Agricultural Systems xxx (2014) xxx–xxx

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Agricultural Systems

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Tillage systems effects on soil carbon stock and physical fractions of soilorganic matter

http://dx.doi.org/10.1016/j.agsy.2014.08.0080308-521X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel./fax: +55 213787 3772.E-mail addresses: erika@ufrrj.br (É.F.M Pinheiro), david.campos@embrapa.br

(D.V.B. de Campos), fabiano.balieiro@embrapa.br (F. de Carvalho Balieiro), lanjosrural@gmail.com (L.H.C. dos Anjos), mgervasiopereira01@gmail.com (M.G. Pereira).

Please cite this article in press as: Pinheiro, É.F.M., et al. Tillage systems effects on soil carbon stock and physical fractions of soil organic matter. Ag(2014), http://dx.doi.org/10.1016/j.agsy.2014.08.008

Érika Flávia Machado Pinheiro a, David Vilas Boas de Campos b, Fabiano de Carvalho Balieiro b,Lúcia Helena Cunha dos Anjos a, Marcos Gervasio Pereira a,⇑a Departamento de Solos, Instituto de Agronomia, Universidade Federal Rural do Rio de Janeiro, BR 465 – km 7, Seropédica, 23.899-000 Rio de Janeiro, Brazilb Embrapa Solos, Rua Jardim Botânico, 1024, 22.460-000 Rio de Janeiro, Brazil

a r t i c l e i n f o

Article history:Received 18 February 2014Received in revised form 11 August 2014Accepted 19 August 2014Available online xxxx

Keywords:No-tillageOrganic fractionsC sequestrationHeavy fractions

a b s t r a c t

Changes in soil management and land use influence soil organic matter (SOM) turnover through changesin quantity and quality of plant residues entering the soil, their seasonal and spatial distribution, the ratiobetween above- and bellow-ground inputs and through changes in soil disturbance. We hypothesizedthat the sequestered C is stored mainly in the mineral associated fraction (C associated in sand, siltand clay fraction). The objective of this study was to evaluate the C stock and stabilization in a tropicalDystrophic Red Latosol (Typic Haplortox) (Paty do Alferes, Brazil) subjected to 6-years soil tillage systemsand soil cover. Treatments included no-tillage (NT), animal traction (AT) and conventional tillage (CT).Two additional treatments were evaluated: grass coverage (GC) and bare soil (BS). After six years crop,soil C stock in the 0–10 cm layer was higher in NT than in CT (17.6 vs. 12.3 Mg ha�1, P < 0.05). It resultedan increase of 5.3 Mg C ha�1 in NT when comparing to CT. In NT, most of the C accumulation compared toCT occurred in the mineral associated fraction. Although, only the C associated in sand fraction was sta-tistically different (6.7 vs. 1.2 g kg�1 soil, P < 0.05). GC had the highest C sequestration and C and N asso-ciated in the mineral fraction (14.9 g C kg�1 and 5.1 g N kg�1) in the 0–5 cm depth. For all treatments,most of the soil organic C was in the heavy fraction (> 55%). GC incorporated to soil annually0.6 Mg C ha�1. C associated with sand fractions was the most sensible mineral associated C fraction com-pared to C in silt and clay fraction, and can be used as a suitable soil quality indicator for sustainable use.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Plants are the main source of C to soils through tissue residuesor via root exudates and symbiotic fungi (Clemmensen et al.,2013). On balance, nearly all the C that enters soil as plant residueseach year either decomposes and returns to the atmosphere or isleached from soils within a few decades to centuries. The rate ofaccumulation and loss of soil C are estimated from two kinds ofinformation: direct observation of change in the amount of organicmatter and inferences based on age measured by radiocarbon(Trumbore and Czimczik, 2008). Soil C can accumulate and be lostat intermediate rates (0.1 to 10 Mg C ha year�1). These rates vary alot and reflect differences in the dominant process contributing tostabilization of SOM.

By stabilization, Sollins et al. (1996) mean a decrease in thepotential for SOM loss by respiration, erosion or leaching. Threemain mechanisms of organic matter stabilization in soils have beenconsidered (Sollins et al., 1996; Von Lutzow et al., 2008): (i) recal-citrance of organic compounds at the molecular level; (ii) organo-mineral interaction on the surface of oxides and phyllosilicates;and (iii) physical protection in soil aggregates against decompos-ers. The first mechanism is no longer seen as the dominant oneand the main attention turned to physical protection and organo-mineral associated interactions (Kleber et al., 2011; Kogel-Knaberand Kleber, 2012; Conceição et al., 2013).

In this way, soil physical fractionation is a useful tool in studiesrelated to organic matter stabilization by organo-mineral interac-tion and physical protection. Physical fractionation of SOM hasbeen useful in distinguishing specific carbon pools that are respon-sive to management, identifying the physical control of organicmatter (Collins et al., 1997). Soil physical fractioning separatesSOM with different size or density fractions characterized by dif-ferent composition and stability as ‘‘free light fraction’’ presentwithin and between aggregates of soil and ‘‘heavy organic matter’’

r. Syst.

2 É.F.M. Pinheiro et al. / Agricultural Systems xxx (2014) xxx–xxx

or ‘‘mineral associated fraction’’ (C associated with clay, silt, andsand fraction) (Christensen, 2000). Physical protection of light frac-tion and its role in aggregate formation and stabilization may becrucial in regulation the proportion of new SOM that is furthertransformed and stabilized in clay and silt fractions. Organic mat-ter associated with soils minerals consists mainly by microbial tis-sues or metabolites that are stabilized by organo-mineralinteraction via adsorption processes (Kleber et al., 2011). C associ-ated with minerals show significantly longer turnover times, sug-gesting protection by minerals association and it might beassumed that the preferential storage in fine fractions resulted ina long-term carbon storage.

There are many investigations trying to understand the rolethat tillage systems play on SOM stabilization An accumulationof SOM under no-tillage (NT) compared to conventional tillage(CT) provides important improvements in soil quality and carbonstabilization. There are mechanisms by which minimum tillageincreases SOM accumulation, for example, reduced soil distur-bance and redistribution of residues, ‘‘the physical protection’’.Increases in aggregation concomitant with increases in organic Chave been observed in NT systems (Six et al., 2000; Pinheiroet al., 2004). Soil tillage has been found to induce a loss of C-richin macroaggregates (>250 lm) and a gain of C-depleted in micro-aggregates (<250 lm) (Six et al., 2000).

In an evaluation of the dynamics of SOM within particle sizefractions of a clayey Typic Haplorthox from Brazil, Torben et al.(1992) suggested that size fractionation of organo-mineral sepa-rates fractions encompass distinct compartments of C with varyingsusceptibility to biological decomposition. Freitas et al. (2000) inthe Cerrado region of Brazil used physical fractionation of SOMand observed more C content in sand fraction in soils under NTthan CT. The objective of this study was to evaluate the C stockand stabilization in a tropical Dystrophic Red Latosol (TypicHaplortox) (Paty do Alferes, Brazil) subjected to 7-year soil tillagesystems and soil cover.

2. Materials and methods

A field experiment was performed in 1995 at the Pesagro-RioExperimental Research Station, Paty do Alferes county, Rio deJaneiro State, Brazil. Soil at the experimental site is classified as aDystrophic Red Latosol (Typic Haplortox) with slope of 30%(Table 1). The regional climate is tropical, classified as CWa by Köp-pen classification system. Annual mean rainfall and temperatureare 1200 mm and 21 �C, respectively.

Plots with dimensions of 22 � 4 m were cultivated with a rota-tion of vegetables, including tomato (Lycopersicon esculentum),green pepper (Capsicum annuum) and beans (Phaseolus vulgaris).The same cropping sequence was followed in all cultivated plots.

The area was previously under degraded pasture (Panicum max-imum L). Treatments consisted of three tillage systems: (1) conven-tional tillage (CT) – in this region, CT consists of one disk plowingdownhill, followed by one light disk harrowing; tillage depth wasnear 0.20 m and harrowing about 0.10–0.15 m. All crop residueswere removed or burned prior to the next rotation cycle; (2) ani-mal traction (AT) – contour tillage with animal traction was used

Table 1Soil chemical properties at the experimental site, in 1998.

Horizon Depth (m) pH (H2O) Clay SOM Cationsa CECb

(g kg�1) (cmolc kg�1)

Ap 0–22 5.8 380 19.5 2.8 8.7Bw 68–92 4.8 450 5.3 0.9 3.1

a Sum of Ca, Mg, K and Na.b CEC = Cations + Al + H.

Please cite this article in press as: Pinheiro, É.F.M., et al. Tillage systems effects(2014), http://dx.doi.org/10.1016/j.agsy.2014.08.008

and all crop residue was left on soil surface; (3) no-tillage (NT) –seedlings were planted directly without tillage and left crops resi-dues in soil surface. Two additional control treatments wereincluded as (4) grass coverage – with P. maximum L (GR) and (5)bare soil (BS). The experimental design was a randomized blockdesign with five treatments and three replications.

Three undisturbed soil samples (0–5 and 5–10 cm) were takenafter beans harvesting in October 1998, by opening one trenchper plot (1.2 m square) for the evaluation of soil bulk density ofeach depth interval (one replicate from each of the four sides ofthe trench from the center of each depth interval). The soil sampleswere taken with Kopeck rings (4.5 cm i.d. and total internal volumeof 101 cm3). The samples were air dried and sieved to pass througha 2 mm sieve. By opening trenches so that soil bulk density of eachdepth interval could be sampled using Kopeck rings. Bulk densitysamples were removed from the Kopeck rings, dried at 105 �C,weighed and then discarded.

Soil samples were analyzed for pH (in water), were extractedwith a solution of potassium chloride (1 mol L�1) to evaluateexchangeable Al, Ca and Mg (Embrapa, 1997). For the analyses oftotal C and N, sub-samples were further ground to a fine powder(<0.15 mm) using a roller mill similar to that described by Arnoldand Schepers (2004). Total organic carbon (TOC) and total nitrogen(TN) analysis was performed on �150 mg aliquots of the samplesusing a total C and N analyzer (LECO model CHN 600, Leco Corp.,St. Joseph, MI).

Physical fractionation of SOM was performed following the pro-cedure of Sohi et al. (2001). After all light fractions (free and intra-aggregate) have been removed, the residual material in the tubewas considered the ‘‘mineral associated fraction’’ or ‘‘heavy frac-tion (HF)’’. The HF samples were shaken overnight in 300 ml ofdeionized water with 0.5 g sodium hexametaphosphate (dispers-ing agent). Sand fraction (2000–53 lm) was isolated by wet-siev-ing directly through a 53 lm sieve. Sand fraction was washedfrom the sieve into pre weighed cups, dried at 60 �C, weighedand finely ground for total C and N determination. The silt + claysuspension was transferred to cylinders placed in a room undercontrolled temperature (25 �C). Water was added to bring the vol-ume to 1000 ml, the suspension was shaken by hand (30 end overend tumbling) and 100 ml of the suspension was immediatelywithdrawn. They constituted an aliquot of the entire 0–53 lm frac-tion (clay + silt). After a settling time of 4 h, 200 ml of the suspen-sion was siphoned (<2 lm aliquot; clay fraction), transferred topre-weighed flasks, dried at 60 �C, and weighed. Once dried, frac-tions were finely ground for total C and N determination. C andN contents in silt fraction were obtained by difference (Gavinelliet al., 1995). TOC in these fractions were analyzed with the drycombustion method using an elemental analyzer.

The total C and N stocks in the soil were estimated using theprocedure recommended by Ellert and Bettany (1995). If in oneplot the soil is compacted more than in another, the profile of for-mer to any specific depth will contain a greater mass of soil. It wasassumed that any differential soil compaction between plots wasmost significant in the surface layers of the profiles so that the Cstock were calculated by subtracting the total C content of theextra weight of soil in the deepest (5–10, 30–40 or 80–100 cmfor calculation of the stocks to 10, 40 or 100 cm, respectively) layersampled in each profile as described by Neill et al. (1997).

The reference profile used in this study was the treatment NT.As proposed by Balesdent et al. (1990) any one treatment (prefer-ably that with the lowest soil mass in the profile) can be used tocorrect the others.

Soil sample was done in 1998, 1999, 2000 and 2001. Soil bulkdensity, as well as all the chemical properties was monitored inthe 1999, 2000 and 2001 years. Soil physical fractionation wasdone only in 2001 years.

on soil carbon stock and physical fractions of soil organic matter. Agr. Syst.

Table 3Soil C stocks (Mg ha�1) in different treatments in the surface layer (0–10 cm depth)for the studied period of 1998 and 2001 and the mean annual C enrichment(Mg C ha�1 year�1).

Treatments C stock (Mg ha�1) DC (Mg C ha�1)a DC (Mg C ha�1 year�1)b

1998 2001 1998–2001 1998–2001

BC 11.5 b 9.8 c �1.7 �0.5CT 16.6 a 12.3 c �4.3 �1.4

É.F.M. Pinheiro et al. / Agricultural Systems xxx (2014) xxx–xxx 3

After data normality was confirmed by the Lilliefors test andhomocedasticity by the Cochran & Barttlet’s test, the results wereanalyzed in a completely randomized design with 5 treatments(NT, CT, AT, GC and BS) and 3 repetitions. Results were analyzedusing SAEG (Sistema de Análises Estatísticas e Genéticas – UFV) sta-tistical package. The F test (P < 0.05) was applied to determine thesignificance of main effects from ANOVA. Significant variables hadtreatment means separated using the Tukey’s test.

AT 17.4 a 13.5 bc �3.8 �1.3NT 19.7 a 17.6 ab �2.2 �0.7GC 18.7 a 20.5 a 1.8 +0.6

Note: Means within a column followed by the same letter are not significantlydifferent at P < 0.05.

a The difference in the mean C evolution between 1998 and 2001 for eachtreatment.

b The mean annual enrichment in C was calculated between 1998 and 2001,considering three cropping cycles.

3. Results and discussion

Carbon distribution was nearly constant within the 0–10 cmlayer of CT soil while stratification with higher contents in the 0–5 cm layer occurred in NT soil. These results are in line with otherstudies (Conceição et al., 2013) and are due to incorporation ofabove and below-ground crop residues by tillage operations inCT. On the other hand, in NT the permanency of shoot residueson surface contributes to accumulate more soil C in this layer(Table 2).

As was to be expected, C concentration of the upper layer (0–5 cm) was higher in NT compared with CT in all years. However,the only significant statistical difference in C content between NTand CT systems was observed in the years 1999 and 2000. In gen-eral, C concentration declined in all treatments evaluated at 5–10 cm depth. The 0–5 cm layer was the most important in termsof C accumulation in the NT system, within the 10 cm depth eval-uated in the study. The positive effect of NT in increasing soilorganic C may extend deeper than 20 cm, in the subtropical Brazil-ian soils (Boddey et al., 2010).

Figueiredo et al. (2010) and Zotarelli et al. (2007) found valuesof 18.8–20.8 g C kg�1 and 17–20 g C kg�1 respectively, at 0–5 cmdepth, both in Oxisols under NT system in Brazil. Minimum tillagepractices cause less soil disturbance than conventional tillage,often resulting in significant accumulation of soil organic carbon(Sá and Lal, 2009). Only in the 0–5 cm layer there was a signifi-cantly (P < 0.01) higher C concentration under NT than CT, exceptfor 1998 and 2001. However, at a depth of 5–10 cm there was nostatistical difference in C concentration under CT compared toNT, reflecting the fact that disk ploughing of the soil buried surfaceresidues to this depth.

Soil C stock was estimated using the soil bulk density data ateach depth interval (Table 2) to calculate equivalent weights of soilto the same depth. When C stock was calculated to a depth of10 cm, it was apparent that there was a statistical significant dif-ference in the quantity of SOM under NT and CT, after six yearsof field experiment (Table 3). It resulted an increase of5.3 Mg C ha�1 in NT when comparing to CT. But, the positive differ-ence for NT system is only a difference and means that CT

Table 2Soil bulk density and organic carbon concentration in 0–5 and 5–10 cm depth, for all periBrazil.

Depth (cm) Treatments Soil bulk density(Mg m�3)

0–5 BS 1.36CT 1.37AT 1.24NT 1.06GC 1.25

5–10 BS 1.36CT 1.19AT 1.31NT 1.27GC 1.42

Note: Means within a column and depth group followed by the same letter are not sign

Please cite this article in press as: Pinheiro, É.F.M., et al. Tillage systems effects(2014), http://dx.doi.org/10.1016/j.agsy.2014.08.008

promoted higher C loss compared to GC and NT did not. Whencomparing NT with AT there were no significantly greater C stocks,although amounting to differences of 4.1 Mg C ha�1 for the sameyear (2001). One explanation could be that changes related toSOM generally occur after long periods of managements and mon-itoring (Zotarelli et al., 2007; Garcia, 2010).

All tillage systems for vegetable crops (NT, AT and CT) showed anegative contribution in soil C stock (�0.7, �1.3, �1.4 Mg C ha�1 -year�1, respectively) compared to grass coverage. Inputs of vegeta-bles crops residues, with low C:N ratio and recalcitrance, both onsoil surface and subsoil, contribute extremely low to C accumula-tion, especially in the tropical soils. Such stabilization mechanismsare no longer seen as the dominant mechanism in this study, so themain attention turned to organo-mineral interactions. Conceiçãoet al. (2013) observed annual C sequestration rate of 0.25 Mg ha�1

in NT relative to CT in subtropical Brazilian soils. By the other side,pasture coverage showed a net positive contribution to C, accumu-lating annually 0.6 Mg C ha�1 and it could be explained by anintensive pasture regrowth, with high C:N relation, lignin and poly-phenols contents, incorporating C on soil surface and in depth byan intense input of roots. Roots stimulated C accumulation andmuch more evidence has been presented to show that root derivedC is far more important as a source of SOM even under situationswhere equal quantities of root and shoot C were added to the soil(Boddey et al., 2010). In a long-term experiment (13 years), Sistiet al. (2004) evaluating the effect crop rotations under zero tillage(ZT) and conventional tillage (CT) on SOM stocks in a clayey Oxisolfrom Southern Brazil and also observed higher C concentration inZT than in CT system, in the surface layer (0–5 cm). But this trendwas not true for 5–10 cm depth. Bayer et al. (2000) evaluate thelong term effect of NT and CT of maize under crop rotation

od studied (1998–2001) in different tillage systems and reference in Paty do Alferes,

Organic carbon (g kg�1)

1998 1999 2000 2001

10.8 b 6.9 c 9.5 d 8.5 c14.5 ab 11.4 bc 15.7 c 10.8 bc17.1 a 16.8 ab 18.9 b 11.9 bc18.1 a 18.5 a 20.5 ab 15.2 ab16.5 a 21.8 a 22.8 a 18.9 a

8.7 b 9.2 b 9.6 c 8.1 b13.5 ab 11.9 ab 15.8 b 10.2 ab12.4 ab 15.4 a 17.5 ab 11.3 ab15.5 a 16.5 ab 16.5 b 14.9 a15.6 a 19.7 a 20.1 a 15.4 a

ificantly different at P < 0.05.

on soil carbon stock and physical fractions of soil organic matter. Agr. Syst.

Table 4Characteristic summary of heavy fractions (HF) of soil organic matter in 2001.

Depth (cm) Treatments Sand Ca Silt Ca Clay Ca HFCb HFN HFC/soil Cc

(g kg�1 soil) (%)

0–5 BS 1.2 c 2.4 a 4.4 b 8.1 2.3 86CT 1.2 c 1.8 a 4.7 ab 7.7 3.0 55AT 5.2 b 0.0 a 5.6 ab 10.8 2.4 57NT 6.7 a 0.0 a 5.5 ab 12.2 2.1 60GC 6.5 a 2.2 a 6.3 a 14.9 5.2 65

5–10 BS 5.6 b 5.6 a 3.1 b 14.3 – 90CT 1.3 c 2.8 ab 5.3 a 9.5 – 60AT 1.5 c 2.2 ab 4.3 ab 8.2 – 57NT 7.2 a 0.0 b 5.1 a 12.3 – 72GC 7.3 a 0.0 b 5.2 a 12.5 – 60

Note: Means within a column and depth group followed by the same letter are not significantly different at P < 0.05.a Organic matter associated to sand, silt and clay.b The proportion of heavy fraction C (HFC) or N (HFN).c Percentage of total organic C in the heavy fraction (HF).

4 É.F.M. Pinheiro et al. / Agricultural Systems xxx (2014) xxx–xxx

(oat/vetch) on soil organic carbon stock in southern Brazil. NT sys-tem stocked more 12 Mg C ha�1 than CT, for the first 30 cm depth.

Significant treatment effects (P < 0.05) were observed on C con-centrations in the heavy fractions (HF) among the depths (Table 4).C concentration in heavy fraction (CHF), in general, decreased shar-ply with sampling depth. Heavy fraction is composed mainly oforganic materials not visually identifiable, tightly bound to soilminerals and, constituting the primary organo-mineral complexes(Christensen, 2000; Sohi et al., 2001). Total fraction mass vary from95.7 to 100.8 g kg�1. These results allow us to conclude thatphysical fractionation was correctly achieved regarding the massbalance.

Soils in NT plots had higher amounts of HFC compared to CT inboth depths (0–5 and 5–10 cm). Averaged across sampling depths,the highest C and N concentrations in heavy fractions wereobserved in soil under GC. More than 55% of the C was present inHF at all depths (Table 4), which is composed of organic materialstrongly linked to minerals fractions. Ashagrie et al. (2007) foundthat 60–90% of the total organic carbon was present in the heavyfraction. Silva and Resck (1997) studied the distribution of organicC associated in the mineral fractions (sand fraction = 2000–53 lm;silt fraction = 53–2 lm and clay fraction <2 lm) of a clayey Ferral-sols from the Brazilian Cerrado region. The authors reported a rel-ative enrichment of organic carbon in the finer fractions (silt andclay fraction) due to cultivation.

Any significant statistically difference (P < 0.05) between treat-ments was observed for silt size fractions (0–5 cm). The same wastrue for organic carbon associated with clay fraction in all vegeta-ble cropping systems and grass coverage (Table 4). Probably, itcould be explained by the high clay contents and Fe and Al oxi-hydroxides concentrations in the soil, enabling strong linkagebetween organic matter and minerals fractions (organo-mineralcomplexes), as well as to a poor C supply by the vegetable crops.The chemical composition of SOM in silt and clay fractions is notdrastically influenced by changes in soil management and landuse (Christensen, 1996). In a review about the physical control ofSOM in the tropics, Feller and Beare (1997) have shown thatorganic matter can be physically protected from decomposerorganisms in microaggregates as plant cell debris and amorphousorganic matter become encrusted in a dense clay fabric.

Organic carbon content in sand fractions was statistically higherfor GC and NT treatments (P < 0.05). About less than 10% of theSOM isolated with sand fractions is found as true primary org-ano-mineral complexes (Christensen, 1996). The major part occursas light fraction (particulate or macro-organic matter) encompass-ing mainly particulate plant and animal residues undergoingdecomposition. A higher statistical C input in sand fractions or

Please cite this article in press as: Pinheiro, É.F.M., et al. Tillage systems effects(2014), http://dx.doi.org/10.1016/j.agsy.2014.08.008

macroorganic matter in NT (6.7 g kg�1) compared to CT (1.2 g kg�1)is probably due to higher maintenance of C input from new cropresidues and a minimum soil disturbance by cultivation. Manystudies have observed that the C in sand fraction are relatively eas-ily decomposable and are greatly depleted upon soil cultivation(Six et al., 1999), indicating their relatively unprotected (biochem-ical and physical) status, whereas it is difficult to observe differ-ences in the clay and silt size fractions (Six et al., 2000). It is welldocumented that a loss of C in sand fraction associated with culti-vation is due to mineralization and to transfer of SOM to silt andclay fractions (Christensen, 1996). A number of studies suggeststhat light fraction isolated from sand fraction and from whole soilsamples may represent identical SOM pools (Gregorich and Ellert,1993; Six et al., 1999, 2000). This fraction has been found to reflectdifferences in the cropping and tillage systems (Cambardella andElliott, 1992). Therefore, a considered C associated with sand frac-tion in NT and GC is a result of newly enrichment in C from therecent crops residues that probably will take part into more stablefractions as result of stable bonds between soil organic carbon andclay and oxides in this soil.

4. Conclusions

In tropical soils, NT is the primary management strategy forincreasing SOM accumulation, even when low input of vegetablescrops residues occurs. After six years of vegetables crop, soil Cstock in the 0–10 cm layer was higher in NT than in CT. Most ofthe C that accumulates in NT soil relative to CT is stored in the min-eral associated fraction, what emphasize the importance of organo-mineral interaction as a stabilization mechanism in NT. Soilorganic C associated with sand fractions was the most sensitivecompared to clay and silt fraction to soil tillage systems in a shortperiod.

Acknowledgments

We are grateful to CNPq – Brazil, FAPERJ and PESAGRO – RJ forthe financial and technical support.

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