Stable carbon isotope depth profiles and soil organic carbon dynamics in the lower Mississippi Basin

Analysis of depth trends of C-13 abundance in soil organic matter and of C-13 abundance from soil-respired CO2 provides useful indications of the dynamics of the terrestrial carbon cycle and of paleoecological change. We measured depth trends of C-13 abundance from cropland and control pairs of soils in the lower Mississippi Basin, as well as the C-13 abundance of soil-respired CO, produced during approximately 1-year soil incubation, to determine the role of several candidate processes on the C-13 depth profile of soil organic matter. Depth profiles of C-13 from uncultivated control soils show a strong relationship between the natural logarithm of soil organic carbon concentration and its isotopic composition, consistent with a model Rayleigh distillation of C-13 in decomposing soil due to kinetic fractionation during decomposition. Laboratory incubations showed that initially respired CO, had a relatively constant C-13 content, despite large differences in the C-13 content of bulk soil organic matter. Initially respired CO2 was consistently C-13-depleted with respect to bulk soil and became increasingly C-13-depleted during I-year, consistent with the hypothesis of accumulation of C-13 in the products of microbial decomposition, but showing increasing decomposition of C-13-depleted stable organic components during decomposition without input of fresh biomass. We use the difference between C-13/C-12 ratios (calculated as delta-values) between respired CO2 and bulk soil organic carbon as an index of the degree of decomposition of soil, showing trends which are consistent with trends of C-14 activity, and with results of a two-pooled kinetic decomposition rate model describing CO2 production data recorded during 1 year of incubation. We also observed inconsistencies with the Rayleigh distillation model in paired cropland soils and reasons for these inconsistencies are discussed. (C) 2005 Elsevier B.V. All rights reserved.