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Waste Management 28 (2008) 678–684

Implication of soil C sequestration on sustainable agricultureand environment

C. Mondini a,*, P. Sequi b

a C.R.A. – Istituto Sperimentale per la Nutrizione delle Piante, sez. di Gorizia, Via Trieste, 23, I-34170 Gorizia, Italyb CRA – Istituto Sperimentale per la Nutrizione delle Piante, Via della Navicella 2, 00184 Rome, Italy

Accepted 6 September 2007Available online 26 November 2007

Abstract

Soil organic matter (SOM) is the largest C stock of the continental biosphere with 1550 Pg. The size of C reservoir in the soil andenvironmental concerns on climate change have recently attracted the attention of scientist and politicians on C sequestration as an effec-tive strategy to tackle greenhouse gas (GHG) emissions. It has been estimated that the potential for C storage in world cropland is rel-evant (about 0.6–1.2 Pg C y�1). However, there are several constraints of C sequestration that raise concern about its effectiveness as astrategy to offset climate change. C sequestration is finite in quantity and time, reversible, and can be further decreased by socio-eco-nomic restrictions. Given these limitations, C sequestration can play only a minor role in the reduction of emissions (2–5% of totalGHG emission under the highest emission scenarios). Yet, C sequestration is still attractive for two main reasons: it is likely to be par-ticularly effective in reducing atmospheric CO2 levels in the first 20–30 yr of its implementation and presents ancillary benefits for envi-ronment and sustainability that make it a real win–win strategy. These beneficial implications are discussed in this paper with emphasison the need of C sequestration not only to offset climatic changes, but also for the equilibria of the environment and for the sustainabilityof agriculture and of entire human society.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Soil organic matter (SOM) is the largest C stock of thecontinental biosphere with 1550 Pg (about three times thatof vegetation – 560–650 Pg, and higher than the atmo-spheric pool – 750 Pg) and has accumulated in soil overhundreds or even thousands of years (Table 1) (Lal, 2003).

The size of the C reservoir in the soil and rising environ-mental concern on climate change have recently attractedthe attention of scientists and politicians on the potentialfor soil C sequestration as a strategy to address greenhousegas (GHG) emissions. As a consequence several researchefforts have been undertaken at different levels on severalaspects of C sequestration. From the results of such studiesit has been estimated that the potential for C storage inworld cropland is about 0.6–1.2 Pg C y�1 (Lal, 2004a)

0956-053X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.wasman.2007.09.026

* Corresponding author. Tel.: +39 0481 522041; fax: +39 0481 520208.E-mail address: claudio.mondini@entecra.it (C. Mondini).

and in the agricultural soils of EU-15 it is of the order of0.45 Pg C y�1 (Smith, 2004).

Nevertheless, there are several indications that raiseconcern about the effectiveness of C sequestration as astrategy to address GHG emissions (Wanderer and Nis-sen, 2004).

C sequestration has a finite potential and a yearlyincrease in SOM can be sustained for only 50–100 yr; assoil tends to approach a new equilibrium, the increase insoil organic C (SOC) slows down and eventually ceases(Smith and Powlson, 2003). In addition, this theoreticalpotential is further decreased by several constraints suchas: land-suitability, unavailability of land and resourcesand socioeconomic restrictions. Due to this limitation itwas calculated that only 20% of the soil C sequestrationpotential is realistically achievable (Freibauer et al., 2004;Smith, 2004).

Another limitation is that C sequestration is not perma-nent and is reversible if appropriate soil management

Fig. 1. Simplified model of annual variations in the soil organicconstituents (Sequi and Pagliai, 1983).

Table 1Distribution of C on continental land (Lal, 2004a)

C pool Size (Pg = 1015 g)

Geologic (coal, oil, gas) 5000Soil organic matter 1550Soil inorganic C (carbonates) 1000Atmospheric 760Biotic 600

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practices are not maintained. This points to the necessity ofenduring policy drivers and the uncertainty associated withthe duration of these policies (Marland et al., 2001).

Therefore, soil C sequestration can play only a minorrole in the reduction of GHG emissions. It is widely recog-nized that if CO2 concentrations are to be stabilized at rea-sonable levels (450–650 ppm), the main solution to avoidclimate change is a drastic reduction in the GHG emissionsby finding new energy technologies that do not emit C. Infact, under the highest emission scenarios soil C sequestra-tion can contribute to a reduction of only 2–5% of totalGHGs emission in EU-15 (Smith, 2004).

Notwithstanding these constraints, C sequestration stillrepresents an attractive strategy for two main reasons.

From the exclusive point of view of climate change, it isrecognized that in order to achieve acceptable levels ofGHGs there is the necessity to obtain a sharp decrease oftheir content in the next period of 20–30 yr (IPCC – Inter-governmental Panel on Climate Change, 2000; Wandererand Nissen, 2004). C sequestration is likely to be particu-larly effective in reducing atmospheric CO2 levels in the first20–30 yr of its implementation and it should be included inany set of measures aimed to reduce atmospheric CO2 con-centration, thus allowing time to be saved while technolo-gies aimed at the reduction of emissions of GHG aredeveloped. During this critical period, C sequestrationhas been shown to be the most cost effective and feasiblemeasure aimed at the reduction of emissions of GHG(Marland et al., 2001).

From a wider perspective it is essential to consider thatsoil C sequestration is only one of the beneficial environ-mental and economic implications of appropriate SOMmanagement. Improved SOM management, necessary toachieve the sequestration of C in soil, presents a range ofother environmental and economic benefits (Lal et al.,1998; Dumanski, 2004).

These beneficial implications will be further discussed inthis paper.

2. Environmental implications of soil organic matter

2.1. Energy conservation

The best way to understand the role of organic matter insoil is to consider its role in the general cycle of C in nature.As it is well known, part of the solar energy (about 0.05–1% of incidental energy) is captured by photosynthetic pro-

cess. Part of this energy is consumed by plants, while theremaining energy is made available for other living organ-isms. It has been estimated that of the total energy madeavailable by plants, 20% is utilized by animals and theremaining energy, constituted by organic residues reachingthe soil, is consumed by soil microorganisms. About 70%of these organic residues are mineralized by soil microor-ganisms and utilized as an energy source, while the remain-ing 30% is incorporate in SOM in the first year (Brady andWeil, 2004) (Fig. 1). Organic C that is incorporated annu-ally in the soil without human intervention has been evalu-ated to range between 4% and 11% of the annual solarradiation (Jenkinson, 1981).

Therefore, taking into account the figures of the overallbalance of organic matter, the function of each pool in thegeneral cycle of C in nature can be deduced (Fig. 2). Pho-tosynthetic organisms capture solar energy, making itavailable to heterotrophic organisms. The main role of soilorganisms is the degradation of residues and the synthesisof increasingly complex and recalcitrant SOM. Soil organicmatter exerts its main function in the conservation of theenergy that is captured by photosynthesis. This storedenergy is fundamental for the long-term and regular func-tioning of the soil ecosystem. The largest pool of SOM con-sists of humic substances, which are recalcitrant enough toendure for long periods of time, but still allow for decom-position and nutrient release to take place. This allows forSOM to exert both a structural role (i.e., improvement ofphysical properties) and a role as a substrate for microbialactivity (Janzen, 2006).

Fig. 2. Carbon content (Mg ha�1) and environmental functions of main C pools on terrestrial land (Sequi, 1989).

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2.2. Biological significance

The amount of living organisms in the soil is by farhigher than that in the upper soil. About 20,000 kg oforganisms live in a hectare of soil, in comparison withabout 300 kg of men and animals in the upper soil (Sequiand Pagliai, 1983). Soil microorganisms play a key role inmaintaining ecosystem functions and productivity by theircontribution to a wide range of essential functions such as:improvement of soil structure through mixing soil horizonsand SOM, decomposition processes and humic substancessynthesis, C and nutrient cycling, enhancement of theuptake of nutrients and water, protection of crops frompests and disease through biological control and bioreme-diation of toxic metals or other hazardous wastes (Van-Camp et al., 2004).

Usually situations favouring the accumulation oforganic matter in soil increase both the amount of soilmicrobial biomass and its proportion of the total SOM(Jenkinson and Ladd, 1981; Powlson et al., 1987). Increasesin the size of soil microbial biomass have been associatedwith enhancement in soil microbial processes (Kennedyet al., 2004) and in soil fertility (Jiang et al., 2006). Onthe other hand, SOM losses induced by tillage directlyaffect the amount of microbial substrates and thus soilmicrobial biomass decreases (Smith and Paul, 1990).

Life in soil is important also from a qualitative point ofview. In fact, soil is one of the most diverse habitats onthe earth and contains one of the most diverse collectionsof living organisms (Powlson et al., 2001); 1 g of soil cancontain many thousands of species of bacteria, as well avariety of other species (Ritz et al., 2003). The main rea-son for this diversity is probably the enormous variationin environmental conditions over short distances thatoccurs in the soil (Powlson et al., 2001); however, SOMalso exerts an important role in determining the degreeof soil biodiversity. The considerable chemical and struc-tural heterogeneity associated with SOM leads to a greatdiversity in the community that feed upon it. Large

amounts of heterogeneous organic C were found to playa significant role in structuring microbial communities(Zhou et al., 2002).

Biodiversity is essential for a proper functioning of thesoil ecosystem. Each soil organism exerts a specific role,and therefore higher diversity favours more processes tobe performed and for them to be performed more effi-ciently. Another consequence of diversity is functionalredundancy, i.e., different microorganisms can performthe same function and this guarantees a higher stabilityof the system against declines in function.

Higher biodiversity was found to increase decomposi-tion of substrates, humus synthesis, biodegradation andbiological control (Ritz et al., 2003). Diverse systems wereshown to be more productive, stable and resilient (Nanni-pieri et al., 2003). There is a close relationship betweenSOM and soil biodiversity, and a rich and varied sourceof organic matter tends to support a wide variety of soilmicroorganisms (Lal, 2002). Soil management practicesthat enhance SOM also increase the biodiversity of soil(Zhou et al., 2002; Kennedy et al., 2004). The ecosystemcan be managed to increase soil biotic diversity by increas-ing OM recycling through crop residue retention, reducingtillage and increasing crop diversity (Kennedy and Smith,1995). On the other hand decrease of SOM is generallyassociated with decline in soil biodiversity.

2.3. Nutrient balance

In the modern societies, the natural cycles of the ele-ments are broken by not replacing the essential nutrientsharvested in crops with the recycling of organic residuesin soil, causing negative environmental and economic con-sequences (Lal, 2004a). In order to attain the equilibriumof nutrient cycles it is enough to follow a very simple rec-ipe: what comes from the soil must be returned to the soil.Appropriate SOM management allows for the closure ofthe cycles of the elements and avoids negative unbalancing(Sequi, 1999).

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2.4. Soil quality, soil health and ecosystem functions

Soil quality may be defined as the capacity of a soil toperform particular ecosystem functions such as supportof plant growth, cycling of elements, provision of a habitatfor soil microorganisms, and regulation of the water cycle(Weil and Magdoff, 2004). SOM is a major factor in deter-mining the soil capacity to perform these functions becauseof its positive role on the physical, chemical and biologicalproperties of soil. As a consequence there is a strong linkbetween soil quality and SOC pool (Lal, 2004b). Long-termstudies have consistently shown the benefit of increasing Cinputs, by manure addition and crop rotation, on main-taining agronomic productivity (Reeves, 1997).

Soil health refers not only to its lack of degradation andcontamination, but on the overall strength for carrying outecosystem functions and responding to environmentalstresses. High levels of SOM are associated with reducedrun off, enhanced soil aggregation and improved filtration,movement and retention of water. Maintenance of soilstructure through aggregation assisted by SOM plays akey role for the prevention of soil degradation through ero-sion and the onset of desertification.

2.5. Environment decontamination

SOM presents buffering, filtering and detoxifying prop-erties that are essentials to maintain the quality of the nat-ural resources and the environment.

Organic matter serves as a buffer in ameliorating theadverse effects of inorganic pollutants. Immobilization ofheavy metals, such as the highly toxic contaminants Pband Cd, through complexation with humified organic mat-ter reduces the concentration of metal ions in the soil solu-tion and consequently their potential to harm (Stevenson,1994). Toxicity of Alþ3 is a widespread phenomenon in sev-eral regions of the United States, Canada and the tropics.Organically complexed forms of Al are less harmful toplants than Alþ3 . Increasing organic matter by amendingthe soil, as well as soil rich in native OM, were found toallow better plant growth (Stevenson, 1994).

Organic matter is a key factor in determining the behav-iour in the soil of many organic chemicals. Organic matteris the soil characteristic most directly related to the sorp-tion of several herbicides. In addition, SOM can act as buf-fer, ion exchanger and chelating agent. SOM thereforeinfluences bioactivity, persistence, biodegradability, leach-ability and volatility of organic pollutants (Kozak, 1996).The degradation of a chemical compound in the soil canoccur through several biotic and abiotic processes, withthe former exerting a major role. Many organic contami-nants are subject to attack by microorganisms and can betransformed into useful microbial substrates (Bouwer,1992), and a positive relationship exists between contentand activity of microorganisms and degradation. Walkeret al. (1992) reported that the rate of degradation of ala-chlor was positively correlated with microbial biomass

and respiration. An increase in organic matter usuallyenhances the rate of degradation of organic contaminantsas SOM acts as a source of energy and nutrients for growthand reproduction of soil microorganisms that take part inthe degradation process (Kozak, 1996).

Another important ecosystem service provided by thesoils is the cleaning of the water. SOM exerts this functionby significantly affecting several components of the hydro-logic cycle, such as water storage, infiltration rate and run-off (Lal, 2004b).

The quality of water depends on the content of sus-pended sediments and dissolved chemicals.

Suspended sediment load is mainly influenced by soilerosion. SOM has a positive role in increasing the abilityof the soil to resist erosion. In particular, the positiveaction of SOM on the aggregation of soil particles increasesaeration, water holding capacity, porosity and permeation.Soil aggregates are not easily carried along by movingwater, and a better soil structure favours water infiltrationand percolation through the soil profile reducing thereforesuperficial runoff (Stevenson, 1994).

Decontamination of dissolved inorganic pollutants ismainly influenced by the content and quality of humic sub-stances and the size and activity of soil microorganisms.

Therefore, the implementation of management practicesenhancing SOM is a strategy that decreases sediment loadin streams and rivers, degrades organic pollutants andexerts a buffering action on inorganic contaminants (Lal,2004b).

3. Soil organic matter and sustainability

Cultivation of natural ecosystems (forests, pastures,etc.) and the intensification of agricultural systems areamong the predominant global changes of the last cen-tury. These transformations were able to satisfy the foodand fibre requirements of global population, but suchrequirements are expected to significantly increase in the21st century due to the foreseen increase in global popu-lation. Most of the best quality land is already used foragriculture, which implies that further possible expansionwould occur on marginal land, unable to sustain highyields (Tilman et al., 2002). Therefore, satisfaction ofthe future needs could be guaranteed mainly by intensiveproduction practices (Powlson et al., 2001). Indeed, agri-cultural intensification, characterized by high mechaniza-tion, chemical inputs, high yielding varieties andirrigation, has been able to increase global food produc-tion for the last 50 years. However, concerns have beenraised over both the long-term sustainability and environ-mental consequences of the intensification of agriculturalsystems and the ability to feed the rapidly growing popu-lation in the 21st century (Matson et al., 1997). There areindications in areas of east and southeast Asia of declin-ing in productivity due to the lack of resources (water)and increased susceptibility to disease and insects pests(Matson et al., 1997; Tilman et al., 2002). Moreover, it

Fig. 4. Estimation of C losses (Pg = 1015 g) from fossil fuel combustionand from soil in post-industrial era (1850–1998) (Lal, 2003).

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is now clear that expansion of agricultural land and inten-sification can have negative environmental consequencessuch as increased erosion, reduced biodiversity, pollutionof water and eutrophication. In particular, with concernon the C cycle, conversion of natural systems to agricul-tural systems and intensive agriculture have resulted in asignificant depletion of the SOC pool.

The rate of decomposition of SOM mainly depends onsoil temperature and moisture. Conversion of natural toagricultural ecosystems increases the maximum soil tem-perature and decreases the soil moisture storage, thereforeincreasing the rate of SOM mineralization. In the last cen-tury the cultivation of new land has led to the mineraliza-tion of about 40% of the C contained in soils into CO2

(Van-Camp et al., 2004).Similarly, the intensification of agriculture practices had

led to a diffuse decrease in SOM in cultivated soil, particu-larly in temperate regions. In Fig. 3 variations in SOM con-tent after the beginning of tillage are reported. InitiallySOM is more uniformly distributed in the ploughed layer,but the higher level of aeration quickly accelerates mineral-ization and reduces SOC content over time down to athreshold that varies according to the type of soil as afunction of new biological equilibrium. Loss of organic Cin cultivated soils is a worldwide phenomenon. Bellamyet al. (2005) estimated a mean C loss from soils acrossEngland and Wales of 0.6% yr�1 between 1978 and 2003.In the Mediterranean area, 74% of the land is covered bysoils containing less than 2% of organic C with a decreaseestimated at around 50% of the original content (Van-Camp et al., 2004). Soil in the tropics and subtropics haslost 60–80% of its SOC pool (Lal, 2006a). The globalcumulative historic loss of C by cultivation is enormousand is estimated at 78 ± 17 Pg by Lal (2003) (Fig. 4) and55 Pg by Cole et al. (1996).

It is important to consider that the reduction in the soilC pool contributes to the increase in CO2 concentration(1 Pg loss of SOC is an equivalent to an atmospheric

Fig. 3. Variation of SOM content after conversion from pasture to maize(Sequi, 1979). 1: original pasture; 2: after the first year of tillage; 3: after 5years of tillage; 4: after 10 years of tillage; 5: after 15 years of tillage.

enrichment of CO2 by 0.47 ppmv), as well as to the startingof an involution in soil fertility and the onset of degrada-tive processes (such as erosion, salinisation, desertification,compaction, nutrients deficiency, etc.). There is consider-able concern that if SOM concentrations in soils areallowed to decrease too much, then the productive capacityof agriculture will be compromised. It has been estimatethat roughly 2 billion ha of land are affected by degrada-tion, which is over 30% of all land on earth (Table 2) (Old-eman, 1994).

At the present rate of increase (1.3% per yr), world pop-ulation is expected to reach 7.5 billion by 2020 and 9.4 bil-lion by 2050 (Lal, 2006b). Most of the future increase inworld population will occur in developing countries whereenvironmental resources are scarce. It has been estimatedthat the number of food insecure people was 850 millionin 2006, and this value is still increasing (Lal, 2006b).Although malnutrition and famine are more related to pov-erty and unbalanced food production per se, many regionsof the world, particularly parts of Africa, are not self suffi-cient in food production (Matson et al., 1997). It has beenestimated that food production must be increased by 778million tons (or by 2.5%) per year between 2000 and2025 to meet the needs of the increasing population (Lal,2006a). In addition, traditional agricultural managementsin developing countries, such as removal of crop residuesand non-utilization of manure in soil, exacerbates soil Closs (Lal, 2006a).

The above considerations imply that present agriculturalecosystems will be requested to meet two apparently con-flicting requirements: avoiding the environmental conse-quences of agriculture intensification and providingsufficient food for the increasing population. To accom-plish such requirements, agricultural systems need to movetoward a more sustainable management, defined as the

Table 2Estimates of the global extent (in million km2) of soil degradation(Oldeman, 1994)

Type of degradation Degree of degradation

Light Moderate Strong + extreme Total

Water erosion 3.43 5.27 2.24 10.94Wind erosion 2.69 2.54 0.26 5.49Chemical degradation 0.93 1.03 0.43 2.39Physical degradation 0.44 0.27 0.12 0.83

Total 7.49 9.11 3.05 19.65

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‘‘soil management that meets the needs of the present with-out compromising the ability of future generations to meettheir own needs from that soil’’ (Smith and Powlson, 2003).

Agriculture, like any other human activity, can bedefined as sustainable if it fulfills simultaneously three dif-ferent requisites (Yunlong and Smit, 1994):

(i) It must guarantee the conservation of environmentalequilibria (sustainability of resources), i.e., the con-tinued productivity and functioning of the ecosys-tem, the maintenance of the resources and theprotection and the conservation of biologicaldiversity.

(ii) It must guarantee full safety to the farmer, in addi-tion to the provision of adequate and safe agricul-tural products to the consumer (sustainability ofhuman health).

(iii) It must guarantee economically convenient produc-tions, i.e., long-term profit to farmers (economicalsustainability).

Appropriate SOM management is essential to increasethe efficiency of intensive agriculture techniques and, con-sidering the above principles, is a fully sustainable practice.In fact:

(i) Concerning sustainability of resources, increased useof organic residues avoids both utilization of non-renewable resources (e.g., fossil fuel, peat) and excessof energy expenses (i.e., production of chemical fertil-izers and pesticides, treatment and disposal oforganic wastes).

(ii) With respect to sustainability of human health, SOMhas the ability to decontaminate the soil from xenobi-otics and pollutants, reduce water pollution andincrease the water quality of streams and rivers.The use of organic amendments avoids an improperand harmful fate to the organic residues, with indirectbenefits for the human society.

(iii) Concerning the economic sustainability, improvedsoil fertility by increased levels of OM avoids excessenergy costs for tillage, irrigation and fertilizationbecause of improvement in soil structure, water hold-ing capacity and fertilizer efficiency. In addition, ade-quate levels of SOM sustain high and constantproductivity, reduce erosion and other degradativeprocesses and eliminate the need for different andexpensive solutions for the disposal of organicresidues.

Since strategies to increase the soil organic C content ofcropland soils present a range of other environmental andeconomic benefits in addition to GHG reduction potential,they are consistent with other measures aimed to improvesoil sustainability (e.g., UN convention on desertificationand biodiversity) and therefore are attractive as part ofintegrated sustainability policies (Smith, 2004).

4. Conclusions

From the above considerations it can be concluded thatsoil C sequestration is desirable, both for its beneficialeffects on GHG reduction and climate change, and for itswider environmental and economic implications. In partic-ular, an increase in the levels of SOM is necessary to coverthe loss of organic C in agricultural soil. The decrease inSOM content led to the decline of several soil propertiesthat are essential for soil protection and conservation fromboth the agronomic and environmental points of view.Proper SOM management is also a prerequisite of a sustain-able agriculture capable of dealing with the increasingdemand of food and the maintenance of the environment.

Appropriate SOM management is therefore an essentialturning point for the equilibria of natural systems and thefuture of the entire human society.

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