Stable Carbon Isotope Depth Profiles and Soil Organic Carbon Dynamics in the Lower Mississippi Basin

Authors

Jonathan G. Wynn, US Geological SurveyFollow
Jennifer W. Harden, US Geological Survey
Terry L. Fries, US Geological Survey

Document Type

Article

Publication Date

3-2006

Keywords

Soil organic carbon, Carbon isotope, Carbon cycle, Decomposition, Depth

Digital Object Identifier (DOI)

https://doi.org/10.1016/j.geoderma.2005.03.005

Abstract

Analysis of depth trends of ¹³C abundance in soil organic matter and of ¹³C 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 ¹³C abundance from cropland and control pairs of soils in the lower Mississippi Basin, as well as the ¹³C abundance of soil-respired CO₂ produced during approximately 1-year soil incubation, to determine the role of several candidate processes on the ¹³C depth profile of soil organic matter. Depth profiles of ¹³C 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 ¹³C in decomposing soil due to kinetic fractionation during decomposition. Laboratory incubations showed that initially respired CO₂ had a relatively constant ¹³C content, despite large differences in the ¹³C content of bulk soil organic matter. Initially respired CO₂ was consistently ¹³C-depleted with respect to bulk soil and became increasingly ¹³C-depleted during 1-year, consistent with the hypothesis of accumulation of ¹³C in the products of microbial decomposition, but showing increasing decomposition of ¹³C-depleted stable organic components during decomposition without input of fresh biomass. We use the difference between ¹³C / ¹²C ratios (calculated as δ-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 ¹⁴C 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.

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No

Citation / Publisher Attribution

Geoderma, v. 131, issues 1-2, p. 89-109

Scholar Commons Citation

Wynn, Jonathan G.; Harden, Jennifer W.; and Fries, Terry L., "Stable Carbon Isotope Depth Profiles and Soil Organic Carbon Dynamics in the Lower Mississippi Basin" (2006). School of Geosciences Faculty and Staff Publications. 228.
https://digitalcommons.usf.edu/geo_facpub/228