How does no-till deliver carbon stabilization and saturation in highly weathered soils?

Research data on the mechanisms of C stabilization and saturation affected by soil management systems in highly weathered soils remain scarce. Past studies have revealed the fundamental role of aggregation promoted by soil conservation practices in the physical protection of C fractions. This study is based on the hypothesis that the increased physical C protection provides sufficient time to strengthen the interaction between C fractions and soil minerals, as being the pathway for C stabilization and accumulation in highly weathered soils. Thus, the objectives of this study were to: i) evaluate the C stocks including labile and mineral-associated C fractions in soil under conventional (CT), no-till (NT) and native vegetation (NV), and, ii) assess the C saturation level in different C fractions through the use of contrasting mathematical models of C accumulation. Soil samples were collected (0-100 cm depth) from agroecosystems established in tropical (Lucas do Rio Verde) and subtropical (Ponta Grossa and Londrina) regions of Brazil. The data show that all C fractions were affected by soil management systems. However, the impact was more pronounced with the labile C fractions than with the mineral associated C fractions. The depletion of C stock of labile fractions in the 0-5 cm layer upon conversion of NV to CT accounted for 86, 89 and 72% of total C in soil of Ponta Grossa, Londrina and Lucas do Rio Verde, respectively. On the other hand, compared to CT, restoration of 89, 15 and 12% of these labile fractions was observed at these respective sites with adoption of NT. The mineral-associated C fraction was the best fit to a C saturation model at all sites. The estimated C saturation level for this fraction was 98.1, 60.2 and 39.1 g C kg(-1) silt + clay at the Ponta Grossa, Londrina and Lucas do Rio Verde sites, respectively, which is still far from the current C content. Thus, the long-term use of NT might be the pathway for physical protection of the labile C fractions as well as strong organo-mineral associations. Together, these processes contribute to C stabilization and accumulation in highly weathered soils.