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

Abstract

Listen

Translate article

Analysis of depth trends of 13 C abundance in soil organic matter and of 13 C abundance from soil-respired CO 2 provides useful indications of the dynamics of the terrestrial carbon cycle and of paleoecological change. We measured depth trends of 13 C abundance from cropland and control pairs of soils in the lower Mississippi Basin, as well as the 13 C abundance of soil-respired CO 2 produced during approximately 1-year soil incubation, to determine the role of several candidate processes on the 13 C depth profile of soil organic matter. Depth profiles of 13 C 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 13 C in decomposing soil due to kinetic fractionation during decomposition. Laboratory incubations showed that initially respired CO 2 had a relatively constant 13 C content, despite large differences in the 13 C content of bulk soil organic matter. Initially respired CO 2 was consistently 13 C-depleted with respect to bulk soil and became increasingly 13 C-depleted during 1-year, consistent with the hypothesis of accumulation of 13 C in the products of microbial decomposition, but showing increasing decomposition of 13 C-depleted stable organic components during decomposition without input of fresh biomass. We use the difference between 13 C / 12 C ratios (calculated as δ -values) between respired CO 2 and bulk soil organic carbon as an index of the degree of decomposition of soil, showing trends which are consistent with trends of 14 C activity, and with results of a two-pooled kinetic decomposition rate model describing CO 2 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.