No abstract

Conservation agriculture can provide a low-cost competitive option to mitigate global warming with reduction or elimination of soil tillage and increase soil organic carbon (SOC). Most studies have evaluated the impact of zero till (ZT) only on surface soil layers (down to 30 cm), and few studies have been performed on the potential for C accumulation in deeper layers (0-100 cm) of tropical and subtropical soils. In order to determine whether the change from conventional tillage (CT) to ZT has induced a net gain in SOC, three long-term experiments (15-26 years) on free-draining Ferralsols in the subtropical region of South Brazil were sampled and the SOC stocks to 30 and 100 cm calculated on an equivalent soil mass basis. In rotations containing intercropped or cover-crop legumes, there were significant accumulations of SOC in ZT soils varying from 5 to 8 Mg ha À1 in comparison with CT management, equivalent to annual soil C accumulation rates of between 0.04 and 0.88 Mg ha À1 . However, the potential for soil C accumulation was considerably increased (varying from 0.48 to 1.53 Mg ha À1 yr À1 ) when considering the soil profile down to 100 cm depth. On average the estimate of soil C accumulation to 100 cm depth was 59% greater than that for soil C accumulated to 30 cm. These findings suggest that increasing sampling depth from 30 cm (as presently recommended by the IPCC) to 100 cm, may increase substantially the estimates of potential CO 2 mitigation induced by the change from CT to ZT on the free-draining Ferralsols of the tropics and subtropics. It was evident that that legumes which contributed a net input of biologically fixed N played an important role in promoting soil C accumulation in these soils under ZT, perhaps due to a slow-release of N from decaying surface residues/roots which favored maize root growth.

No abstract

The area under no-till (NT) in Brazil reached 22 million ha in 2004-2005, of which approximately 45% was located in the southern states. From the 1970s to the mid-1980s, this region was a source of carbon dioxide to the atmosphere due to decrease of soil carbon (C) stocks and high consumption of fuel by intensive tillage. Since then, NT has partially restored the soil C lost and reduced the consumption of fossil fuels. To assess the potential of C accumulation in NT soils, four long-term experiments (7-19 yr) in subtropical soils (Paleudult, Paleudalf, and Hapludox) varying in soil texture (87-760 g kg(-1) of clay) in agroecologic southern Brazil zones (central region, northwest basaltic plateau in Rio Grande Sul, and west basaltic plateau in Santa Catarina) and with different cropping systems (soybean and maize) were investigated. The lability of soil organic matter (SOM) was calculated as the ratio of total organic carbon (TOC) to particulate organic carbon (POC), and the role of physical protection on stability of SOM was evaluated. In general, TOC and POC stocks in native grass correlated closely with clay content. Conversely, there was no clear effect of soil texture on C accumulation rates in NT soils, which ranged from 0.12 to 0.59 Mg ha(-1) yr(-1). The C accumulation was higher in NT than in conventional-till (CT) soils. The legume cover crops pigeon pea [Cajanus cajan (L.) Millsp] and velvet beans (Stizolobium cinereum Piper & Tracy) in NT maize cropping systems had the highest C accumulation rates (0.38-0.59 Mg ha(-1) yr(-1)). The intensive cropping systems also were effective in increasing the C accumulation rates in NT soils (0.25-0.34 Mg ha(-1) yr(-1)) when compared to the double-crop system used by farmers. These results stress the role of N fixation in improving the tropical and subtropical cropping systems. The physical protection of SOM within soil aggregates was an important mechanism of C accumulation in the sandy clay loam Paleudult under NT. The cropping system and NT effects on C stocks were attributed to an increase in the lability of SOM, as evidenced by the higher POC to TOC ratio, which is very important to C and energy flux through the soil.

We examined the influence of land use change, tillage system and soil texture on organic carbon (C) stocks and on organic matter composition of tropical and subtropical soils from Brazil at four long-term experiments (11-25 years) based on fine-and coarse-textured soils. Soil samples were collected from the 0-5, 5-10 and 10-20 cm layers of conventional tillage (CT) and no-till (NT) plots, and of the adjoining soil under native vegetation (NV) of Cerrado (tropical) or grassland (subtropical). Conversion of NV to CT resulted in losses of 7-29% of the original C stock of 0-20 cm; conversion to NT increased this C stock by 0-12% compared with CT. Organic matter composition of the 0-5 cm layer, assessed by solid state CPMAS-13 C-NMR, ESR and laser induced fluorescence spectroscopies, was affected by land use and tillage systems. Conversion of NV to CT decreased O-alkyl and increased aromatic, carbonyl, aromatic/O-alkyl ratio, free radicals concentration and fluorescence signal. The opposite trend was observed when NT replaced CT. The relative losses and gains of C and qualitative changes resulting from land use and tillage were less evident in fine-than in coarse-textured soils, suggesting a greater resistance and a smaller resilience of fine-compared with coarse-textured soils. The direct relation between increase in C stock and increase in potentially labile moieties (e.g. O-alkyl) and the decrease in more recalcitrant moieties (e.g. aromatics) in NT soils suggests that spatial inaccessibility by aggregates is playing a major role, compared with selective preservation, in promoting C accumulation in NT soils.