Soil organic carbon dynamics under different tillage systems in rotations with perennial pastures

Physical fractionation and 13C determinations are useful techniques for soil organic carbon (SOC) dynamics studies. Changes in SOC content, distribution and origin were assessed after 9.5-year crop-perennial (C3 species) rotation on a Uruguayan Mollisol under conventional tillage (CT) and no-tillage (NT). Soil samples were collected at depths of 0–6, 6–12 and 12–18 cm in 1994 and 2003. Determinations were made of total SOC, particulate organic matter C (POM-C) and mineral-associated organic matter C (MAOM-C). In addition, 13C determinations were made on the total sample and the different particle size fractions. None of the studied variables were affected significantly by the tillage system. SOC levels in 2003 did not differ significantly from those of 1994 at any of the studied depths. However, changes were found in fraction distribution. Within 0–18 cm of the soil surface, POM-C decreased by 63%, whereas MAOM-C did not vary significantly. After 9.5 years, only 14.5% of SOC within 0–18 cm of the soil surface was young SOC. The largest proportion was incorporated within 0–6 cm of the soil surface and in the coarsest physical fractions of organic matter. Only 17% of the estimated C input from crops for the study period was retained by the topsoil. The estimated half-life of SOC within the upper 18 cm of soil was 28 years. Within this layer, the C half-life varied from less than 5 years for POM-C to more than 400 years for MAOM-C. These results suggest that agricultural rotation systems including perennial pastures are capable of maintaining SOC levels even under CT. However, C cycling and other ecosystem processes may be altered due to the significant loss of labile organic matter. The use of 13C analysis enabled the estimation of parameters relevant to the modeling of SOC dynamics.