Combined role of no-tillage and cropping systems in soil carbon stocks and stabilization

Soil & Tillage Research 129 (2013) 40–47

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Combined role of no-tillage and cropping systems in soil carbon stocks and stabilization Paulo Cesar Conceic¸a˜o a, Jeferson Dieckow b, Cime´lio Bayer c,* a

Campus Dois Vizinhos, Universidade Tecnolo´gica Federal do Parana´, 85660-000 Dois Vizinhos-RS, PR, Brazil Departamento de Solos e Engenharia Agrı´cola, Universidade Federal do Parana´, 80035-050 Curitiba, PR, Brazil c Departamento de Solos, Universidade Federal do Rio Grande do Sul, 90001-970 Porto Alegre, RS, Brazil b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 27 May 2012 Received in revised form 14 January 2013 Accepted 18 January 2013

Increases in carbon (C) input and stabilization are key processes to turn soils into sinks of atmospheric CO2–C and help mitigating global warming. We hypothesized (i) that C sequestration in no-tillage soil is further enhanced by high input cropping systems and (ii) that the sequestered C is stored mainly in the mineral associated fraction. The objective of this study was to assess, in 2003, the C sequestration and stabilization in a subtropical Acrisol (Eldorado do Sul, Brazil) subjected to 18-year conventional tillage [CT] and no-tillage [NT] combined with two cropping systems: black oat (Avena strigosa Schreb) as winter cover crop – maize (Zea mays L.) as summer grain crop [Ot/M]; and black oat plus vetch (Vicia villosa Roth) as winter cover crops – maize in summer intercropped with cowpea (Vigna unguiculata (L.) Walp) cover crop [Ot+V/M+C]. Soil C stock in the 0–20 cm layer was higher in NT than in CT, either in Ot/M (31.1 vs. 27.8 Mg ha1, P < 0.05) or Ot+V/M+C (37.3 vs. 32.8 Mg ha1, P < 0.05). Annual C sequestration rate in NT relative to CT was 0.25 Mg ha1 in Ot+V/M+C and 0.18 Mg ha1 in Ot/M, in agreement to the higher biomass-C addition and legume cover crops inclusion in Ot+V/M+C (7.6 vs 4.0 Mg ha1 year1) and to our first hypothesis. Increase in the proportion of large macroaggregates (9.51–4.76 mm) and of mean weight diameter occurred in NT soil up to 10 cm depth, both in Ot/M and Ot+V/M+C. In NT, most of the C accumulation relative to CT occurred in the mineral-associated fraction, showing the importance of organo-mineral interaction in C stabilization and supporting our second hypothesis. However, the physical protection by aggregates played equally important role by stabilizing the occluded particulate organic matter (occluded-POM) before it was further stabilized by organomineral interaction. No-tillage is recommended as a sustainable soil management, but to increase soil C accumulation, the potential of cropping systems such those based on legume cover crops must be concurrently explored. ß 2013 Elsevier B.V. All rights reserved.

Keywords: Organic fractions Legume cover-crops Soil aggregation Acrisol C sequestration Occluded POM

1. Introduction Most of the knowledge with respect to organic matter stabilization is associated to studies carried out in temperate regions (Ko¨gel-Knabner et al., 2008; Ko¨gel-Knabner and Kleber, 2012; Six et al., 2002; Sollins et al., 2007; von Lu¨tzow et al., 2006), in soils predominantly constituted by high activity clays. Much more, however, remains to be investigated for tropical and subtropical soils in which kaolinite and Fe/Al oxides are the major clay constituents, and especially for those subjected to conservation management systems that promote C sequestration and contribute to mitigate climate change. Three main mechanisms of organic matter stabilization in soils have been considered (Sollins et al., 1996; von Lu¨tzow et al., 2006):

* Corresponding author. Tel.: +55 51 33166040; fax: +55 51 33166050. E-mail address: [email protected] (C. Bayer). 0167-1987/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.still.2013.01.006

(i) selective preservation due to recalcitrance of organic compounds at the molecular level; (ii) organo-mineral interaction on the surface of Fe-, Al-, Mn-oxides and phyllosilicates; and (iii) physical protection due to spatial inaccessibility (occlusion) of organic matter against decomposers. The selective preservation of recalcitrant components is no longer seen as the dominant mechanism and, thus, the main attention turned to physical protection and organo-mineral interactions (Dieckow et al., 2009; Kleber et al., 2011; Ko¨gel-Knabner and Kleber, 2012; Marschner et al., 2008; von Lu¨tzow et al., 2006, 2008). In this sense, soil physical fractionation is a useful tool in studies related to organic matter stabilization by organo-mineral interaction and physical protection. Golchin et al. (1994) developed an approach that is coherently linked to changes that organic material undergoes upon entering the soil. According to this model, litter or root residues follows through free particulate organic matter (POM), to occluded POM inside macro and microaggregates until finally becoming part of the mineral-associated organic matter

P.C. Conceic¸a˜o et al. / Soil & Tillage Research 129 (2013) 40–47

pool. The free POM keeps much of the characteristics of the original litter or root residues, is located between stable aggregates, and it is subjected neither to spatial inaccessibility nor to organo-mineral interactions; but here recalcitrance may play some role, at least during the early decomposition stages (von Lu¨tzow et al., 2006). The occluded POM is physically protected by occlusion, but not (or at least very little) by organo-mineral interaction (Golchin et al., 1994). The mineral-associated organic matter is derived mainly from microbial tissues or metabolites (Guggenberger et al., 1994; Lehmann et al., 2007) that are stabilized by organo-mineral interaction via adsorption processes (Kleber et al., 2007; Ko¨gelKnabner and Kleber, 2012). However, clay sized particles that adsorb organic matter actually do not exist as isolated entities, but are assembled into silt- and clay-sized microaggregates that additionally stabilize the adsorbed organic matter by physical entrapment (spatial inaccessibility) (Chenu and Plante, 2006; Virto et al., 2008). Effort must be applied on the investigation of the role that tillage and cropping systems play on the organic matter stabilization mechanisms. In most cases, no-tillage (NT) indeed allows carbon (C) accumulation in soil due to decreases in organic matter decomposition rate. Average annual C sequestration rates of 0.35 Mg ha1 and 0.48 Mg ha1 were estimated for NT soils, respectively, for the tropical (Cerrado) and subtropical regions of Brazil (Bayer et al., 2006). Six et al. (1999, 2000) suggested that the main reason for C increases in NT soils is the lower turnover rate of macroaggregates and thus the higher stabilization of microaggregates where ultimately the organic matter will be stabilized in the long-term. For two clayey subtropical Ferralsols with mineralogical composition of kaolinite and Fe-oxides, Denef et al. (2007) showed that the mineral-associated C of these occluded microaggregates is a very important long-term stabilized fraction of soil organic C and also the most responsive to changes in tillage system (NT vs. CT), so that it represents a diagnostic fraction.

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The adoption of NT as an isolated strategy, however, may not produce the expected positive results with respect of soil C accumulation. The establishment of diversified and high input cropping systems based on legume cover crops might be another important management strategy that should be associated to NT. Sisti et al. (2004) have observed that after 13 years under NT, the wheat (Triticum aestivum L.)–soybean (Glycine max (L.) Merr.)– vetch (Vicia villosa Roth)–maize (Zea mays L.) cropping system increased the total C stocks to 100 cm depth of a southern Brazilian Oxisol by nearly 1.3 Mg C ha1 year1 when compared with conventional tillage (CT); while no C accumulation in NT soil was observed under the less diversified wheat-soybean system. We hypothesized (i) that C sequestration in no-tillage soil is further enhanced by high input cropping systems based on legume cover crops and (ii) that the sequestered C is stored mainly in the mineral associated fraction. The objective of this study was to assess the contribution of tillage systems combined with cropping systems on soil aggregation, C accumulation and C stabilization in an sandy clay loam subtropical Acrisol. 2. Materials and methods 2.1. Field experiment A long-term field experiment (18 years) in Eldorado do Sul (RS), Brazil (308510 S and 518380 W), was used in this study. Detailed characteristics about soil and climate are given in Table 1. Until 1969 the area was under native grassland (mainly Paspalum spp. and Andropogon spp.) that represented the common native vegetation in this southern region of Brazil. In the following 16 years, soil was conventionally tilled with plowing and disking for annual cropping, leading to serious problems of soil physical degradation (Bayer et al., 2000). The experiment was established in 1985, and we selected for this study two tillage systems, namely conventional tillage (CT)

Table 1 Soil and climate characteristics of the long-term field experiment (18 years) used in this study, comparing conventional tillage (CT) and no-tillage (NT) which were combined with two cropping systems: (i) black oat (Avena strigosa Schreb) as winter cover crop followed by maize (Zea mays L.) as summer grain crop; and (ii) oat plus vetch followed by maize in summer intercropped with cowpea cover crop (Vigna unguiculata (L.) Walp). The soil under native grassland vegetation (NG), mainly Paspalum spp. and Andropogon spp., adjacent to the experiment was evaluated as a reference and reliable representative of the original soil conditions. Eldorado do Sul, Brazil: 308510 S and 518380 W. Characteristic

Data

Altitude Mean annual temperature Variation of the monthly temperatures Mean annual rainfall precipitation Variation of the monthly precipitation Ko¨ppen Climate Classification Soil group (WRB)a Particle size distribuiton Clay Silt Sand

96 m 19.4 8C 13.9–24.9 8C 1440 mm 96–168 mm Cfa Acrisol Experiment

NG

220 g kg1 240 g kg1 540 g kg1

248 g kg1 212 g kg1 540 g kg1

Mineralogyb Feoc Fedd Feo/Fed Gt/(Gt+Hm)e Soil bulk densityf(Mg m3) 0–5 cm 5–10 cm 10–20 cm a b c d e f

0.9 g kg1 of soil 11.8 g kg1 of soil 0.08 0.21 NG

CT

NT

1.50 1.57 1.63

1.51 1.57 1.60

1.43 1.67 1.65

World Reference Base for Soil Resources (IUSS, 2006). Original data presented by Inda-Junior et al. (2007). Ammonium oxalate soluble Fe. Dithionite-citrate-bicarbonate soluble Fe. Gt: goethite; Hm: hematite. Original data presented by Lovato (2001) to NG (native grassland) and Soler (2003) for CT (conventional tillage) and NT (no-tillage) plots.

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P.C. Conceic¸a˜o et al. / Soil & Tillage Research 129 (2013) 40–47

and no-tillage (NT) which were combined with two cropping systems: (i) black oat (Avena strigosa Schreb) as winter cover crop followed by maize (Zea mays L.) as summer grain crop [Ot/M]; and (ii) oat plus vetch followed by maize in summer intercropped with cowpea cover crop (Vigna unguiculata (L.) Walp) [Ot+V/M+C]. The experiment was arranged in a randomized complete block design with split-plot (tillage system in main plots and cropping system in subplots), and three field replicates. Each block was divided into two strips, one without N application and other with 180 kg N ha1 applied to the maize crop as urea, but we selected only the strip without N application. The soil under native grassland vegetation (NG) adjacent to the experiment was evaluated as a reference and reliable representative of the original soil conditions. The soil texture in the NG is similar to that of the experimental area (Table 1). In CT, one disk plowing to 17 cm depth and two disking operations to 10–13 cm depth were undertaken on spring each year, before maize planting. In NT, oat and vetch cover crops were rolled down with knife-roller at flowering. After maize harvesting in Ot+V/M+C the intercropped cowpea was allowed to grow and spread over the entire plot, until frosts ceased its cycle. 2.2. Soil sampling Soil samples of the 0–5, 5–10, and 10–20 cm layers were collected in October (spring) 2003, before plowing and disking in CT treatment. In each plot, soil was collected with spatula in a 20 cm  40 cm area (one per plot) and used for total C analysis. Soil samples with structure preserved in blocks of 10 cm wide  20 cm long were collected on the wall of the opened trench. These blocks were used for analysis of aggregate size distribution and physical fractionation of soil organic matter. 2.3. Soil organic carbon analysis and biomass carbon addition Soil samples were air dried and crushed with a wood roller to pass a 2 mm mesh and a subsample of approximately 20 g was further crushed in a mortar to pass a 250 mm mesh. About 400 mg was analyzed in a dry combustion Shimadzu TOC-VCSH analyzer. To calculate the total organic C stock in the 0–5, 5–10, and 10– 20 cm layers, the equivalent soil mass approach was adopted, following the procedure employed by Ellert and Bettany (1995). This approach normalizes the effect of management on soil bulk density. We used the values of bulk density previously obtained by Soler (2003) for CT and NT soil and by Lovato (2001) for NG soil. The soil under NG was the reference for the equivalent mass. Data of annual C addition by cropping systems (shoot plus root, being root considered 30% of shoot) was compiled from Zanatta et al. (2007), based on a survey since the beginning of the experiment. That study presents the average C addition for Ot/M (4.0 Mg C ha1 year1) and Ot+V/M+C (7.6 Mg C ha1 year1) across CT and NT, since biomass addition was the same in both tillage systems. 2.4. Aggregate size distribution The blocks of soil samples with preserved structure were manually and gently disrupted in the planes of weakness to obtain aggregates < 9.51 mm diameter, which were air dried. Duplicates of about 50 g of these aggregates were capillary wetted overnight in a funnel-folded filter paper, gently transferred into a 1000 mL cylinder (7.2 cm diameter  23.5 cm height) containing 500 mL water, and end-over-end shaken during two minutes at 16 rotations per minute, according to an adaptation of the method proposed by Tisdall et al. (1978) and Carpenedo and Mielniczuk (1990). The whole suspension was gently poured on the top of a

nest of five sieves (4.76, 2.00, 1.00, 0.50 and 0.25 mm mesh) immersed in a cylindrical vessel containing 12 L of water. The nest was attached to a lift mechanism and then mechanically raised and lowered in 3 cm amplitude, 42 oscillations per minute during 15 min. After the removal of the nest, the suspension of the cylinder vessel was poured through 105 mm and a 53 mm sieves and then treated with 50 mL potassium alum (5% w/v) to promote flocculation for recovery of < 53 mm particles. The aggregates in each sieve were transferred to aluminum pans, dried at 105 8C for 24 h and weighed. The mean weight diameter was calculated according to the description of Kemper and Rosenau (1986). 2.5. Physical fractionation of soil organic matter Three physical fractions were obtained according to adaptations of the density physical fractionation approach of Golchin et al. (1994): free particulate organic matter (free-POM), occluded particulate organic matter (occluded-POM), and mineral-associated organic matter (min-OM). Ten grams of air dried soil aggregates < 9.51 mm diameter were placed in a 100 mL centrifuge tube containing 80 mL of sodium polytungstate (SPT) solution of density 2.0 Mg m3 (Conceic¸a˜o et al., 2007, 2008). The use of soil aggregates avoids the underestimation of occluded-light fraction (Tomazi et al., 2011). The tube was closed with a rubber stopper and gently inverted five times to completely detach the aggregate mass from the bottom and release the free-POM contained between aggregates. The suspension was centrifuged at 2000  g during 90 min and then the supernatant was filtered (Whatman GF/C) under vacuum to recover the free-POM. The SPT solution was returned to the tube containing the aggregates ‘‘pellet’’ in the bottom and ultrasonic vibrations at an energy level of 240 J mL1 were carried out in order to disperse soil aggregates. This energy level was previously defined in a specific test and proved to be sufficient to disperse microaggregates > 2 mm in order to obtain 99% of total clay fraction (IndaJunior et al., 2007). After dispersion, the suspension was centrifuged at 2000  g during 90 min and the supernatant was filtered to recover the occluded-POM, in a similar manner as described for free-POM. The free- and occluded-POM fractions were weighed and analyzed by dry combustion for organic C, in the same equipment used for the bulk soil analysis. The C content in min-OM (heavy fraction) was obtained from the difference between total organic C and free- and occluded-POM. 2.6. Statistical analysis Results were subjected to analysis of variance (ANOVA) and the least significant difference between means was tested by Tukey test (P < 0.05). The normality of distributions and the homogeneity of variances, two important assumptions of ANOVA, were significant at a level a of 0.05, according to results of Kolmogorov–Smirnov test and Bartlett’s test, respectively. The results from NG were not included in the statistical analysis because this treatment was not part of the experimental design. 3. Results and discussion 3.1. Total organic carbon Carbon distribution was nearly constant within the 0–20 cm layer of CT soil while stratification with higher contents in the 0– 5 cm layer occurred in NT and NG soils (Fig. 1). These results are due to incorporation and homogenization of above- and belowground crop residues by plowing and disking operations in CT and to the permanency of shoot residues on surface and of roots in the

P.C. Conceic¸a˜o et al. / Soil & Tillage Research 129 (2013) 40–47 -1

C (g kg ) 0,0

6

8

10

12

14

16

18

20

22

Soil Depth (cm)

0

5

10 CT Ot/M CT Ot+V/M+C NT Ot/M

15

NT Ot+V/M+C NG

20

Fig. 1. Total organic carbon (C) content in the 0–5, 5–10 and 10–20 cm layers of a sandy clay loam Acrisol subjected to conventional tillage (CT) or no-tillage (NT) in combination with two cropping systems: oat/maize (Ot/M) and oat + vetch/maize + cowpea (Ot+V/M+C). The native grassland (NG) soil represents the original soil condition. Each horizontal bar represents the least significant difference (LSD) according to Tukey test (P < 0.05). The NG was not included in statistical analysis for not being part of the experimental design. Eldorado do Sul, Brazil: 308510 S and 518380 W.

0–5 cm layer of NT and NG soil. The only significant difference in C content and stock between tillage systems was observed in the 0– 5 cm, with higher content and stocks in NT than in CT, both in Ot/M and Ot+V/M+C (Fig. 1 and Table 2). The 0–5 cm layer was the most important in terms of C accumulation in the NT soil, within the 20 cm depth evaluated in the study. Studies in temperate soils showed the importance of considering the whole plow layer when comparing the effects of NT versus

43

CT and indicated that overall increments in C stock due to NT may be neutral because of the C redistribution within the plow layer (VandenBygaart and Angers, 2006). This finding, however, could not be confirmed in our study, since C content and stock of 5–10 and 10–20 cm layers in CT was not significantly higher than in NT (Fig. 1 and Table 2). On the other hand, there are studies indicating that positive effects of NT at increasing the organic carbon may extend deeper than 20 cm in subtropical Brazilian soils (Boddey et al., 2010; Diekow et al., 2005; Sisti et al., 2004). The soil C stock in 0–20 cm was higher in NT than in CT, either in Ot/M or Ot+V/M+C (Fig. 2); so that the annual C sequestration rate in NT relative to CT was 0.25 Mg ha1 in Ot+V/M+C and 0.18 Mg ha1 in Ot/M. The higher sequestration rate in Ot+V/ M+C is due to its higher annual biomass C input (7.6 Mg C ha1, compared with 4.0 Mg C ha1 in Ot/M). But the sequestration rates obtained in this study are lower than the average rate of 0.48 Mg ha1 or 0.68 Mg C ha1 estimated in reviews of Bayer et al. (2006) and Bernoux et al. (2006), respectively, for subtropical Brazilian croplands, and the rate obtained by Carvalho et al. (2009) for soils in cerrado region of the Brazilian Amazon (0.38 Mg ha1 year1). The influence of cropping system on C content and stock was not restricted only to the 0–5 cm layer, as observed for tillage system, but extended to 10 cm depth in NT and 20 cm in CT (Fig. 1 and Table 2). In the whole 0–20 cm, the difference of C stocks between Ot+V/M+C and Ot/M (5.0 and 6.2 Mg C ha1 in CT and NT, respectively) were higher than the differences between NT and CT (3.3 and 4.5 Mg C ha1 in Ot/M and Ot+V/M+C, respectively). Considering Ot/M as the baseline system, the annual C sequestration rate in Ot+V/M+C was 0.28 Mg C ha1 in CT and 0.34 Mg C ha1 in NT, and both rates were higher than those found for NT against CT (0.18 and 0.25 Mg C ha1 year1). These results suggest that changing to high input cropping systems might

Table 2 Organic carbon (C) stock in free particulate organic matter (free-POM), occluded particulate organic matter (occluded-POM) and mineral-associated organic matter (min-OM) fractions, in the 0–5, 5–10 and 10–20 cm layers of a sandy clay loam Acrisol subjected to conventional tillage (CT) or no-tillage (NT) in combination with two cropping systems: oat/maize (Ot/M) and oat+vetch/maize+cowpea (Ot+V/M+C). The native grassland (NG) soil represents the original soil condition. Eldorado do Sul, Brazil: 308510 S and 518380 W. Tillage system

CT NT

Cropping system

NT

NT

NT NG a

Free-POM

Occluded-POM

Min-OMa

5.4 bB 6.0 bA 7.4 aB 8.9 aA 10.0

0–5 cm 6.8 bB 8.5 bA 10.4 aB 14.1 aA 12.7

7 aA 10 aA 8 aA 13 aA 7

14 19 21 24 15

aA aA aA aA

79 71 71 63 79

aA aA bA bA

aB aA aA aA

5.8 aB 6.6 aA 5.9 aB 7.1 aA 8.4

5–10 cm 6.9 aB 8.3 aA 6.9 aB 8.4 aA 9.9

2 4 2 2 3

aB aA aA bA

14 16 14 14 12

aA aA aA aA

84 80 85 84 85

aA aB aA aA

2.0 2.1 1.4 1.4 1.7

aA aA bA bA

11.8 13.7 12.3 13.2 14.6

aB aA aB aA

10–20 cm 14.0 aB 16.1 aA 13.8 aA 14.8 aA 16.7

2 2 1 1 2

aA aA bA bA

14 aA 13 aA 10 aA 9 aA 10

84 85 89 89 88

aA aA aA aA

4.0 5.0 4.5 6.0 4.8

aB aA aB aA

23.0 26.3 25.5 29.1 33.0

aB aA aB aA

0–20 cm 27.8 bB 32.8 bA 31.1 aB 37.3 aA 39.3

3 4 3 6 4

aA aA aB A

14 15 15 16 12

83 80 82 78 84

aA aB aA B

Min-OM

Ot/M Ot+V/M+C Ot/M Ot+V/M+C

0.5 aAb 0.9 bA 0.8 aB 1.8 aA 0.8c

1.0 1.6 2.2 3.5 1.9

bA bA aB aA

Ot/M Ot+V/M+C Ot/M Ot+V/M+C

0.1 0.3 0.1 0.2 0.3

aB aA aA bA

1.0 1.3 0.9 1.2 1.2

Ot/M Ot+V/M+C Ot/M Ot+V/M+C

0.3 0.3 0.1 0.2 0.4

aA aA bA bA

Ot/M Ot+V/M+C Ot/M Ot+V/M+C

0.8 1.5 1.1 2.1 1.5

aA aA aB aA

NG

CT

Total

Occluded-POM

NG

CT

Distribution of total C (%)

Free-POM

NG

CT

C stock (Mg ha1) a

aA aA aB A

Values to min-OM were obtained by the difference between total C minus free-POM and occluded-POM. Lowercase letters compare tillage systems, within the same cropping system and layer; and uppercase letters compare cropping systems, within the same tillage system and layer, according to Tukey test (P < 0.05). c The NG (native grassland) was not included in statistical analysis for not being part of the experimental design. b

P.C. Conceic¸a˜o et al. / Soil & Tillage Research 129 (2013) 40–47

44

50

NT

0

Tillage and cropping systems Fig. 2. Total organic (C) stock in the 0–20 cm layer of a sandy clay loam Acrisol subjected to conventional tillage (CT) or no-tillage (NT) in combination with two cropping systems: oat/maize (Ot/M) and oat + vetch/maize + cowpea (Ot+V/M+C). The native grassland (NG) soil represents the original soil condition. The lowercase letters above the bars compare the two tillage systems within the same cropping system, while the uppercase letters compare the two cropping systems within the same tillage system according to Tukey test (P < 0.05). Vertical bars refer to the standard deviation. The NG was not included in statistical analysis for not being part of the experimental design. Eldorado do Sul, Brazil: 308510 S and 518380 W.

be more effective for C sequestration than changing to no-tillage system, but surely the optimal strategy is combining both: NT with high input cropping system. Contrasting the two extreme treatments of NT under Ot+V/M+C versus CT under Ot/M, the difference in C stock was 9.5 Mg ha1 and the C sequestration rate was 0.53 Mg ha1 year1. This C gain is considerably higher than the individual gains when only CT is changed to NT or only Ot/M is changed to Ot+V/M+C. Those findings also confirm our first hypothesis that C sequestration in NT soil is further enhanced by high input cropping systems. Regarding to changes in cropping system, it is important to highlight the contribution that legume species like vetch and cowpea in Ot+V/M+C system had in adding N into the soil. This N addition contributes to higher phytomass production of non-legume species like oat and maize (Lovato et al., 2004) and, according to Drinkwater et al. (1998), is more effectively immobilized in microbial biomass and soil organic matter than mineral N addition, thus also contributing to retention of carbon. In a global estimate of soil C change across several climate regimes, West and Six (2007) estimated that the average increase of C stocks after conversion of CT to NT was higher (16%) than the increase promoted by improvements in rotation intensity (6%). Our findings, however, suggested that for soil C increments improvements in tillage system (adoption of NT instead of CT) was not more important than improvements in cropping system (adoption of Ot+V/M+C instead of Ot/M). High input cropping systems are concrete possibilities for tropical or subtropical regions when the potential for high net primary productivity and C input of native grass and legume species are deeply and rationally explored. Although reduced C oxidation rates occur when CT is converted to NT, this can be considered as a limited gain if compared with benefits that countless spatio-temporal arrangements of cash- and cover-crops may offer in terms of net primary productivity and C inputs in cropping system plans. The benefits of high input cropping systems in promoting C sequestration in NT was shown in a previous study (Diekow et al., 2005) where the average C sequestration rate of legume-based cropping systems (including pigeon pea and maize in summer) was 0.48 Mg C ha1 year1 in the 0–17.5 cm layer and 1.01 Mg C ha1 year1 in the 0–107.5 cm layer.

Percentage of the total Soil mass (%)

CT

40

NG

10

aA

60 bA

Ot+V/M+C

Ot+V/M+C

20

Ot/M

C stock (Mg ha-1)

30

27.8 bB

80

39.3

31.1 aB

Ot/M

32.8 bA

40

37.3 aA

aA

(a) 0-5 cm aA aA bA

bB

bA

CT Ot/M CT Ot+V/M+C NT Ot/M NT Ot+V/M+C NG aA aA

20

bA

bA

0 80 aA

60

aA

(b) 5-10 cm

bA aA

40

aA aA bA

aA aA aA aA aA

20 0 80 (c) 10-20 cm 60

aA

aA aA aB

40

aA

aA aA

aA aA aA aA aA

20

9.51-2.00

2.00-0.25

<0.25

Aggregates size-classes (mm) Fig. 3. Aggregate size-classes distribution in the (a) 0–5, (b) 5–10 and (c) 10–20 cm layers of a sandy clay loam Acrisol subjected to conventional tillage (CT) or notillage (NT) in combination with two cropping systems: oat/maize (Ot/M) and oat + vetch/maize + cowpea (Ot+V/M+C). The native grassland (NG) soil represents the original soil condition. The lowercase letters above the bars compare the two tillage systems within the same cropping system, while the uppercase letters compare the two cropping systems within the same tillage system according to Tukey test (P < 0.05). The NG was not included in statistical analysis for not being part of the experimental design. Vertical bars refer to the standard deviation. Eldorado do Sul, Brazil: 308510 S and 518380 W. For simplification, the original eight size classes of aggregates were composed only into three.

3.2. Soil aggregation The aggregate-size distribution was affected by tillage systems in the 0–5 and 5–10 cm layers, but not in the 10– 20 cm (Fig. 3). The lower percentage of 9.51–4.76 mm macroaggregates in CT compared with NT (Fig. 3a and b) resulted from their fragmentation into smaller aggregates of 2.00–0.25 mm and microaggregates < 0.25 mm. The fragmentation to aggregates is induced directly or indirectly by plowing and disking operations of CT and is in agreement to the model of Six et al. (2000) suggesting higher turnover for macroaggregates in CT than in NT. In the 0–5 cm layer, the mean weight diameter (MWD) of 1.9 mm in CT Ot/M and 2.8 mm in CT Ot+V/M+C was increased respectively to 3.3 mm (74% increase) and 3.9 mm (39% increase) in NT soil (Fig. 4). In the 5–10 cm layer, the increase in MWD due to NT was not great as in the top layer but ranged from 26% in Ot/M to 13% in Ot+V/M+C. The effects of cropping systems on soil aggregates distribution were not as evident as the effects of tillage systems. The only significant changes observed were that Ot+V/M+C, compared Ot/ M, increased the 9.51–4.76 mm class in the 0–5 cm layer of CT soil (Fig. 3a), but decreased this same size–class and the MWD in the 10–20 cm layer of NT soil (Fig. 3c, Fig. 4). This minor MWD could be due to a fragmentation of larger aggregates by more intensive wetting-drying cycles in the rhizosphere of Ot+V/M+C (higher evapotranspiration) (Bradfield, 1937).

P.C. Conceic¸a˜o et al. / Soil & Tillage Research 129 (2013) 40–47

6

5

aA aA

MWD (mm)

CT Ot/M CT Ot+V/M+C NT Ot/M NT Ot+V/M+C NG aA aA

4

aA

aA bA aA

aA

bA

3

aB

bA 2

1

0 0-5

5-10

10-20

Layer (cm) Fig. 4. Mean weight diameter (MWD) of aggregates in the 0–5, 5–10 and 10–20 cm layers of a sandy clay loam Acrisol subjected to conventional tillage (CT) or notillage (NT) in combination with two cropping systems: oat/maize (Ot/M) and oat + vetch/maize + cowpea (Ot+V/M+C). The native grassland (NG) soil represents the original soil condition. The lowercase letters above the bars compare the two tillage systems within the same cropping system, while the uppercase letters compare the two cropping systems within the same tillage system according to Tukey test (P < 0.05). The NG was not included in statistical analysis for not being part of the experimental design. Vertical bars refer to the standard deviation. Eldorado do Sul, Brazil: 308510 S and 518380 W.

3.3. Carbon distribution in physical fractions The only layer in which NT promoted C accumulation in comparison to CT was the top 0–5 cm (Fig. 1). In this layer, the C stocks in free-POM of NT (0.8 and 1.8 Mg ha1) were almost twice of that of CT (0.5 and 0.9 Mg ha1) (Table 2), and was expected considering that aboveground residue accumulates on the surface of NT while is incorporated in CT soil. The same was observed for occluded-POM, so that while 21% and 24% of the total organic C was in occluded-POM fraction in NT soil, these percentages were lower in CT soil (14% and 19%) (Table 2). These results are related to the higher proportion of stable macroaggregates in NT (Fig. 3a) and their role in stabilizing the occluded POM through the spatial inaccessibility mechanism.

45

Of the total increment in C stocks due to NT in the 0–5 cm layer, 10%–17% took place in the free-POM, 33%–34% in the occludedPOM and 50%–56% in the min-OM fraction (Table 3). This higher accumulation in min-OM fraction confirms our second hypothesis that the sequestered C is stored mainly in the mineral associated fraction. A previous study in two subtropical Brazilian Oxisols also showed that the mineral-associated C (in microaggregates inside macroaggregates) stored most of the C accumulation obtained by adopting NT against CT (Denef et al., 2007), leading to the conclusion that this was a very important long-term stabilized fraction for C sequestration. The greater storage of organic C in the mineral-associated pool (mainly related to microaggregates) compared with storage in occluded-POM after conversion of degrading to aggrading soil management systems is a well reported fact for temperate conditions (Denef et al., 2004; Jastrow, 1996; Six et al., 2000). If considering the whole 0–20 cm layer, the contribution of min-OM raised to 63–77% (Table 3). However, those results do not mean that min-OM is the most important fraction for C stabilization, because up to one third of the C accumulation in the 0–5 cm layer and 16 and 22% in the whole 0–20 cm layer still occurred in the POM fraction occluded inside aggregates. Furthermore, Balabane and Plante (2004) speculate that the effective role of physical protection by macroaggregates is ‘‘offering the time for organic matter to mature in the soil and to interact with specific adsorption sites on mineral surfaces’’. This agrees with Six et al. (1999, 2000) when they suggest that the main reason for C increases in NT soils is the lower turnover rate of macroaggregates and thus the higher stabilization of the occluded microaggregates wherein organic matter is finally stabilized at a higher degree. In microaggregates, organo-mineral interactions by surface adsorption actually take place simultaneously with the physical entrapment of this adsorbed organic matter (Chenu and Plante, 2006). Most of the C accumulation due to Ot+V/M+C in comparison to Ot/M also occurred in the heavy fraction and this became more evident from the top layer, in which the heavy fraction held 39– 40% of the C accumulation, in CT and NT respectively, to the deepest layer, whereby 93–94% of the C increments occurred in the heavy fraction (Table 4). The C accumulation in occluded-POM and min-OM fractions of CT soil cropped with Ot+V/M+C confirms the beneficial role of high input cropping systems in contributing to effective C sequestration, even under non-favorable conditions created by the soil tillage disturbances.

Table 3 Carbon (C) accumulation and percentage of this accumulation in the free particulate organic matter (free-POM), occluded particulate organic matter (occluded-POM) and mineral-associated organic matter (min-OM) fractions, in the 0–5, 5–10 and 10–20 cm layers of a sandy clay loam Acrisol due to adoption of no-tillage (NT) in comparison to conventional tillage (CT), in two cropping systems: oat/maize (Ot/M) and oat + vetch/maize + cowpea (Ot+V/M+C). Eldorado do Sul, Brazil: 308510 S and 518380 W. Cropping system

Free-POM Ot/M Ot+V/M+C

Percentage of total DC (%)a

DCNTCT (Mg ha1)

0.35  0.29b 0.95  0.67

Occluded-POM

Min-OM

1.24  0.25 1.88  0.54

0–5 cm 2.02  1.25 2.86  1.56

Total

Free-POM

Occluded-POM

Min-OM

3.61  1.28 5.69  1.72

10 17

34 33

56 50

Ot/M Ot+V/M+C

0.02  0.03 0.16  0.03

0.06  0.28 0.16  0.27

5–10 cm 0.08  0.42 0.42  1.77

0.01  0.40 0.10  1.60

– –

– –

– –

Ot/M Ot+V/M+C

0.10  0.04 0.10  0.08

0.65  0.41 0.74  0.60

10–20 cm 0.47  1.27 0.47  2.86

0.29  0.85 1.31  3.06

– –

– –

– –

Ot/M Ot+V/M+C

0.23  0.23 0.69  0.72

0.54  0.76 0.98  0.77

0–20 cm 2.56  2.30 2.81  6.07

3.33  1.73 4.48  6.31

7 15

16 22

77 63

a The percentage of total DC was calculated by dividing the DCNTCT of the fraction by the total DCNTCT. This calculation was done only for layers where DCNTCT values were positive, and so the 5–10 and 10–20 cm layers were not included. b Values after  refer to the standard deviation.

P.C. Conceic¸a˜o et al. / Soil & Tillage Research 129 (2013) 40–47

46

Table 4 Carbon (C) accumulation and percentage of this accumulation in the free particulate organic matter (free-POM), occluded particulate organic matter (occluded-POM) and mineral-associated organic matter (min-OM) fractions, in the 0–5, 5–10 and 10–20 cm layers of a sandy clay loam Acrisol due to adoption of oat + vetch/maize + cowpea cropping system (Ot+V/M+C) in comparison to oat/maize (Ot/M) cropping system, in two tillage systems: conventional tillage (CT) and no-tillage (NT). Eldorado do Sul, Brazil: 308510 S and 518380 W. Tillage system

DCOt + V/M + COt/M (Mg ha-1) Free-POM

Percentage of total DC (%)a

Occluded-POM

Min-OM

Total

Free-POM

Occluded-POM

Min-OM

1.64  0.72 3.73  0.16

24 27

36 33

39 40

CT NT

0.39  0.38b 1.00  0.55

0.60  0.15 1.24  0.62

0–5 cm 0.65  0.64 1.49  0.76

CT NT

0.17  0.04 0.03  0.00

0.35  0.15 0.25  0.22

5–10 cm 0.83  0.68 1.18  0.80

1.35  0.71 1.45  0.94

13 2

26 17

61 81

CT NT

0.03  0.07 0.03  0.04

0.11  0.37 0.02  0.13

10–20 cm 1.88  1.05 0.95  1.41

2.03  1.46 1.01  1.58

1 4

5 2

93 94

CT NT

0.60  0.33 1.06  0.59

1.06  0.29 1.51  0.33

0–20 cm 3.37  2.17 3.62  1.94

5.03  2.27 6.19  2.40

12 17

21 24

67 58

a b

The percentage of total DC was calculated by dividing the DCOt + V/M + COt/M of the fraction by the total DCOt + V/M + COt/M. Values after  refer to the standard deviation.

4. Conclusions The potential for soil C accumulation in subtropical soils depends on the establishment of cropping systems with diverse cash and cover crops, rather than on the simple conversion of CT to NT. This is in line to our first hypothesis that C sequestration in NT soil is further enhanced by high input cropping systems. No-tillage influences organic matter stabilization by reducing decomposition rate, but on the other hand, improvements in cropping system to further increase the C addition might have a greater potential for soil C stock enhancement. This is particularly true for tropical and subtropical regions where a large diversity of cash and cover crops are available to be chosen. In this subtropical Acrisol, most of the C which accumulates in NT soil, relative to CT, is stored in the mineral associated fraction, what highlights the importance of organo-mineral interaction as a stabilization mechanism in NT soil. This is in line to our second hypothesis that the sequestered C is stored mainly in the mineral associated fraction. However, the occlusion of C inside aggregates is also an equally important stabilization mechanism by serving as a passage for the residue C to be later stabilized by organo-mineral interaction. Acknowledgements This project was funded by the Brazilian Council for Scientific and Technologic Development (CNPq) and Foundation of Research Support of Rio Grande do Sul State (FAPERGS). References Balabane, M., Plante, A.F., 2004. Aggregation and carbon storage in silty soil using physical fractionation techniques. European Journal of Soil Science 55, 415–427. Bayer, C., Martin-Neto, L., Mielniczuk, J., Pavinato, A., Dieckow, J., 2006. Carbon sequestration in two Brazilian Cerrado soils under no-till. Soil and Tillage Research 86, 237–245. Bayer, C., Mielniczuk, J., Amado, T.J.C., Martin-Neto, L., Fernandes, S.V., 2000. Organic matter storage in a sandy clay loam Acrisol affected by tillage and cropping systems in southern Brazil. Soil and Tillage Research 54, 101–109. Bernoux, M., Cerri, C.C., Cerri, C.E.P., Siqueira-Neto, M., Metay, A., Perrin, A.S., Scopel, E., Razafimbelo, T., Blavet, D., Piccolo, M.D., Pavei, M., Milne, E., 2006. Cropping systems, carbon sequestration and erosion in Brazil, a review. Agronomy Sustainable Development 26, 1–8. Boddey, R.M., Jantalia, C.P., Conceic¸a˜o, P.C., Zanatta, J.A., Bayer, C., Mielniczuk, J., Dieckow, J., Santos, H.P., Denardin, J.E., Aita, C., Giacomini, S.J., Alves, B.J.R., Urquiaga, S., 2010. Carbon accumulation at depth in Ferralsols under zero-till subtropical agriculture. Global Change Biology 16, 784–795.

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