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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.

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RESUMO O C fotossintetizado adicionado ao solo pelos resíduos vegetais (C-resíduo), as emissões de C na forma de dióxido de carbono (C-CO

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.