Decadal cycling within long-lived carbon pools revealed by dual isotopic analysis of mineral-associated soil organic matter
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Long-lived soil organic matter (SOM) pools are critical for the global carbon (C) cycle, but challenges in isolating such pools have inhibited understanding of their dynamics. We physically isolated particulate (>53 μm), silt-, and clay-sized organic matter from soils collected over two decades from a perennial C3 grassland established on long-term agricultural soil with a predominantly C4 isotopic signature. Silt- and clay-sized fractions were then subjected to a sequential chemical fractionation (acid hydrolysis followed by peroxide oxidation) to isolate long-lived C pools. We quantified 14C and the natural 13C isotopic label in the resulting fractions to identify and evaluate pools responsible for long-lived SOM. After removal of particulate organic matter (~14% of bulk soil C) sequential chemical treatment removed 80% of mineral-associated C. In all mineral-associated fractions, at least 55% of C4-derived C was retained 32 years after the switch to C3 inputs. However, C3–C increased substantially beginning ~25 years after the switch. Radiocarbon-based turnover times ranged from roughly 1200–3000 years for chemically resistant mineral-associated pools, although some pools turned over faster under C3 grassland than in a reference agricultural field, indicating that new material had entered some pools as early as 14 years after the vegetation switch. These findings provide further evidence that SOM chemistry does not always reflect SOM longevity and resistance to microbial decomposition. Even measureable SOM fractions that have extremely long mean turnover times (>1500 years) can have a substantial component that is dynamic over much shorter timescales.
KeywordsΔ14C C4–C3 switch Recalcitrance Acid hydrolysis Peroxide oxidation
SLO was supported by a Department of Energy Global Change Education Program Graduate Research Environmental Fellowship. This work was supported by the US Department of Energy, Office of Science, Office of Biological and Environmental Research, Climate and Environmental Sciences Division under contract DE-AC02-06CH11357 to Argonne National Laboratory and a grant from the National Science Foundation to MAG-M. A portion of this work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. We thank Fermilab personnel who established and maintained the site. We are grateful to a number of interns and technicians for help collecting soil samples and to Kelly Moran at Argonne and Mannette Sandor at the UIC stable isotope lab for assistance in the lab. We also acknowledge Colleen Iversen and two anonymous reviewers for helpful comments on earlier drafts of this manuscript.
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