Soil & Tillage Research 109 (2010) 161–168

Soil carbon and nitrogen mineralization kinetics in organic and conventionalthree-year cropping systems

S. Marinari a,*, A. Lagomarsino b, M.C. Moscatelli a, A. Di Tizio a, E. Campiglia c

a Dipartimento di Agrobiologia e Agrochimica, Universita degli studi della Tuscia, via S. Camillo de Lellis, Viterbo 01100, Italyb Dipartimento dell’Ambiente Forestale e delle sue Risorse, Universita degli Studi della Tuscia, Via S. Camillo de Lellis, Viterbo 01100, Italyc Dipartimento di Produzione Vegetale, Universita degli studi della Tuscia, Viterbo 01100, Italy

A R T I C L E I N F O

Article history:

Received 2 July 2009

Received in revised form 31 May 2010

Accepted 7 June 2010

Keywords:

Soil mineralization

Organic fertilizers

Green manure

Nitrogen availability

Dystric Fluvisol

Typic Xerofluvent

A B S T R A C T

The scientific literature regarding the use of C and N mineralization kinetics as a tool to highlight the effects

of different cropping systems on soil C and N release is scarce. In this study we aimed to assess the

effectiveness of these parameters in evaluating soil C and N potential release in organic (ORG) and

conventional (CONV) three-year cropping systems. A long-term field study was established in 2001 at the

University of Tuscia experimental farm (Viterbo, Italy) in a randomized block design. The soil is classified as

Typic Xerofluvent or Dystric Fluvisol. In the CONV system the Good Agricultural Practice is adopted, whereas

the ORG system is managed following the Regulation 2092/91/EEC. Both systems had a three-year crop

rotation (pea – Pisum sativum L.; durum wheat – Triticum durum Desf.; tomato – Licopersicum esculentum

Mill.). One of the main differences between the two systems is the soil N fertilization program: organic

fertilizers (Guano: 6% N, 32% organic carbon and DIX10: 10% N, 42% organic carbon, both produced by

Italpollina, Italy) and mineral nitrogen fertilizers (NH4NO3) were applied to ORG and CONV fields,

respectively. Moreover, the rotation in the ORG system included common vetch (Vicia sativa L.) and

sorghum (Sorghum vulgare L.) as green manure crops. Our results supported the hypotheses in that the two

systems differed significantly on potentially mineralizable C (C0) in 2008 and on potentially mineralizable

N (N0) as nitrate form (N0-NO3�) in 2006 (318 mg C-CO2 g�1 28 d�1 vs. 220 mg C-CO2 g�1 28 d�1;

200 mg N-NO3� g�1 vs. 149 mg N-NO3

� g�1 in ORG and CONV, respectively). The reduction of N0 in soil

during the crop rotation period could reflect the N microbial immobilization since a negative correlation

between microbial biomass N:total N ratio and N0 as ammonium form (N0-NH4+) (P < 0.001) as well as a

positive correlation between N0-NH4+ and C:N ratio of microbial biomass (P < 0.05) were observed.

Moreover, a lower potential mineralization rate of N was observed in soil with Guano (25%) than in soil with

DIX10 (35%); nevertheless the former fertilizer might cover a longer period of crop N demand as a more

gradual release of N0 was observed. In this work we demonstrated that the use of mineralization kinetics

parameters can offer a potential to assess the mineralization–immobilization processes in soils under

different climatic and management conditions. Moreover, they can be used to evaluate the most suitable N

release pattern of organic fertilizers used in various cropping systems.

� 2010 Elsevier B.V. All rights reserved.

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1. Introduction

The conversion of agricultural systems to improve theirsustainability can be achieved with a gradual change fromstrictly conventional practices (e.g., monocultures with massivemineral fertilization) through Good Agricultural Practices (GAPs)to the adoption of organic farming principles (Abubaker et al.,2008). Good Agricultural Practice (GAP) is based on the principlesof risk prevention, risk analysis, and sustainable agriculture by

* Corresponding author.

E-mail address: marinari@unitus.it (S. Marinari).

0167-1987/$ – see front matter � 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2010.06.002

means of Integrated Pest Management (IPM) and Integrated CropManagement (ICM) to continuously improve farming systems.GAP is of utmost importance in protecting consumer health andrequires ensuring safety throughout the food chain (Akkaya et al.,2006). Organic farming systems rely on the management oforganic manures and soil organic matter (SOM) to enhance soilchemical, biological, and physical properties, to optimize cropproduction (Watson et al., 2002) and to return nutrients back tothe soil. Therefore, organic management of agricultural soilspositively influences soil properties (Tu et al., 2006) since theaddition of organic amendments improves SOM accumulation byincreasing carbon (C) pools with a slow turnover time (Lal andKimble, 1997).

S. Marinari et al. / Soil & Tillage Research 109 (2010) 161–168162

SOM is considered to be a key attribute of soil quality (Gregorichet al., 1994) and is usually considered as one of the most importantproperties of soils because of its impact on ecosystem sustainabilityand its effects on other soil physical, chemical, and biologicalcharacteristics (Reeves, 1997). Moreover SOM can provide mineralnutrients for plants and micro-organisms through the mineraliza-tion process or biochemical oxidation of organic substrates. For thisreason SOM content is the result of an equilibrium between theprocesses supplying new organic inputs and the rate of mineraliza-tion of the existing SOM (Stockdale et al., 2002). Cycles of C andnitrogen (N) are strongly linked in organic cropping systems. N isfrequently considered to be one of the key crop growth limitingfactors (Berry et al., 2002; Cavigelli et al., 2008; Moller et al., 2008).However, the synchronization between the supply of N bymineralization of organic manures and crop N demand cannot beeasy to achieve (Pang and Letey, 2000; Gaskell and Smith, 2007).Simultaneously, readily degradable C compounds added to the soilcan lead to an immobilization of mineral N (Dosch and Gutser, 1996).Mineralization and immobilization are thus soil microbial processesgoverned by C availability and are closely linked to both SOM qualityand its active fractions (Hassink, 1994). Therefore, an active soilmicroflora and a considerable pool of accessible nutrients are twoimportant priorities in organic cropping systems (Wander et al.,1994). Differences between composition of SOM in organically andconventionally managed soils have been demonstrated by someauthors (Elmholt, 1996; Wander and Traina, 1996; Marinari et al.,2007). A rapid mineralization process of labile SOM consumingcellulose structures has been reported in soil under organic farming,while the attack of soil native organic matter has been observed inconventionally managed soil (Marinari et al., 2007). The microbiallymediated N mineralization in organically managed soil is alsoimportant in order to sustain plant productivity without the use ofmineral fertilizers. Several studies show that organic farming leadsto higher soil quality with greater microbiological activity andenhanced nutrient availability when compared with conventionalfarming, due to versatile crop rotations, application of organicfertilizers, and absence of pesticides (Hansen et al., 2001; Shannonet al., 2002; Marinari et al., 2006). The increase of microbial biomassand activity under organic management leads to increased nutrientavailability for plants (Zaman et al., 1999; Tu et al., 2003; Wang et al.,2004; Marinari et al., 2006). In fact, Tu et al. (2003) showed thatenhanced soil microbial biomass and activity were associated withhigh net N mineralization rates, which resulted in larger Navailability. Nevertheless, the crop growth in organic systems isvery often N limited, and not C limited, which means that the soil Csupply is largely sufficient to sustain long-term soil fertility.

In 2001, an experiment was established at the University ofTuscia experimental farm (Viterbo, Italy) to study crop yield in athree-year crop rotation (pea – Pisum sativum L.; durum wheat –Triticum durum Desf.; tomato – Licopersicum esculentum Mill)under organic and conventional management, and to studydifferences in soil quality and nutrient cycling between the twosystems. Results from the fourth year reported by Lagomarsinoet al. (2009) showed that the effect of organic fertilization was cropspecies dependent considering only one year of the three-yearrotation. To escape this short-term bias of crop species in this paperwe investigated the impact of organic management on soil C and Ncycling considering the whole crop rotation. The mineralizationkinetics parameters were used as a tool to assess C and N potentialaccumulation and mineralization during the whole crop rotation.The scientific literature regarding the use of C and N mineralizationkinetic parameters as a tool to highlight the effects of differentcropping systems on soil C and N release or to evaluate N potentialmineralization organic fertilizers is scarce. For these reasons andwith the aim to contribute to fill a gap in this specific field ofagricultural research, the objectives of this study were: (i) to

evaluate differences between organic and conventional croppingsystems in terms of C and N potential mineralization; and (ii) toassess the potential mineralization kinetics of N in the twofertilizers applied in the organic cropping system in order toevaluate the nitrogen availability of the specific fertilizers, allowedin organic farming, through their time-dependent release.

2. Materials and methods

2.1. Site description

A long-term field study was established in 2001 at theUniversity of Tuscia experimental farm (Viterbo, Italy, 428260N,128040E, 310 m above see level), in order to compare organically(ORG) and conventionally (CONV) managed soil in a randomizedblock design with three replications. The soil is a clay loam andclassified as Typic Xerofluvent (Soil Survey Staff, 2006) or Dystric

Fluvisol (WRB, 2006). Soil characteristics at the start of theexperiment were: clay loam texture according to USDA classifica-tion, pH 6.9, total nitrogen 0.12%, total organic carbon 0.85%. In theconventional cropping system (CONV) the Good AgriculturalPractice (GAP), including the use of chemical fertilizers andpesticides, was adopted according to the Italian and Europeanregulations (D.M. April 19, 1999 and Regulation 2078/92/EEC,respectively) (EC, 1991). The organic cropping system (ORG)system was managed following the Regulation 2092/91/EEC (EC,1991). Both systems have a three-year crop rotation (pea – P.

sativum L.; durum wheat – T. durum Desf.; tomato – L. esculentum

Mill.). Tomato was irrigated according to potential evapo-transpiration replacement (3500–4000 m3 water ha�1). In theORG, the rotation started with common vetch (Vicia sativa L.) as acover crop and sorghum (Sorghum vulgare L.) as a catch crop, bothused as green manure before tomato transplanting and peaplanting, respectively. Organic fertilizers (Guano: 6% N, 32%organic carbon, 15% P2O5, 3% K2O, 3.5% humic acids, 7.5% fulvicacids and DIX10: 10% N, 42% organic carbon, 3% P2O5, 3% K2O, 3.0%humic acids, 7.0% fulvic acids both produced by Italpollina, RivoliVeronese, Italy) were used only in the ORG and mineral nitrogen(NH4NO3) fertilizers were applied in CONV. In Table 1 total C and Ninputs (green manure + straw + fertilizers) supplied at each rota-tion cycle are reported.

The three species were simultaneously cropped in theexperimental field that included 18 plots: 2 systems � 3 crops � 33 replicates.

2.2. Soil sampling

Soil samples were collected in February 2006 and 2008 in orderto verify the effect of soil management on soil C and Nmineralization potential. After removal of the litter layer two soilcores were taken inside each plot (0–20 cm depth) and then pooledtogether. Soils were sieved (<2 mm) in order to ascribe theobserved potential mineralization activity only to microbialbiomass excluding other living organisms. Soil samples were keptat 4 8C prior to analyses. Soil water content was adjusted to 60% ofwater holding capacity (WHC); then soil samples were left toequilibrate at room temperature in the dark for 24 h beforeincubation for C and N mineralization assays. Soil sieving affectsaggregate size and stability, but as we reported above it is not a realmeasurement and thus does not take into account neither the realclimatic conditions nor soil structure.

2.3. Chemical and biochemical analyses

Microbial biomass carbon (Cmic) was determined following theFumigation Extraction (FE) method (Vance et al., 1987). Microbial

Table 1Total soil C and N inputs in conventional and organic cropping systems of the experimental site. Values of C input produced by straw are calculated on dry-weight crop

residues biomass basis during the three-year crop rotation.

Crop Cropping systems

Conventional Organic

Nitrogen

(kg N ha�1 year�1)

Carbon

(kg C ha�1 year�1)

Nitrogen

(kg N ha�1 year�1)

Carbon

(kg C ha�1 year�1)

Durum wheat (Triticum durum Desf.) 30 Ca(NO3)2

90 (NH4NO3)

16 (straw)

3618 (straw) 21 (Guano)

59 (DIX10)

14 (straw)

112 (Guano)

247 (DIX10)

3039 (straw)

Pea (Pisum sativum L.) 65 (straw) 1699 (straw) 53 (straw)

39 (sorghuma

green manure)

1623 (straw)

1074 (sorghuma

green manure)

Tomato (Licopersicum esculentum Mill.) 100 (NH4NO3)

44 (straw)

1512 (straw) 40 (Guano)

46 (straw)

159 (vetchb

green manure)

211 (Guano)

1715 (straw)

3488 (vetchb

green manure)

Total inputs in a three-year crop rotation (kg ha�1) 345 6829 431 11,509

Inputs C:N ratio 19 26

a Sorghum vulgare L.b Vicia sativa L.

S. Marinari et al. / Soil & Tillage Research 109 (2010) 161–168 163

biomass was calculated as follows: Biomass C = EC: kEC where EC isthe difference between organic C extracted from fumigated soilsand organic C extracted from non-fumigated soils and kEC = 0.38(Joergensen, 1996). Microbial biomass nitrogen (Nmic) wasestimated using ninhydrin reagent solution (Sigma) on fumigatedand non-fumigated samples following the method reported byJoergensen and Brookes (1990). Microbial quotients (Cmic:Corg andNmic:TN) were calculated as the percentage of microbial biomass Cor N to the respective total organic C or N. Water soluble carbon(WSC) was determined after extraction of 15 g of soil in 30 mL ofdeionised cold water. Total C in the extracts was measured usingthe dichromate digestion method (Burford and Bremner, 1975).Total organic carbon (Corg) and total nitrogen (TN) were deter-mined using an elemental analyzer (Thermo Soil NC – FlashEA1112).

2.4. Soil carbon and nitrogen mineralization

Microbial respiration was measured at 28 8C. The CO2 evolvedwas trapped after 1, 3, 7, 10, 14, 21, 28 days of incubation, in 2 mL1 M NaOH and determined by titration of the excess NaOH with0.1 M HCl (Badalucco et al., 1992). The CO2 evolved during the 28thday of incubation was used as the basal respiration value (MRbasal).The kinetic parameters of C mineralization were obtained from thefirst order kinetic model [Cm = C0(1 � e�Kt)] (Riffaldi et al., 1996),where Cm is the cumulative value of mineralized carbon during t

days and C0 is the potentially mineralizable carbon. Furtherderived parameters were calculated: C0:Corg, the ratio of poten-tially mineralizable C to total organic C and C0:Cmic, the potentiallymineralizable C per unit of microbial biomass.

Nitrogen mineralization activity was assessed following themethod of Stanford and Smith (1972) with a 30-week aerobicincubation. Ten-gram soil aliquots mixed with quartz sand (1:1, w/w) were incubated at 28 8C in a combined filtration–incubationcontainer. Soil water content was maintained at 60% WHC duringall the incubation period. The mineral N produced was leached atpredetermined intervals (after 2, 4, 8, 12, 16, 22 and 30 weeks) with50 mL 0.01 M CaSO4. In order to prevent any limiting effects due tothe absence of other nutrients, after each leaching 20 mL of anutrient solution, minus N, was added to the soil (0.002 M CaSO4,0.002 M MgSO4, 0.005 M Ca(H2PO4)2, 0.0025 M K2SO4). After eachleaching, ammonium (N-NH4

+) and nitrate (N-NO3�) produced

during the incubation time were determined colorimetrically,

following Anderson and Ingram (1993) and Cataldo et al. (1975),respectively. The kinetic parameters of N mineralization, such asmineralized N (Nm) and potentially mineralizable N (N0), obtainedfrom the first order kinetic model [Nm = N0(1 � e�Kt)], wereexpressed as nitrate (N-NO3

�) and ammonium (N-NH4+) produc-

tion during t days. As for C kinetics, the ratio of potentiallymineralizable N to total N was calculated (N0:TN). In bothmineralization kinetic models of C and N, k is the rate constantof labile pool mineralization. At the same time, N mineralizationactivity was also assessed for fertilizers used only in the organiccropping system (Guano and DIX10). In this case, before theincubation, 10 g of ORG soil was fertilized with 0.67 and 0.40 g ofGuano and DIX10, respectively (4 mg g�1 of N equivalent dose) inthree replicates. The N mineralization of both fertilizers wasperformed separately in order to assess their specific time-dependent kinetics.

2.5. Statistical analysis

Data were averaged across crops within each system with theaim to focus the study, and the discussion of the results, on theoverall influence of two different cropping systems (ORG vs. CONVmanagement) on soil C and N mineralization rate in 2006 and2008. The effects of the main two factors (management and time)and their interactions were assessed (n = 9, 3 crops � 3 replicates)using two-way ANOVA procedure (Systat 11.0 SPSS Inc., 2004).Pearson correlation coefficients were used to assess the signifi-cance of the interrelationships between soil C and N pools (n = 36).

3. Results

3.1. Carbon pools and microbial biomass of soil

The total organic carbon (Corg) content of soil in 2006, after fiveyears of organic management, was not significantly differentbetween the two systems (ORG and CONV), whereas this differencebecame significant (P < 0.05) after seven years (2008). In fact, theCorg content of soil showed an opposite temporal trend in the twosystems with an increase in the ORG soil and a decrease in theCONV soil in the period 2006–2008 (Table 2). The water soluble C(WSC) was significantly higher in ORG than in CONV soil in 2006,but it decreased with time reaching a similar level in 2008 in thetwo cropping systems. WSC was inversely correlated with kC

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( mg

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C

(mg

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Cm

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(mg

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Cm

ic:C

org

(%)

Cm

(mg

C-C

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12

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C0

(mg

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(10

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(mg

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C0/C

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[(Fig._1)TD$FIG]

Fig. 1. C:N ratio of soil (A) and microbial biomass C:N ratio (B) in the year 2006 and

2008 in a Dystric Fluvisol under organic and conventional management in Central

Italy (428260N, 128040E). Data related to the year 2006 are from Lagomarsino et al.

(2008). Bars indicate standard error (n = 9). Different letters indicate significant

differences between treatments and years (Fisher’s LSD post hoc test, P � 0.05).

S. Marinari et al. / Soil & Tillage Research 109 (2010) 161–168164

(P < 0.05) or k-NO3� (P < 0.001) and positively correlated

(P < 0.001) with potentially mineralizable N pools (Table 4). Onthe contrary the C pool extracted with K2SO4 (ExC) showed anincrease with time reaching the highest value in ORG soil (Table 2).Even if in 2006 the microbial biomass C (Cmic) in the ORG soil wasslightly higher (P < 0.1) than in CONV soil, after two years similarvalues of Cmic and Cmic:Corg were observed in the two systems. Infact the microbial biomass increased with time particularly in theCONV soil. Soil C:N ratio did not change with time ranging from 9.5to 10 in both systems (Fig. 1A). Conversely, the soil microbialbiomass C:N ratio decreased with time in both systems and in theORG soil became significantly lower than in CONV soil in 2008 (3.4vs. 4.8) (Fig. 1B).

3.2. Soil carbon mineralization kinetics

The soil C mineralization kinetics in 2006 were similar in thetwo cropping systems, while in 2008 they significantly increased inthe ORG soils when compared with CONV (Fig. 2A and D). Thekinetic parameters of C mineralization are reported in Table 2. Thepotentially mineralizable C (C0), was strongly affected by thecropping systems only in 2008, reaching the highest value in soilunder ORG management. The same pattern was observed for Cm,which represents the cumulative value of carbon mineralizedduring 28 days (Fig. 2A and D). The cropping systems inducedsignificant changes in CO2 production patterns with the highestvalues in the ORG soil in 2008. The mineralized C (Cm) becamesignificantly different between the two systems in 2008 as apositive trend in the ORG soil and a slight decrease in the CONV soilwere observed (Table 2). The cropping system did not affect thepercentage of potentially mineralizable carbon to total organic C(C0:Corg) (Table 2). The basal respiration of microbial biomass

[(Fig._2)TD$FIG]

Fig. 2. Mineralization kinetics of carbon [C] (small letters) and nitrogen [N] (capital letters) in a Dystric Fluvisol under organic and conventional management in Central Italy

(428260N, 128040E). expressed as cumulative CO2, nitrate (N-NO3�) and ammonium (N-NH4

+) production per gram of soil in 2006 (A–C) and 2008 (D–F). Bars indicate standard

error (n = 9).

S. Marinari et al. / Soil & Tillage Research 109 (2010) 161–168 165

(MRbasal) decreased with time in the CONV soil; as for Cm and C0 itwas significantly different between the two cropping systems onlyin 2008. A significant positive correlation was found betweenMRbasal and the percentage of C0 to Corg (P � 0.001, Table 4).

Table 3Nitrogen pools and nitrogen mineralization kinetic parameters of a Dystric Fluvisol in Cen

and the end of the experimental period (2006–2008). Data in brackets [..] are from Lag

TN (%) Nmic

(mg N g�1)

ExN

(mg N g�1)

Nmic:TN

(%)

Nm (m

NO3�

2006 Organic [0.13]a [28.0]a [13.6]a 2.2b 106a

s.e. 0.01 4.1 1.19 0.3 5.2

Conventional [0.13]a [28.8]a [12.0]a 2.3b 96a

s.e. 0.01 4.2 0.89 0.4 5.0

2008 Organic 0.15a 81.6b 14.69a 6.9a 96a

s.e. 0.02 15.5 1.46 1.7 5.9

Conventional 0.12a 61.1b 13.45a 5.6a 81b

s.e. 0.01 6.8 1.36 1.1 2.6

Analysis of variance

Management (M) n.s n.s. n.s n.s. *

Year (Y) n.s *** n.s *** *

M�Y n.s n.s n.s n.s n.s

TN: total nitrogen, Nmic: microbial biomass nitrogen, ExN: extractable nitrogen, Nmic:

incubation. N0: potentially mineralizable nitrogen, k�1000: rate constant of labile n

nitrogen.

n.s. not significant. Different letters within column indicate significant differences (Fis# Significant difference at P�0.1.* Significant difference at P�0.05.** Significant difference at P�0.01.*** Significant difference at P�0.001.

3.3. Soil N pools

Soil total nitrogen (TN) and the N pool extracted with K2SO4

(ExN) did not differ from the fifth to the seventh years, both of

tral Italy (428260N, 128040E) under organic and conventional management beginning

omarsino et al. (2008). s.e. indicates standard error (n = 9).

g N g�1) N0 (mg N g�1) k�1000

(1000 d�1)

N0/TN

NH4+ NO3

�/NH4+ NO3

� NH4+ NO3

� NH4+ NO3

� NH4+

17.6a 6b 200a 20.8a 4.4b 9.0a 15.7b 1.7a

0.8 25.6 0.6 0.6 0.7 1.8 0.1

18.6a 5b 149b 21.2a 5.7b 9.9a 11.0a 1.7a

0.8 19.2 1.3 0.8 0.8 1.3 0.2

6.73b 14a 104b 8.61b 10.8a 9.6a 8.4a 0.7b

0.64 8.5 0.54 0.7 2.2 1.5 0.1

5.78b 14a 87b 6.53b 12.2a 11.6a 7.1a 0.6b

0.59 1.7 0.64 1.4 1.2 0.6 0.1

n.s n.s # n.s n.s. n.s. # n.s.*** *** *** *** *** n.s ** ***

n.s n.s n.s n.s n.s n.s n.s n.s.

TN: microbial nitrogen quotient, Nm: mineralized nitrogen after 30 weeks of soil

itrogen mineralization, N0/TN: percentage of potentially mineralizable N to total

her’s LSD Post Hoc test, P�0.05).

[(Fig._4)TD$FIG]

Fig. 4. Potentially mineralizable nitrogen (N) in a Dystric Fluvisol (428260N, 128040E)

fertilized with Guano and DIX10 during 30 weeks of incubation. Data are expressed

as mg N g�1 or as percentage of N0 to total N. Bars indicate standard error (n = 3).

S. Marinari et al. / Soil & Tillage Research 109 (2010) 161–168166

them were not significantly different between the two systems(ORG and CONV) at both sampling dates (Table 3). The nitrogen ofmicrobial biomass (Nmic) as well as the: microbial nitrogenquotient (Nmic:TN) showed an increase with time but no differencewas observed between the two cropping systems (Table 3).

3.4. Soil N mineralization kinetics

Soil nitrogen mineralization was measured as NO3� and NH4

+

production during 30 weeks of soil incubation. The inorganic Nproduction significantly decreased in 2008 as well as the N-NO3

�/N-NH4

+ ratio (Table 3 and Fig. 2C and F). In fact the ammoniumproduction of both soils decreased more than nitrate in the period2006–2008, thus the two ion forms ratio changed from 5–6 to 14.

The potentially mineralizable N (N0) showed a similar trend ofNm reaching the lowest values in both soils in 2008 (Table 2).Moreover, significant differences between ORG and CONV soilswere evident: the potentially mineralizable N as nitrate (N0-NO3

�)(Fig. 2B and E), was in fact positively affected by the organicmanagement only in 2006. On the contrary for Nm, whichrepresents the cumulative value of mineralized N, soil manage-ment induced significant changes in N-NH4

+ production patternswith the highest values in CONV soil at the first sampling date(Table 2). Furthermore, in CONV soil the Nm/N0 ratio was higherthan in ORG soil at both sampling dates (2006 and 2008). Thecropping system did not affect the percentage of potentiallymineralizable N to total organic N (N0/TN) (Table 2). Thepotentially mineralized nitrogen as ammonium (N0-NH4

+) wasnegatively correlated with the percentage of Nmic to total N(Nmic:TN) (P < 0.001) and positively correlated with C:N ratio ofmicrobial biomass (P < 0.05) (Table 4).

The nitrogen mineralization kinetics of soils with Guano andDIX10 are shown in Fig. 3A and B. The peak of N release wasobserved after four and three weeks, respectively. The potentiallymineralized N, after 30 weeks of incubation, was 722 and 1133 mgN-NO3 g�1 plus 166 and 314 mg N-NH4 g�1 in soils amended withGuano and DIX10, respectively (Fig. 4). Hence the amount ofmineralized N accounted for 25% and 35% of the total N added tothe soil with the two organic fertilizers, Guano and DIX10,respectively (Fig. 4).

4. Discussion

4.1. Effect of cropping systems and fertilization on soil C and N pools

Seven years of organic management in a Mediterraneanenvironment caused significant changes of soil C and N pools, aswell as of their mineralization patterns. Increases in SOM contentfor soils under organic management are widely reported (Stock-dale et al., 2000) as well as increases of microbial biomass and its

[(Fig._3)TD$FIG]

Fig. 3. Nitrogen (N) mineralization kinetics in a Dystric Fluvisol (428260N, 128040E) fe

activity as cumulative respiration (Diez et al., 1991; Bachinger,1995; Fließbach and Mader, 2000; Stockdale et al., 2000; Maderet al., 2002; Marinari et al., 2006; Tu et al., 2006; Melero et al.,2006). In this study significant increases of Corg, Cm and C0 wereobserved after seven years of organic management (2008)supporting the first hypothesis of this work aiming to verify theeffectiveness of C and N mineralization kinetics in discriminatingbetween different cropping systems. Similarly the values ofmicrobial biomass and Cmic:Corg ratio are consistent with datareported for other agricultural soils in the Mediterraneanenvironment (Melero et al., 2006; Moscatelli et al., 2007). Theseresults suggested that C mineralization activity is particularlyenhanced under organic management which provides a largerfraction of mineralizable carbon available for microbes (C0/Cmic).The total organic C showed a similar behaviour of C0, therefore theC0/Corg ratio did not change between the two cropping systems.The percentage of C0 to Corg may also represent the C poolsupporting the microbial basal metabolism since a significantpositive correlation was found between C0/Corg and MRbasal

(P < 0.001, Table 4). In view of the fact that soil C:N ratio alsodid not change, a similar composition of SOM is supposed tocharacterize the two soils. Nevertheless, the relatively higher rateconstant (k) values in CONV soil would suggest that microbialrespiration metabolized organic compounds that have a differentdegree of degradability. The rate constant kC was inversely relatedto C0 or WSC as well as kN to N0, reflecting a dependence of theseparameters on the potentially mineralizable C and N pools or oneasily degradable C source (Wang et al., 2003). For this reason thevalues of C0/Corg and N0/TN may reflect either the size or thequality of the C and N pools (Moscatelli et al., 2007).

In this study the ORG system had a significant effect on thequality of the soil N pools with respect to the CONV system: a

rtilized soil with Guano (A) and DIX10 (B). Bars indicate standard error (n = 3).

Table 4Pearson correlation coefficient of soil carbon and nitrogen pools (n = 36).

Microbial C:N ratio Cmic:Corg Nmic:TN WSC MRbas

C0 n.s. n.s. n.s. n.s. 0.85***

k n.s. n.s. n.s. �0.36*

Cm/Corg n.s. n.s. n.s. n.s. 0.92***

C0/Corg n.s. n.s. n.s. n.s. 0.88***

C0/Cmic n.s. �0.69*** n.s. n.s. 0.41*

N0-NO3� n.s. n.s. n.s. 0.60*** n.s.

k-NO3� n.s. n.s. 0.41* �0.71*** n.s.

N0-NO3�/TN n.s. n.s. n.s. 0.61*** n.s.

N0-NH4+ 0.47** �0.35* �0.56*** 0.74*** n.s.

k-NH4+ n.s. 0.40* n.s. n.s. n.s.

N0-NH4+/TN 0.54*** n.s. �0.38* 0.67*** n.s.

Microbial C:N ratio: C:N ratio of microbial biomass, C0:Corg potentially mineraliz-

able C to total organic carbon, Cmic:Corg: microbial quotient, C0:Cmic fraction of

mineralizable carbon available for microbes, N0:TN potentially mineralizable N to

total N, Nmic:TN: microbial nitrogen quotient, WSC: water soluble carbon, MRbas:

basal respiration.

n.s., not significant.* Significant at P�0.05.** Significant at P�0.01.*** Significant at P�0.001.

S. Marinari et al. / Soil & Tillage Research 109 (2010) 161–168 167

higher percentage of N0-NO3 to TN and a lower microbial biomassC:N ratio were observed in 2006 and 2008, respectively. Casestudies of N flows on organic farms also reveal lower mean Ninputs to organic (90 kg N ha�1) than to conventional systems(165 kg N ha�1) over a whole crop rotation period (Kirchmann andRyan, 2004). In our trials the assessed N inputs in the ORG systemwere higher than in the CONV one and were also over the average Namounts reported in literature. Nevertheless, it can be questionedwhether the potentially available form was comparable to theinorganic N applied as mineral fertilizer to the conventionalsystem. The increase of potentially mineralizable N recorded in thisstudy might not however meet the effective N demand of non-leguminous crops as also reported by other authors (Gaskell andSmith, 2007). In fact, the organic N applied to this system caused anincrease of the soil potentially mineralizable N-NO3 only by 50 and17 mg N g�1 in 2006 and 2008, respectively. These results wereprobably due to the low N0/TN ratio of the yearly added organicfertilizers (35% and 25% for DIX10 and Guano, respectively).Moreover, since a more gradual release of N-NO3 was observedfrom Guano than from DIX10, the former fertilizer might cover alonger period of crop N demand also preventing nitrate losses(Stopes et al., 2002).

The significant inverse relationship between Nmic:TN and N0-NH4

+ (P < 0.001) as well as the positive correlation between N0-NH4

+ and C:N ratio of microbial biomass (P < 0.05) confirm thatammonium form is preferentially taken up by soil microbes(Haynes and Naidu, 1998).

4.2. Temporal trend of soil C and N mineralization

Soil C mineralization kinetic parameters were affected byorganic management mainly at the second sampling date (2008).Conversely, N mineralization kinetic parameters showed a slightchange due to the cropping system at first sampling date (2006).From these trends, it can be hypothesized that the ORG system (i)requires longer term study to make clear the trend of the potentialC mineralization with time, and (ii) affects C and N potentialmineralization showing an opposite pattern with time. Regardingthe temporal variation, the Cmic:Corg ratio increased in CONV soil in2008, suggesting C substrate immobilization into the microbialbiomass. Conversely, in ORG soil the Cmic:Corg did not change withtime since both C pools, Cmic and Corg increased constantly.Cmic:Corg has been used as an early indicator of C accumulation insoil (Anderson and Domsch, 1989) therefore the Good Agricultural

Practice adopted in the CONV system may take longer to reach highlevel of organic C in soil when compared to the ORG system. At thesecond sampling date the C immobilization in ORG soil probablyreached a temporary steady state, while the lower microbialbiomass C:N ratio suggests a massive use of N to build up cellcomponents, promoting N immobilization. This result could be dueby the high C:N ratio of input in the ORG system (26, Table 1),because an excess supply of C in ORG system may result in atemporary strong immobilization of soil N (causing N deficiency incrops) (Amberger, 1989). However, as shown by the generaldecrease with time of microbial biomass C:N ratio and N0, in bothsystems N immobilization into microbial biomass occurred during2006–2008 period. For this reason the reduction of potentiallymineralizable N in soil may reflect the N immobilization intomicrobial biomass and further investigation should be carried outto understand the competition for nitrogen between soil micro-rganisms and plants.

5. Conclusion

C and N mineralization kinetics parameters were effective indiscriminating between organic and conventional systems in athree-year crop rotation, even if the parameters of C and N showedan opposite pattern with time. While a more clear effect of organicmanagement on C potential mineralization was evident only at thesecond sampling date, the N potential mineralization was differentbetween systems at the first sampling date. The reduction ofpotentially mineralizable N in soil may reflect the N immobiliza-tion into microbial biomass.

As far as the N mineralization kinetics of organic fertilizer isconcerned, a lower potential mineralization rate of N wasobserved in Guano (25%) than in DIX10 (35%). Nevertheless theformer fertilizer might cover a longer period of crop N demand inthe organic system as a more gradual release of potentiallymineralizable N was observed. The use of the C and Nmineralization kinetics parameters was proved to be effectivein discriminating between organic and conventional croppingsystems in terms of mineralization–immobilization processesdynamic during the crop rotation period. Moreover, it offers apotential to assess the most suitable N release pattern of fertilizersused in the organic cropping system. However, it should beemphasized that the above considerations can be applied to soilsof the Mediterranean areas with similar physico-chemicalproperties to those of this study. Further investigations, on thebehaviour of other organic fertilizers, should be advisable in orderto extend our conclusions to a wider range of organic orsustainable agricultural systems.

Acknowledgements

The authors wish to thank Mr. Claudio Stefanoni for technicalassistance; Dr. Francesco Biondi for providing soil classificationand Prof. Paolo De Angelis for allowing the use of the elementalanalyzer.

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