Dynamics of 13C-labeled mustard litter (Sinapis alba) in particle-size and aggregate fractions in an agricultural cropland with high- and low-yield areas

The application of C-13-labeled litter enables to study decomposition processes as well as the allocation of litter-derived carbon to different soil C pools. (13)Carbon-labeled mustard litter was used in order to compare decomposition processes in an agricultural cropland with high-yield (HY) and low-yield (LY) areas, the latter being characterized by a finer texture and a lower organic-C (OC) content. After tracer application, C-13 concentrations were monitored in topsoil samples in particulate organic matter (POM) and in fine mineral fractions (silt- and clay-sized fractions). After 568 d, approximately 5% and 10% of the initial C-13 amount were found in POM fractions of LY and HY areas, respectively. Higher amounts were found in POM occluded in aggregates than in free POM. Medium-term (0.5-2 y) storage of the initial C-13 in fine silt- and clay-sized fractions amounts to 10% in HY and LY soils, with faster enrichment but also faster disappearance of the C-13 signal from LY soils. Amounts of 80%,90% of the added C-13 were mineralized or leached in the observed period. Decomposition of free POM was faster in HY than in LY areas during the first year, but the remaining C-13 amounts in occluded-POM fractions were higher in HY soils after 568 d. High-yield and low-yield areas showed different C-13 dynamics in fine mineral fractions. In LY soils, 13C amounts and concentrations in mineral-associated fractions increased within 160 d after application and decreased in the following time period. In HY areas, a significant increase in C-13 amounts did not occur until after 568 d. The results indicate initially faster decomposition processes in HY than in LY areas due to different soil conditions, such as soil texture and water regime. The higher silt and clay contents of LY areas seem to promote a faster aggregate formation and turnover, leading to a closer contact between POM and mineral surfaces in this area. This favors the OC storage in fine mineral fractions in the medium term. Lower aggregate formation and turnover in the coarser textured HY soil leads to a delayed C stabilization in silt- and clay-sized fractions.