G Model STILL-2815; No. of Pages 9 Soil & Tillage Research xxx (2010) xxx–xxx Contents lists available at ScienceDirect Soil & Tillage Research journal homepage: www.elsevier.com/locate/still Soil carbon stocks under no-tillage mulch-based cropping systems in the Brazilian Cerrado: An on-farm synchronic assessment Marcos Siqueira Neto a, Eric Scopel b,c,*, Marc Corbeels b,c, Alexandre Nunes Cardoso c, Jean-Marie Douzet d, Christian Feller e, Marisa de Cássia Piccolo a, Carlos C. Cerri a, Martial Bernoux e a Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, PO Box 96, 13400-970 Piracicaba, SP, Brazil CIRAD PERSYST, UPR SCA, Avenue Agropolis 34398, Montpellier Cedex 5, France Embrapa-Cerrados, PO Box 8233, 73301-970 Planaltina, DF, Brazil d CIRAD PERSYST, URP SCA, PO Box 230, 110 Antsirabe, Madagascar e IRD, UMR 210 Eco&Sols, Inra-IRD-SupAgro, SupAgro-Bat 12, 2 place Viala, 34060 Montpellier Cedex 1, France b c A R T I C L E I N F O A B S T R A C T Article history: Received 28 November 2008 Received in revised form 13 July 2010 Accepted 19 July 2010 No-tillage mulch-based (NTM) cropping systems have been widely adopted by farmers in the Brazilian savanna region (Cerrado biome). We hypothesized that this new type of management should have a profound impact on soil organic carbon (SOC) at regional scale and consequently on climate change mitigation. The objective of this study was thus to quantify the SOC storage potential of NTM in the oxisols of the Cerrado using a synchronic approach that is based on a chronosequence of fields of different years under NTM. The study consisted of three phases: (1) a farm/cropping system survey to identify the main types of NTM systems to be chosen for the chronosequence; (2) a field survey to identify a homogeneous set of situations for the chronosequence and (3) the characterization of the chronosequence to assess the SOC storage potential. The main NTM system practiced by farmers is an annual succession of soybean (Glycine max) or maize (Zea mays) with another cereal crop. This cropping system covers 54% of the total cultivated area in the region. At the regional level, soil organic C concentrations from NTM fields were closely correlated with clay + silt content of the soil (r2 = 0.64). No significant correlation was observed (r2 = 0.07), however, between these two variables when we only considered the fields with a clay + silt content in the 500– 700 g kg 1 range. The final chronosequence of NTM fields was therefore based on a subsample of eight fields, within this textural range. The SOC stocks in the 0–30 cm topsoil layer of these selected fields varied between 4.2 and 6.7 kg C m 2 and increased on average (r2 = 0.97) with 0.19 kg C m 2 year 1. After 12 years of NTM management, SOC stocks were no longer significantly different from the stocks under natural Cerrado vegetation (p < 0.05), whereas a 23-year-old conventionally tilled and cropped field showed SOC stocks that were about 30% below this level. Confirming our hypotheses, this study clearly illustrated the high potential of NTM systems in increasing SOC storage under tropical conditions, and how a synchronic approach may be used to assess efficiently such modification on farmers’ fields, identifying and excluding non desirable sources of heterogeneity (management, soils and climate). ß 2010 Elsevier B.V. All rights reserved. Keywords: Cover crops Chronosequence Intensive agriculture Tropics Oxisols 1. Introduction The Cerrado is a large savannah biome occupying 25% of Brazil’s land area in the central part of the country (Ribeiro and Teles Walter, 1998). About 46% of the Cerrado soils are oxisols that are suitable for mechanized and intensive agriculture after correction * Corresponding author at: CIRAD PERSYST/Embrapa-Cerrados, Km 18, BR 020 – Rodovia Brası́lia/Fortaleza, PO Box 08223, CEP 73310-970 Planaltina, DF, Brazil. Tel.: +55 61 3388 9849; fax: +55 61 3388 9879. E-mail address: eric.scopel@cirad.fr (E. Scopel). for soil acidity and phosphorous deficiency (Reatto et al., 1998; Yamada, 2005). Since the 1970s a rapid expansion of a large-scale commercial agriculture took place thanks to the intensive use of lime and chemical fertilizers. The region accounts for 28% of Brazil’s total grain production (IBGE, 2004). To face the challenges of soil degradation, whilst at the same time reducing production costs (Borges, 1993), farmers opted from the early 1980s onwards for no-tillage cropping systems with a permanent mulch of crop residues (in this study referred to as notillage mulch-based (NTM) cropping systems). A major challenge under the tropical humid conditions of the Cerrado was to develop NTM cropping systems that produce sufficient amounts of crop 0167-1987/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2010.07.010 Please cite this article in press as: Neto, M.S., et al., Soil carbon stocks under no-tillage mulch-based cropping systems in the Brazilian Cerrado: An on-farm synchronic assessment. Soil Tillage Res. (2010), doi:10.1016/j.still.2010.07.010 G Model STILL-2815; No. of Pages 9 2 M.S. Neto et al. / Soil & Tillage Research xxx (2010) xxx–xxx residues to effectively protect the soil surface from erosion and to maintain adequate levels of soil organic carbon (SOC) (Séguy et al., 1996; Bolliger et al., 2006). In addition to efficiently tackling the problem of soil erosion (Scopel et al., 2005; Bertol et al., 2007), mulching with crop residues also enhances the water retention capacity of the topsoil layer (Fischer et al., 2002; Findeling et al., 2003) and soil biological activity (Kladivko, 2001; Blanchart et al., 2007) thereby improving on the long term soil porosity (Kladivko, 1994; Singh et al., 2007). Microbial biomass activity can be enhanced under NTM in the short (Alvarez and Alvarez, 2000) and long-term (Wang et al., 2008), because of better soil physical conditions and more important organic returns. All these modifications eventually result in higher crop yields (Séguy et al., 1996; Scopel et al., 2005). Several studies have been realized in the Cerrado on oxisols stating that NTM cropping systems increase SOC in the topsoil layer in the long term (Sá et al., 2001; Bayer et al., 2006; Bernoux et al., 2006; Carvalho et al., 2009). There is, however, no unanimity on this point; other studies (Roscoe and Buurman, 2003; Bernoux et al., 2006; Metay et al., 2007) found no significant differences between SOC stocks under NTM and conventional tillage management. There are no reports of long-term diachronic studies on SOC changes under NTM management in the Cerrado. Quite a number of synchronic studies exist that compare NTM with conventional tillage, but they mostly relate to NTM systems that had been implemented for only a couple of years (Metay et al., 2007; Carvalho et al., 2009) and often consider only one single point in time (Bayer et al., 2006). Most of these studies describe researcher-managed experiments and thus do not allow an assessment of the SOC storage potential of NTM systems as practiced by farmers. For an on-farm synchronic assessment of SOC storage under NTM, it is possible to use pairs of conventionally and NTM managed farmer’s fields (Blanco-Canqui and Lal, 2008), or a chronosequence of farmers’ fields managed under NTM for different lengths of time (Sá et al., 2001). Whatever the approach, all the conditions potentially linked with SOC dynamics must be the same for all the selected fields (Hewitt et al., 2001). It means that the fields must have similar climatic conditions (Alvarez et al., 2001; Fang and Moncrieff, 2001), the same soil mineralogy and texture (Parfitt et al., 1997; Six et al., 1999; Zinn et al., 2005), and a similar land-use history. Given the vast areas under NTM in the Cerrado it is necessary to gain a good estimate of their potential to store SOC in order to quantify the environmental impacts of this technology on a regional and even global scale. The aim of this study was to estimate the long-term evolution of SOC stocks under NTM. The study was undertaken in a pioneer region (southwest Goiás State), where the first NTM cropping systems appeared in 1985. We hypothesized that a synchronic study that considers and stratifies the diversity of situations under which NTM systems are adopted by farmers, starting from a large number of situations and then zooming in a few steps on a final homogeneous chronosequence of fields of different time periods under NTM, was an effective approach for the assessment of long-term SOC storage under NTM, for a given set of specific soil and weather conditions. 2. Materials and methods 2.1. Study area and sites description The study was carried out between 2001 and 2002 in the municipalities of Rio Verde, Montividiu and Santa Helena de Goiás, in the southwestern part of the Goiás state (Fig. 1). The three municipalities cover an area of 11,391 km2 of which 7150 km2 is agricultural land. The climate in the region is of the Aw type (Köppen classification), with a mean annual temperature of 23 8C and an annual precipitation of around 1500–1800 mm, with a dry season from May to September. The natural vegetation in the study area has been classified as Cerrado sensu stricto (tree dominated scrub of shrubs and trees of 3–8 m height with grass under story) Fig. 1. (a) Localization of the three municipalities (Montividiu, Rio Verde and Santa Helena de Goiás) in the southwest of Goiás state, with (b) localization of the selected farms in phase 1 of the study, and (c) selected fields in phase 2 (& pasture, ~ cerrado, * field with CT, * fields under NTM). Please cite this article in press as: Neto, M.S., et al., Soil carbon stocks under no-tillage mulch-based cropping systems in the Brazilian Cerrado: An on-farm synchronic assessment. Soil Tillage Res. (2010), doi:10.1016/j.still.2010.07.010 G Model STILL-2815; No. of Pages 9 3 M.S. Neto et al. / Soil & Tillage Research xxx (2010) xxx–xxx Table 1 Successive phases of the study, tools and objectives. Sample Tools/measurements Objective Phase 1: Farm survey 50 Farms Interviews with farmers Characterize variability in farmers’ NTMa management Phase 2: Field survey 48 Fields (from zero to up to 12 years under NTM) Rapid characterization of the 0–20 cm top soil layer (2–3 points/field) Characterize soil natural variability and its impacts on SOC storage Phase 3: Homogeneous chronosequence characterization 8 Fields/areas (2 Cerrados, 1 Pasture, 1 Conventional and 4 NTM from 1 to 12 years) Detailed characterization of C storage on the 0–30 cm profile (3 sub-areas per field and 6 points per sub-areas) Characterize the impact of NTM management on SOC storage a NTM: no-tillage mulch based cropping systems. and Cerradão (dry semi deciduous woodland) (Coutinho, 1978; Reatto et al., 1998). 2.2. Structuration of the study in successive phases The study comprised three phases (Table 1). Firstly, we characterized the NTM systems as adopted by the farmers and their regional variability because no information was available on local systems, whilst this information was necessary to select a homogeneous chronosequence in terms of management. We then selected and characterized a large number of fields that were managed under the most typical NTM system of the region to characterize soil natural variability and its consequences on SOC storage. Finally, from this large sample we selected a chronosequence of homogeneous fields of varying time periods under NTM and determined their soil C stocks for the assessment of SOC storage under this system. 2.2.1. Phase 1: Farm survey The first phase of the work involved a farm survey by means of a questionnaire to farmers and farm managers. Questions related to field characteristics, the history of land use, and the current management (area cultivated, type of cropping systems, crops used in succession and/or rotation, use of fertilizers and other inputs). Interviews were carried out on fifty randomly selected farms in the region. All fields of each farm of the survey were considered in the questionnaire. The 50 farms covered by the survey together represented 33,374 ha of cultivated land, or almost 5% of the cultivated area in the three municipalities. 2.2.2. Phase 2: General field survey In the second phase of the study, we selected 48 fields that were managed under the most common cropping systems of the region (i.e. as identified in the first phase). These fields were under NTM for varying time periods (from zero to 12 years). All fields were selected for having the same dominant soil type: (dark) red yellow latossol (Brazilian soil classification) or typic acrustoxes (USDA Soil Taxonomy). They were also chosen in restricted areas of the region in order to minimize the effects of the local spatial variability: i.e. all selected fields were located on the plateau of the landscape. We also excluded fields in the municipality of Santa Helena de Goiás because of the dryer climate (Fig. 1). 2.2.3. Phase 3: Chronosequence selection and characterization In the third phase of the study we established a chronosequence of NTM fields of different age. For comparison, also two plots under Cerrado vegetation (CE) and one plot under a pasture of Brachiaria decumbens (Stapf) (PA) were sampled. The chronosequence consisted of one soybean field managed under conventional tillage (CT) and four fields that had been managed for 1, 4, 8 and 12 years, respectively, under the most common NTM systems of the region (i.e. soybean–cereal succession each year). All arable fields were established on former native savannah vegetation between 20 and 30 years ago and had been under soybean monoculture with conventional tillage during at least 10 years prior to NTM adoption. The CT field that had been under conventional tillage for 23 years was considered as a reference i.e. ‘‘year zero’’ under NTM. All fields had similar soils (Table 2) within the same textural range (500–700 g kg 1 of clay + silt). The color of the wet soils (Munsell Company Inc, 1954) corresponded in all fields to the 2.5YR matrix that is characteristic for the red yellow latosols of the Cerrado. The mineral composition of the soils showed also a clear similarity in all eight fields, the main minerals being quartz (SiO4), kaolinite (Al2Si2O5(OH)4), gypsite (Al(OH)3), hematite (Fe2O3) and titanium oxide (TiO2) (data not shown). Table 2 Main characteristics of the fields/areas selected for the final chronosequence. Site Rotation system General Slope Color of B1 pedologic horizona Texture of 0–20 cm layer Sand (%) Silt (%) Clay (%) Cerrado Native vegetation 5% 2.5YR 3/6 48 4 48 Cerrado Native vegetation <3% 2.5YR 3/6 41 5 54 Pasture – 17 years Extensive Brachiaria sp. pasture <3% 2.5YR 3/6 27 5 68 Conventional tillage – 23 years 1 Cycle/year soybean (maize every 2 years) 3% 2.5YR 4/7 40 4 56 NTMa –1 year 2 Cycles/year soybean + (maize or sorghum or millet) 5–7% 2.5YR 4/6 36 4 60 NTM – 4 years 2 Cycles/year soybean + (maize or sorghum or millet) 5–7% 2.5YR 4/6 37 6 57 NTM – 8 years 2 Cycles/year soybean + (maize or sorghum or millet) 3% 2.5YR 4/6 31 4 65 NTM – 12 years 2 Cycles/year soybean + (maize or sorghum or millet) <3% 2.5YR 3/6 25 9 66 a NTM: no-tillage mulch-based cropping systems. Please cite this article in press as: Neto, M.S., et al., Soil carbon stocks under no-tillage mulch-based cropping systems in the Brazilian Cerrado: An on-farm synchronic assessment. Soil Tillage Res. (2010), doi:10.1016/j.still.2010.07.010 G Model STILL-2815; No. of Pages 9 4 M.S. Neto et al. / Soil & Tillage Research xxx (2010) xxx–xxx 2.3. Soil sampling and analysis 2.3.1. Phase 2: General field survey In each field three geo-referenced points were chosen. Each point was considered as an individual observation without aiming at characterizing the whole field. Soil samples were taken at 0–5, 5–10 and 10–20 cm depth at each point for SOC determination. An additional composite sample, from four other points situated 2 m around the geo-referenced sampling point, was taken from the 0– 20 cm layer for determination of pH in water, clay, silt and sand contents. The soil samples were air-dried, homogenized and sieved through a 2 mm mesh. The pH was determined in a 1:2.5 (soil:water) solution. Soil texture was determined by densitometry after aggregate dispersion with hexametaphosphate and digestion of the organic material in H2O2. Fine and coarse sand components were separated by wet sieving (EMBRAPA, 1997). Soil organic C was determined by wet oxidation (Walkey and Black, 1934). Finally, undisturbed samples were taken at each geo-referenced sampling point for bulk density (0–5, 5–10 and 10–20 cm) by the core method (Blake and Hartge, 1986) using stainless steel cylinders of 5 cm height and 8.5 cm diameter (253.7 cm3) for the two first layers and 10 cm height and 8.5 cm diameter (567.5 cm3) for the third one. 2.3.2. Phase 3: Chronosequence Soil sampling on the chronosequence was carried out in three geo-referenced sub-areas (10 m  20 m) of each field, not necessarily the same from phase 2 but the most similar ones. In each sub-area 0–5, 5–10, 10–20, 20–30 cm samples were collected at six different points and analysed separately. All soil samples were collected with stainless steel cylinders. Two cylinders of 5 cm height and 8.5 cm diameter (253.7 cm3) were used for each upper layers 0–5 and 5–10 cm and one cylinder of 10 cm height and 8.5 cm diameter (567.5 cm3) for the layers 10–20 and 20–30 cm, resulting in similar amounts of soil for each layer sampled. The soil samples were air-dried, homogenized and passed through a 2 mm sieve to remove any visible plant material. A sub-sample from this 2-mm sieved soil was finely ground (<150 mm) using a stainless steel ball-mill grinder before analysis for organic C by dry combustion in a LECO1 CN-2000 analyser without any comparison with SOC measured during phase 2 because of the differences in sampling and analyzing methods. Soil organic C stocks were calculated from the SOC concentrations with the bulk density sampled for each layer (Bernoux et al., 1998). To correct for differences in bulk density along the NTM chronosequence, SOC stocks were estimated based on an equivalent soil mass-depth basis (Ellert et al., 2002) that is, as the total C content of the same weight of soil as that present to 30-cm depth of the situation under native Cerrado vegetation. Finally, only one sample of each soil horizon was taken at each field without repetition for the determination of 13C/12C and 15 N/14N isotopic composition, just to verify tendencies between the situations of different age in NTM. Total C and N concentrations and the isotopic composition (d13C e d15N) were determined by dry combustion in an oxidizing environment in a Carbo Erba1 EA-1110 analyzer. The gases produced were separated by gas chromatography and taken by continuous flux to a Finnigan Delta Plus1 mass spectrometer. 3. Results 3.1. Phase 1: Variability of farms and NTM cropping systems The farms in our survey were representative of the region having large land areas varying from 200 to more than 3000 ha. While crop production was the main activity in the region, on 70% Table 3 Types of NTM cropping systems identified through a survey on 50 farms in southwest Goiás state, Brazil, 2001. First commercial crop Second crop Area (ha) (%) Soybean Soybean Soybean Maize Maize Maize Othersa Maize/sorgum/millet Fallow Beans/coton/sunflower Fallow Maize/sorgum/millet Beans/coton/sunflower 14,961 6,899 1,587 4,110 2,898 2,046 873 33,374 44.8 20.7 4.8 12.3 8.7 6.1 2.6 100.0 a Others: beans, cotton and rice as commercial crop. of the farms some form of livestock production existed. The vast majority of the cultivated area in the three municipalities (91%) was under NTM management, with a number of fields that have been continuously under NTM for 14 years. No-tillage mulch-based cropping systems had typically two distinct crop cycles: a first crop grown during the rainy season from October to February, followed by a second crop grown from February to May. The various types of crop successions that were recorded in the survey are shown in Table 3. The region showed to be strongly specialized in soybean production. Soybean occupied around 70% of the total area planted in the first crop cycle of the year. The remaining 30% was almost exclusively under maize. In the second cycle, cereal crops (maize, sorghum and millet) were grown on about 54% of the area. Other crops such as cotton, beans, wheat and sunflower were only grown on 11% of the area. The remainder of the area (35%) was left as fallow during the second crop cycle. The two main types of NTM systems, representing close to 85% of the cultivated area in the region, were: (1) soybean or maize followed by a cereal crop (maize, sorghum or millet) (System A), accounting for 54% of the area; and (2) soybean or maize followed by a fallow period (System B) representing 33% of the area. All the other cropping systems combined represented only 13% of the cropped area in study region. System A is being used by the majority of the soybean producers. In this system, maize is only grown as the first crop from time to time (once every 5 years on average) to diversify the cropping system, since maize is economically less attractive than soybean. Management of a given crop (chemical fertilizer application rates, weed and disease control) was generally very similar between fields and even between farms. As can be seen from Table 4, the standard deviations of the doses of fertilizers, herbicides and insecticides were very small for both soybean and maize. In all cases, crop management was intensive with high levels of inputs, in general resulting in high grain yield with corresponding high levels of straw production. For the next phases of the study we selected only fields under System A, since it was the most practiced cropping system in the study area. Table 4 Crop management practices and grain yields for soybean and maize fields on 50 farms in southwest Goiás state, Brazil, 2001. 1 N application (kg ha ) P2O5 application (kg ha 1) K2O application (kg ha 1) Total amount of biocides applied (kg ha Total number of biocide applications Grain yields (t ha 1) 1 ) Soybeana Maize 8 (4) 70 (11) 69 (9) 6.0 (0.9) 5.6 (1.7) 3.62 (0.35) 112 (24) 78 (14) 70 (16) 7.8 (1.9) 3.9 (1.0) 8.16 (0.92) a Value represents mean for total sample n = 48 for soybean and n = 40 for maize, standard deviations are reported between brackets. Please cite this article in press as: Neto, M.S., et al., Soil carbon stocks under no-tillage mulch-based cropping systems in the Brazilian Cerrado: An on-farm synchronic assessment. Soil Tillage Res. (2010), doi:10.1016/j.still.2010.07.010 G Model STILL-2815; No. of Pages 9 5 M.S. Neto et al. / Soil & Tillage Research xxx (2010) xxx–xxx Table 5 Main physical and chemical characteristics under different land use practices in southwest Goiás state, Brazil. Attributes C content (g kg Site 1 ) Bulk density (g cm pH water Clay + silt (%) Fine sand (%) Coarse sand (%) a 3 ) Depth Natural Cerradoa (n = 8) Pasture (n = 2) CT fields (n = 18) NTM fields from 1 to 3 years (n = 24) NTM fields from 4 to 6 years (n = 24) NTM fields from 7 to 9 years (n = 48) NTM fields more than 10 years (n = 27) 0–5 cm 5–10 cm 10–20 cm Integrated value on 0–20 cm 36.0 21.1 19.7 25.6 23.4 18.9 18.7 20.3 18.6 17.7 15.6 17.3 19.3 17.7 15.9 17.7 22.5 19.0 17.1 19.5 22.9 19.3 17.8 20.0 24.9 20.1 18.7 21.2 0–5 cm 5–10 cm 10–20 cm Integrated value on 0–20 cm 0–20 cm 0–20 cm 0–20 cm 0–20 cm (5.8) (4.1) (2.5) (3.6) 0.97 1.08 1.10 1.05 (0.17) (0.15) (0.07) (0.12) 4.48 (0.31) 58.9 (7.9) 26.0 (5.7) 15.1 (2.5) (0.8) (0.5) (2.0) (1.1) 0.99 1.10 1.16 1.08 (0.02) (0.02) (0.03) (0.01) 5.80 (2.67) 77.1 (29.1) 14.3 (9.3) 8.6 (4.5) (3.2) (3.1) (3.4) (3.1) 1.12 1.20 1.25 1.19 (0.14) (0.11) (0.13) (0.12) 5.57 (0.27) 59.6 (13.3) 26.3 (8.9) 14.1 (6.2) (4.1) (3.1) (3.0) (3.2) 1.07 1.20 1.25 1.17 (0.14) (0.11) (0.08) (0.10) 5.42 (0.27) 57.3 (13.0) 28.4 (7.8) 14.3 (6.8) (3.3) (2.2) (1.5) (2.0) 0.98 1.14 1.19 1.10 (0.14) (0.11) (0.08) (0.10) 5.56 (0.24) 62.2 (8.8) 26.2 (7.0) 11.6 (3.3) (3.8) (3.1) (3.1) (3.2) 1.01 1.14 1.18 1.11 (0.17) (0.14) (0.14) (0.13) 5.57 (0.30) 64.6 (12.1) 23.1 (9.1) 11.7 (5.5) (3.7) (2.4) (1.5) (3.2) 0.94 1.08 1.13 1.05 (0.11) (0.09) (0.04) (0.05) 5.39 (0.37) 66.0 (7.1) 24.1 (6.6) 9.9 (2.7) Value represents mean and standard deviation between brackets; n = number of sampled points. 3.2. Phase 2: Variability of soil conditions and its impacts on SOC We identified 48 fields cultivated with System A for further characterization together with two sites under native vegetation and one site under pasture. Their soil characteristics are shown in Table 5. Soil pH in water was on average 5.4 with a fairly wide range (3.9–6.7). The lowest pH values were found under CE. They are characteristic for the Cerrado soils, while soils from cultivated fields have higher pH values due to lime amendments. The average clay + silt content was 626 g kg 1 soil, with values varying from 226 to 828 g kg 1. These results demonstrate that despite the fact that the fields had the same soil type and were located on a similar topography, soil texture was variable. Average SOC concentrations decreased with depth, being higher in the 0–5 cm layer and lower in the 10–20 cm layer. The highest SOC values for all depths were observed under CE, while the lowest values were found in the fields under CT. Soil organic carbon concentrations had high variation coefficients (23%, 16% and 17% for the 0–5, 5–10 and 10– 20 cm depth layers, respectively). Only a light non significant tendency of increase in the upper art of the profile (0–10 cm) of SOC content was observed with time of NTM application (Table 5). Because of the strong aggregation processes in the oxisols of the Cerrado, we preferred to use the clay + silt content for the estimation of the fine-texture fraction of the soil rather than the clay content alone (Reatto et al., 1998). As expected, SOC concentration (0–20 cm) increased significantly with soil clay + silt content (Fig. 2a). There was no need to normalize the data despite the fewer points in the sandy texture range compared to the more clayey one. On the other hand, our data from fields within the 500–700 g kg 1 clay + silt range showed no significant relationship between texture and SOC concentration (r2 = 0.07). It can be noticed from Fig. 2a that in the 500–700 g kg 1 clay + silt range three specific points showed very low SOC concentrations. These points corresponded to three fields under CT management. We observed that these fields showed very strong soil erosion losses which partly explained the low SOC levels. These three specific points did not have a strong weight; the correlation between SOC and texture in the 500–700 g kg 1 clay + silt range was even weaker (r2 = 0.01) without them. For fields with a clay + silt content of between 500 and 700 g kg 1 (without a significant correlation between texture and organic SOC), age of NTM and soil C concentration were significantly correlated with an average increase of 0.2 g C kg soil 1 year 1 (Fig. 2b). On the basis of these results, we focused on the sub-sample with a clay + silt content in the range of 500–700 g kg 1 for our final NTM chronosequence. Thus, taking into account the main cropping system of the region (System A), the location in the landscape (plateau), soil type (red yellow latosols), and texture (500–700 g kg 1 of clay + silt), we selected five fields of different age under NTM (including zero years, i.e. conventional tillage), Fig. 2. Relationships between (a) soil organic carbon content and clay + silt content (200–900 g kg 1 and 500 and 700 g kg 1 of clay + silt content ranges) and, (b) soil organic carbon content and years under no-tillage mulch-based management in southwest Goiás state – Brazil. Please cite this article in press as: Neto, M.S., et al., Soil carbon stocks under no-tillage mulch-based cropping systems in the Brazilian Cerrado: An on-farm synchronic assessment. Soil Tillage Res. (2010), doi:10.1016/j.still.2010.07.010 G Model STILL-2815; No. of Pages 9 6 M.S. Neto et al. / Soil & Tillage Research xxx (2010) xxx–xxx Fig. 3. Soil organic carbon stocks (kg m 2) in the top 0–30 cm soil layer under different land-use systems and from fields with different years under NTM management in southwest Goiás State, Brazil. (PA: pasture; CT: conventional tillage; NTM: no-tillage mulch-based systems). Each bar represents the mean for each field (n = 18  s.d.). Treatments with the same letter did not differ in a T-test (p < 0.05). along with three reference areas: two Cerrado vegetation (CE) locations and a permanent grassland pasture (PA) for the next phase of our study. ranged between 4.2 and 6.7 kg m 2. The largest SOC stocks (p < 0.05) were found in areas under CE and 12-years old NTM, and the lowest values were found under PA, CT and 1 to 4-year-old NTM fields. Using CE as the baseline, SOC stocks were about 20% lower after 15 years of pasture, and 24% lower after 23 years of CT. On the contrary, in the areas under NTM, SOC increased linearly (r2 = 0.97 excluding CT treatment) with time, with an annual accumulation rate of 0.19 kg C m 2. After 12 years of NTM, SOC stocks returned to the levels of those under natural vegetation, with no significant difference between SOC stocks under CE and 12-year-old NTM. d13C values observed for the superficial ‘‘A’’ soil horizon varied between land use systems (Table 6). The most negative values were observed under CE ( 25%), that has typically a high proportion of C3 plants (Bernoux et al., 1998). In the cultivated fields, where the d13C values were derived from a mixture of C3 and C4 vegetation, the isotopic signature was closest to the C4 vegetation signal, presumably due to the higher contribution of residues from cereals crops. With increasing soil depth higher d13C values were found. Values of d15N were similar between the land use systems and tended to increase in the soil profile (5%) from the ‘‘A’’ soil horizon to the ‘‘B2’’ soil horizon. 3.3. Phase 3: Potential SOC storage under NTM 4. Discussion The SOC stocks in the 0–30 cm upper layer for all the fields of the chronosequence are shown in Fig. 3. Soil organic C stocks Table 6 Isotopic composition 13C/12C and 15N/14N (%) of the soil horizons for different land use systems and a chronosequence of fields of different years under NTMa in southwest Goiás state – Brazil. d13C (%) d15N (%) Site Horizons Depth (cm) Cerrado A A AB B1 B2 0–13 13–38 38–91 >91 25.37 24.15 24.39 23.74 5.06 7.36 9.79 10.63 Cerrado B A AB B1 B2 0–6 6–36 36–79 >79 24.21 19.73 17.65 17.84 4.84 7.28 10.10 10.06 Pasture A AB B1 B2 0–27 27–36 36–76 >76 15.78 13.82 13.83 15.92 7.22 8.87 11.64 10.94 Conventional tillage – 23 years A AB B1 B2 0–13 13–24 24–75 >75 14.43 12.46 12.01 12.33 6.46 8.70 10.76 12.97 NTMa – 1 year A AB B1 B2 0–17 17–32 32–62 >62 17.05 14.05 13.08 13.62 7.39 10.34 10.91 12.50 NTM – 4 years A AB B1 B2 0–15 15–28 28–61 >61 16.06 13.55 12.32 13.75 6.42 8.90 10.18 11.98 NTM – 8 years A AB B1 B2 0–16 16–29 29–90 >90 16.36 13.55 12.76 14.22 6.37 8.28 12.03 11.90 NTM – 12 years A AB B1 B2 0–11 11–36 36–72 >72 15.83 13.86 12.89 12.58 6.95 8.04 9.83 11.37 a NTM: no-tillage mulch-based cropping systems. Our study confirmed the fact that changing the land use, i.e. from native vegetation to conventional mono-cropping with tillage or from soybean mono-cropping to NTM cropping with two crops per year has a significant impact on SOC storage in the Brazilian Cerrado conditions (Bustamante et al., 2006; Corbeels et al., 2006). The observed SOC stocks under the different land-use systems (Fig. 3) were comparable with values reported in other studies in the Brazilian Cerrado (Corazza et al., 1999; Roscoe and Buurman, 2003; Leite et al., 2004; Silva et al., 2004; Wilcke and Lilienfein, 2004; Bayer et al., 2006; Bernoux et al., 2006; Bustamante et al., 2006; Carvalho et al., 2009). Bernoux et al. (2001) reported that the average C stock for the 0–30 cm layer of soils of the Goiás state under native vegetation was 4.1  0.2 kg C m 2. However, this value represents an average value for all types of soil and native vegetation in this region. Recent studies show that C stocks in oxisols under Cerrado vegetation range from 3.8 to 6 kg C m 2 for the 0–20 cm layer and from 3.5 to 9.5 kg C m 2 for the 0–30 cm soil layer (Salton, 2005; Bayer et al., 2006; Dieckow et al., 2009; Jantalia et al., 2007; Leite et al., 2009; Marchão et al., 2009; Maia et al., 2010). The C stocks of between 6 and 7 kg C m 2 (0–30 cm) observed in our case under Cerrado vegetation (Fig. 3) correspond to the higher values reported in the literature, probably because of the clayey texture of these situations. In the same studies C stocks under pasture are reported to vary between 4.1 and 6.6 kg C m 2 for the 0–20 cm soil layer, mainly depending on fertility management. In our study the C stock of around 5 kg C m 2 (0– 30 cm) under pasture is relatively low in such a clayey oxisol, probably because of the low productivity of this pasture as a result of the extensive management without fertilization, which is a common practice in the region (Marchão et al., 2009). Finally, in most of the published studies, C stocks are lowest after several years of cropping under CT that involves tillage with a harrow disk, ranging between 1.9 and 5.4 kg C m 2 (0–20 cm). The C stock of around 5 kg C m 2 (0– 30 cm) under CT in our study is the higher end of the published results, probably because of the clayey texture of the soils and a reduced erosion impact. All the fields currently under NTM in our study had been cultivated for more than 10 years under CT before their conversion to NTM. Other studies have shown that 10 years is a reasonable time period for SOC levels to stabilize under CT following conversion from CE (Séguy et al., 1996; Resck et al., 2000). Please cite this article in press as: Neto, M.S., et al., Soil carbon stocks under no-tillage mulch-based cropping systems in the Brazilian Cerrado: An on-farm synchronic assessment. Soil Tillage Res. (2010), doi:10.1016/j.still.2010.07.010 G Model STILL-2815; No. of Pages 9 M.S. Neto et al. / Soil & Tillage Research xxx (2010) xxx–xxx However, it was not possible to determine whether the selected CT field in our chronosequence had a similar SOC content than the initial (after >10 years under CT) SOC contents of the NTM fields. For example, the 1 and 4-years old NTM fields (NTM-1 and NTM-4) showed smaller or similar SOC stocks as the CT field. Through discussions with the respective owners of the fields, we found out that both fields had suffered from severe erosion in the past. Therefore, it is possible that their initial SOC stocks (under CT) were lower than that of the reference CT field in our chronosequence. After 12 years of NTM practice, soils had almost recovered the SOC levels as found under CE (Fig. 3). The SOC accumulation rates under NTM that were observed in this study are of the highest reported in the Cerrado region. Reviews on impact of land use changes on C stocks in the Cerrado region (Bernoux et al., 2006; Bustamante et al., 2006; Batlle-Bayer et al., 2010) reported values of SOC storage from 0.013 to 0.191 kg C m 2 when converting from conventional to no-tillage cropping systems. The high rate observed in our study was probably due to a combination of factors such as, the relatively high clay + silt contents of the soil, the complete absence of soil disturbance, and the high productivity of the NTM cropping systems with high inputs of C to the soil (Sá et al., 2001; Lovato et al., 2004; Sisti et al., 2004; Corbeels et al., 2006). Our study showed that, despite their quite recent appearance in the region, intensive NTM cropping systems have been widely adopted by large-scale, mechanized farmers in the Cerrado (Table 3). The introduction of these cropping systems had important consequences for farm organization and crop management (Séguy et al., 1996). Their adoption was facilitated by the fact that soybean is the main crop grown in the region and that it is relatively simple to produce it under no-tillage management (Yamada, 2005). The drivers behind the dominance of soybean production are the good access to markets and credits, the high profitability and the existence of high productive short-cycle cultivars that can be grown during the first crop cycle of the year (Bertrand et al., 2004). There is little diversity in the region with regard to the type of crop successions practiced by the farmers. The dominant cropping system is based on soybean or maize followed by a cereal crop (maize, sorghum or millet) and covers almost half of the study area (Table 3). Crop management in the region is intensive (Table 4), with high inputs of fertilizers and pesticides according to the model for soybean production in the region (Bertrand et al., 2004). The intensive NTM cropping systems with two crops per year are very productive, both in terms of grains and total plant biomass production. It has been estimated in other studies that total biomass production increases from 6 to 8 Mg ha 1 year 1 in a soybean-fallow cropping system to 15 Mg ha 1 year 1 in a soybean-cereal NTM cropping system (Scopel et al., 2004; Maltas et al., 2009). The higher biomass production under NTM makes these new cropping systems much more efficient in terms of SOC storage (Corbeels et al., 2006). We think that the existing NTM cropping systems could be more diversified, and perhaps even more productive, if cropping and livestock activities were integrated at the farm level. Such integration usually leads farmers to produce fodder plants as cover crops, thus diversifying the second-cycle crops (Garcia-Préchac et al., 2004). In our study the effect of the second-cycle crop on SOC levels is corroborated by the tendency of d13C increase in the topsoil under NTM since the main source of C4 vegetation in the NTM cropping systems are the second-cycle cereal crops (Bernoux et al., 1998). In a previous study, Scopel et al. (2004) showed that second-cycle cereal crops produced on average 50–75% of the total biomass in NTM systems. Our results also show that there is a significant signature of C4 vegetation in the case of CT, which can be explained by the higher frequency of maize grown under CT compared to NTM in rotation with soybean (every two years under CT compared 7 to every five years under NTM as reported by the farmers). The ratio of C3–C4 crop residues under CT was about the same as under NTM, but with far lower quantities of C inputs to the soil in the CT systems, because only one crop per year is grown (Table 6). The higher values of d13C found in the deeper soil layers in all fields (Table 6), can be explained by changes in the vegetation (C3–C4– C3) that occurred after the last glacial period, when the climate became drier (Desjardins et al., 1996; Salgado-Labouriau et al., 1997; Pessenda et al., 1998; Freitas et al., 2001). The low diversity in cropping systems was an advantage for selecting a homogeneous chronosequence of fields of different age. However, the soil characteristics of the farmers’ fields in the region appeared to be quite variable. Results from our study showed that, although the fields for the NTM chronosequence were selected on the basis of similar environmental conditions (same soil type, same topography and same rainfall), important differences in soil properties that may affect SOC dynamics occurred between the selected fields. Soil pH and texture in particular were highly variable, and the latter was strongly correlated with SOC. The relationship between clay + silt and SOC is well documented in the literature and attributed to the formation of organic–mineral complexes (Feller and Beare, 1997; Zinn et al., 2005). For this study it was very important to precisely characterize this interaction since i) many important mechanisms are related to it: soil biology, physical state and porosity of the soil, and chemical fertility (Kladivko, 1994; Fischer et al., 2002; Yamada, 2005; Singh et al., 2007), ii) we needed to select a specific textural range to efficiently study the long-term impact of NTM systems. The numerous fields sampled in phase two of the study enabled us to identify a sub-sample of fields with a textural range (500– 700 g kg 1 clay + silt) for which the relation between soil texture and SOC was not significant. This study tested a methodology for selecting a homogeneous chronosequence of fields of different time under NTM in order to assess the SOC storage potential. The methodology consists of three phases: (1) a farm survey; (2) a field survey and (3) a chronosequence characterization. It enables in this way a progressive stratification of the diversity of conditions under which NTM systems are practiced by farmers in the region, prior to the selection of the fields for the chronosequence and the soil sampling for SOC stock determination. We consider these three phases as very useful to avoid heterogeneous conditions (management, climate, soil) that would misrepresent the effect of NTM management on SOC storage over the years. For example, we were able to identify and eliminate a zone in the study area (that corresponded with a somewhat drier climate) where most farmers did not always grow a second-cycle crop. At the same time, our methodology clearly shows that the obtained results are valid for well defined situations of NTM management, climate and soil. Therefore, extrapolation of the results has to be done with care. From our study we can assume that the contrasting results on NTM effects on soil C storage in the Cerrado found in the literature (Bernoux et al., 2006; Bustamante et al., 2006) are to a large extent due the variability in the conditions under which these systems are practiced. Despite all the methodological precautions to select the most homogeneous chronosequence, there may still have been a residual field to field variability in the initial (prior to NTM management) soil conditions, that is hard to capture. For example, it was only after the somewhat surprising results on SOC stocks on the 1 and 4-year-old NTM fields that we investigated further and found that these fields had probably encountered more C losses through erosion than the other fields during their previous CT period. As with paired plots where there is no real repetition of the treatments (Blanco-Canqui and Lal, 2008), the non-homogeneity of the initial soil conditions can create a serious bias in synchronic Please cite this article in press as: Neto, M.S., et al., Soil carbon stocks under no-tillage mulch-based cropping systems in the Brazilian Cerrado: An on-farm synchronic assessment. Soil Tillage Res. (2010), doi:10.1016/j.still.2010.07.010 G Model STILL-2815; No. of Pages 9 8 M.S. Neto et al. / Soil & Tillage Research xxx (2010) xxx–xxx studies. In addition, based on other studies, we assumed that SOC stocks had stabilized after more than 10 years of CT. This seemed realistic but has not been demonstrated in our case. This means that the SOC storage rate (0.19 kg of C m 2 year 1) found in this study may have been overestimated due to the low initial stocks in NTM-1 and NTM-4. Probably, this bias could have been partially avoided if more precise historical information had been collected on each field and even more homogeneous topographic conditions had been chosen. Complementary diachronic studies on the fields of the same chronosequence should allow to draw more conclusive statements on the capacity of soil C storage of NTM systems in the Cerrado. 5. Conclusion Our study clearly showed that in the Brazilian Cerrado, a change in land use from natural savannah vegetation to conventional agriculture induces a total loss of about 1.5 kg of C m 2 after more than 10 years of continuous cropping. Today, conventional production systems that are based on tillage have been abandoned and NTM cropping systems are widely adopted. Those systems are based on growing two crops per year with little diversity in crop rotation and crop management. Under such NTM systems with intensive management and high biomass production, SOC increased with 0.19 kg m 2 year 1and after 12 years of NTM soil C stocks reached the levels of that under natural Cerrado. These results suggest that NTM cropping systems may be efficient in controlling SOM degradation processes and in mitigating partly climate change. To obtain such results we suggested a methodology for the assessment of the soil C storage potential using an onfarm synchronic approach. A 3-phases approach – farm survey, field survey and final chronosequence characterization – aims at identifying and excluding non desirable sources of heterogeneity. This methodology can be applied for the impact assessment of other land management or land use changes on soil C storage potential. Acknowledgements This work was supported by the Fond Français pour ĺEnvironnement Mondial (FFEM-Agroécologie), Conselho Nacional de Desenvolvimento Cientı́fico e Tecnológico (CNPq), Fundação Agrisus (Project 116/04), Fundação de Amparo a Pesquisa do Estado de São Paulo (Fapesp – Project 2004/15538-7), French Ministry of Foreign Affairs (MAE) and The Global Environmental Facility (GEF – Project GFL-2740-02-43-81). We also thank the farmers from the Grupo de apoio à pesquisa de Rio Verde (GAPES) for their collaboration. Special thanks go to Dr. Plı́nio Carmargo, Dr. Marcelo Moreira and Sérgio Luiz de Jesus for helping with the soil analyses and Sophie Primot and Nélia Doucene for participating in the fieldwork and with the data analysis. References Alvarez, C.R., Alvarez, R., 2000. Short-term effects of tillage systems on active soil microbial biomass. Biol. Fertil. Soils 31 (2), 157–161. Alvarez, R., Alvarez, C.R., Lorenzo, G., 2001. 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