification of C storage or CO2 balance. All green-house gas fluxes must be computed at the plot level in CCO2or CO2 equivalents, incorporating as many emission sourcesand sinks as possible across the entire soil-plant system (Fellerand Bernoux, 2008). For example, results of Ginting et al.(2003) showed that fluxes of CH4 and nitrous oxide (N2O)were nearly zero after 4 years of manure and compost appli-cations. This evidence indicates that the residual effects hadno negative influence either on soil C and N storage, or globalwarming. However, there is a lack of information and a strongneed to further understand the greenhouse gas fluxes as relatedto organic material input dynamics in the soil.


The reviewed results suggest that the process of organic car-bon depletion can be reversed by long-term organic amend-ment application. Soil carbon sequestration should be consid-ered as a winwin strategy for increasing soil fertility andpreventing soil erosion processes (Snchez-Monedero et al.,2008).

3.2. Heavy metal concentration in the agro-ecosystem

Despite the numerous benefits, the agricultural utilizationof raw and composted wastes could also induce adverse im-pacts on the environment. This is particularly due to the typesof toxic elements contained in these organic materials thatmight enter the food chain, since they may be taken up byplants from soil.

The concentration of heavy metals in compost is generallyhigher than the normal concentration in soil, so the possibilityexists of metal accumulation when the compost is repeatedlyapplied (Zhang et al., 2006). On the other hand, little researchis available from studies lasting decades and longer about theavailability of metals applied as constituents of composted or-ganic matter. Over a 10-year trial period, the most abundantmetals in the uppermost soil horizon were copper (Cu), zinc(Zn) and lead (Pb). Cadmium (Cd) was the least plentiful, cor-responding to the mean metal concentrations in the municipalsolid waste compost applied (Businelli et al., 2009). This issupported by the opposite findings of Bergkvist et al. (2003)who found, during a period of 41 years, 92% of applied Cdwas recovered in the topsoil in sludge treatment, indicatingmeasurable losses by both downward movement and crop up-take.

Six-year consecutive applications of a swine compost re-sulted in significantly higher concentrations of Cu and Zn at1020 cm depth of the compost-amended soil, relative to thecontrol, with an increase from 102.8 to 127.4 mg kg1 for Cuand from 111.9 to 165.7 mg kg1 for Zn (Zhao et al., 2006).On the other hand, Bartl et al. (2002) found that 32t ha1 ofbiowaste compost did not influence the total contents of Cd,manganese (Mn), molybdenum (Mo) or nickel (Ni) in soil, in5 years. The total soil contents of Zn and Pb were significantlyhigher in soils with compost treatment than in the unfertilizedsoils.

From experiments longer than 10 years, it is possible to sug-gest that sludge and composted sludge showed a high accumu-lation of Zn, Cu and chromium (Cr), probably due to the no-tably higher concentration of these metals in the raw materials(Saviozzi et al., 1999; Kunito et al., 2001). The soil enzyme ac-tivities, dehydrogenase, urease and -D-glucosidase were alsofound to be adversely affected by the metals derived from theaddition of sewage sludge (Kunito et al., 2001).

Notwithstanding, the concentration of pollutants in com-posts may be reduced through the correct separation of or-ganic wastes at source, which offers the opportunity of high-quality input material for aerobic biodegradation (Montemurroet al., 2009). For the long-term protection of the environment,it is also necessary to develop and implement other prelimi-nary treatments for potentially polluting wastes. Erhart et al.

(2008) showed that, after 10 years of application, the use ofhigh-quality biowaste compost gives no variation in either to-tal heavy metal concentrations or available fractions. In partic-ular, with total applications of 95, 175 and 255 t biowaste com-post ha1, no heavy metal significant increase was measurable,except for Zn in the treatment with the highest application rate.

However, the environmental hazard is strictly linked to themobility of metals, and to their concentration in the soil solu-tion rather than to the total soil concentration. According toBusinelli et al. (2009), metal mobilization is not an imme-diate process, but it involves various equilibria that controltheir adsorption and desorption. These behaviors depend onsoil characteristics and climatic conditions and they involvebiochemical processes responsible for the microbial degrada-tion of autochthonous and compost-derived organic matter.This highlights the importance of long-term experimentationwhen studying the environmental fate of heavy metals, par-ticularly in compost-amended soils on large scales (Businelliet al., 2009).

Within this context, according to Tittarelli et al. (2007), themain factors that affect the environmental behavior of heavymetals are: (i) cation exchange capacity, as an index of thesoil capacity to adsorb and hold metal cations; (ii) humic sub-stances, that can interact with heavy metals, forming com-plexes with different solubility, and consequently mobility, and(iii) the water and thermic regime of the soil, which affect theorganic matter decomposition.

It can also be generally assumed that extractability and up-take of heavy metals decline as the soil pH becomes more al-kaline, especially after repeated compost application. By con-trast, low pH in the soil, caused by more than 60 years of ricestraw compost applications, may have enhanced the concen-tration of metals in the water-soluble fraction (Takeda et al.,2005).

The aerobic composting processes increase the complexa-tion of heavy metals in organic waste residues. In this condi-tion the metals are strongly bound to the compost matrix andorganic matter, limiting their solubility and potential bioavail-ability in soil (Smith, 2009). However, 10 years after munici-pal solid waste compost application, a share of the heavy met-als was further re-mobilized in the soil profile, also leading toa decrease in the percentage distribution of organically-boundheavy metals with time. According to Businelli et al. (2009),this metal mobilization was primarily influenced by organicmatter dynamics. On the contrary, Sukkariyah et al. (2005)reported that in aerobically digested sewage sludge-amendedsoils, the extractability of the heavy metals steadily declinedover 17 years, despite a significant decline in organic matterconcentration in the amended plots. This outcome showed thatthe mineralization of exogenous applied organic matter did notincrease heavy metal availability due to loss in metal bindingcapacity associated with organic matter.

Monitoring concentrations of various heavy metals, theirfractionation and the percentage contribution of each fractionto the total concentration are important analytical tools. In fact,in soil receiving long-term application of compost or other or-ganic materials with relatively high concentrations of metals,these determinations can produce important information at the


present time and help to determine what might happen in thefuture (Zhao et al., 2006; Businelli et al., 2009).

In any case, moderate compost doses applied in currentagriculture will not cause any risk of toxicity for plants oranimals, and therefore it is reasonable that application ratesalways have to be chosen on the basis of limited heavy metalloadings. However, no adverse effect on plant growth or exces-sive amounts of plant metal uptake were noted 17 to 19 yearsafter biosolid application, despite the high application rate,equal to 210 t ha1, containing concentrations of Cu and Znthat exceeded the pollutant concentration limits (Sukkariyahet al., 2005).

In general, heavy metal uptake by crops increases in leafyplants and it is higher in cereal leaves than in grain. Lettuce,for example, had higher assimilative capacity for Zn and Cduptake than other non-leafy crops (Sukkariyah et al., 2005).Mantovi et al. (2005) reported that biosolid applications sig-nificantly increased the content of Zn and Cu in wheat grainand of only Cu in both sugarbeet roots and maize grain. Cad-mium is one of the most significant potential contaminants offood supplies on arable lands and may limit sewage sludgesuitability for soil amendment, because this organic materialpresents a large amount of this metal (Singh and Pandeya,1998). Being relatively soluble in soils, it is readily taken upby crops and it is quite toxic to humans (Miller and Miller,2000). This behavior is particularly evident at low soil pH. Cdsolubility in equilibrium extracts of Ca(NO3)2 increased, dur-ing the 41-year trial above mentioned, by a factor of 20 in thesludge treatment compared with the control (Bergkvist et al.,2003). This was reflected in the Cd concentration of the strawfraction in barley, which was almost doubled in the sewagesludge treatment, compared with the control. The grain frac-tion, on the other hand, showed no significant increase in Cdconcentration.

It can be concluded that there is no tangible evidencedemonstrating negative impacts of heavy metals applied tosoil, particularly when high-quality compost was used fora long period. Composting processes also inherently reducemetal availability compared with other organic waste stabiliza-tion methods (Smith, 2009).

3.3. N pool fate

Proper use of organic amendments requires the capacity topredict the release in the soil of inorganic nutrients from or-ganic forms. Sikora and Szmidt (2001) suggested that a betterunderstanding of coupled mineralization-nitrification is essen-tial to manage the soil N pool with an environmentally soundapplication of amendments.

The C/N ratio, which is an important tool of amendmentevaluation, cannot explain all differences in N mineralization,since organic materials with similar C/N ratios may miner-alize different amounts of N. This behavior is probably dueto other differences in their chemical compositions. Repeatedlong-term applications of organic amendments not only gener-ally increase the size of the soil organic N pool, but also cause

remarkable changes in soil characteristics, that influence N dy-namics and can lead to a residual effect.

Habteselassie et al. (2006a) found an 89% increase in to-tal soil N content after 5 years when dairy-waste compost at200 kg N ha1 was applied. Conversely, Zaman et al. (2004)observed, after 23 years of 240 kg Nha1 sewage sludge plussawdust compost application, an increase in total N contentof approximately 14%, as compared with a chemical fertilizersupply. The increase in total soil N after compost, biosolidsor compost-plus-N was closely related to the build-up of or-ganic matter in the soil over time and may be attributed tothe direct effect of organic inputs (Mantovi et al., 2005; Roset al., 2006a). After 7 years of application, the soil N con-centration in the topsoil accounted for 33% of compost N ap-plied (Sullivan et al., 2003). Antil et al. (2005) showed thatthe highest total soil N content increase, by 70% in respect toNPK mineral fertilizer, was found after 38 years of animal-manure liquid applications, as compared with solid animal-manure, cattle slurry and half cattle slurry plus straw. More-over, in the plots after 13 years of cropping, compared withthose kept fallow for the same period, there was a clear impactof N removal due to both the harvest and N losses, followingenhanced mineralization due to tilling.

The N uptake by crops depends on several factors such asthe N content and C/N ratio of the amendment, soil charac-teristics, climatic conditions and, obviously, on the plants Nrequirements. Research on an average of 10 years by Hartland Erhart (2005) pointed out that the ratio of N output byharvested plant parts to total N input in the source-separatedorganic waste compost treatments ranged from 3 to 7% of theapplied N. This result was lower compared with mineral fer-tilization (11 to 15%), even if it was not reflected by a higherproportion of NO3N in the soil profile at the end of the grow-ing season.

Data collected on N recovery from different composts, re-lated to N uptake or yield, showed that the N effect of com-post application does not generally exceed 1520% of totalN supply in the first year, while the residual compost N poolis mineralized at rates of 38% in following years. Continu-ous compost amendments and crop rotations with high nutri-ent demand may increase N mineralization (Amlinger et al.,2003). Eghball (2000) found that of the organic N appliedto provide corn N requirements, 151 kg available N ha1 foran expected 9.4 t ha1 grain yield, about 11% was mineral-ized from composted beef cattle manure and 21% from non-composted manure during the succeeding growing season. Alower N availability from compost reflects the presence of sta-ble N compounds.

The organic matter mineralization process increases theamount of ammonium and nitrate in the soil. However, theNO3N is minimally adsorbed by the soil particles becauseit is very mobile and is susceptible to losses into ground- andsurface waters by infiltrating water. Within this context, it isessential to remember that the synchronization of N supplywith crop N demand, together with a proper application rate,is the best way to avoid the accumulation of soil mineral N,thus reducing the risk of NO3N leaching (Montemurro andMaiorana, 2008).


The long-term application effects of raw and compostedorganic materials on nitrate leaching have been evaluated byseveral researchers, but the results of the studies are often dis-cordant. In fact, Mallory and Griffin (2007) in a 13-year exper-iment found that, despite similar NH4N inputs and rates ofNH4N consumption for manure and fertilizer N treatments,NO3N accumulation was slower in the manure treatment.This is because the N from manure became available moreslowly than fertilizer N. On the contrary, Basso and Ritchie(2005), in a 6-year maizealfalfa rotation, observed the high-est amount of NO3N leaching of 681 kg ha1 in the manuretreatment, followed by compost (390 kg ha1), inorganic N(348 kg ha1) and control (311 kg ha1). They suggest that, al-though manure applications can be valuable for organic matterincrease, attention needs to be given to the possible environ-mental impact without benefiting from yield increase.

From several experiments longer than 8 years conductedunder different soil and climate conditions, it might be con-cluded that compost fertilization resulted in equal or lowerNO3N leaching losses to groundwater than correspondingmineral supply (Hartl et al., 2001; Hartl and Erhart, 2005;Erhart et al., 2007). These findings are observed even withhigher amounts of amendments than used in practical farm-ing. Nevens and Reheul (2003) reported that, at the end of a4-year trial, the increase in residual soil NO3N was smallerwith vegetable-fruit-garden waste compost and compost pluscattle slurry applications, in respect to mineral fertilizer treat-ment.

Regarding the effect of the application rates, over a 5-yearfield trial with permanent rye, the treatments receiving 20 to40 t ha1 of biowaste compost per single application showedsmaller amplitudes in the soil NO3N levels in fall thanthe treatments receiving 60 t ha1 (Hartl et al., 2003). Otherstudies have noted that compost produced more nitrate thanneeded for plant use throughout a 5-year period. This is par-ticularly evident at a high rate of application. The contin-uing mineralization of the organic material, even after har-vest, led to postharvest nitrate production and a consequentdownward movement (Habteselassie et al., 2006a). Further-more, measurements of gross N transformation rates areimportant to properly understand N cycling in agriculturalsoils, where both productive and consumptive processes occur.Habteselassie et al. (2006b) reported that, by applying dairy-waste compost and liquid dairy-waste annually for 6 years,the mean gross N mineralization rates were about 5.7 and2.9 mg N kg1 day1. Conversely, the gross nitrification rateswere 10.2 and 1.6 mg N kg1 day1, respectively, and the netmineralization rates were less than 35% of gross rates. Theratio of gross nitrification to mineralization for dairy-wastecompost-treated plots were more than 100%, indicating a largeincrease in nitrifying capacity of the soil following

long-term effects of organic amendments on soil fertility. a review

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