Carbon isotopes for estimating soil decomposition and physical mixing in well‐drained forest soils

Abstract

[1] Carbon isotopes are applied to estimate soil decomposition and physical mixing in well-drained forest soils by coupling new isotope and soil organic carbon (SOC) data with literature meta-analysis and carbon isotope mass balance modeling. New soil data results are presented for old-and second-growth forests in Southern Appalachia, USA and the Blue Mountains, Australia. The soils exhibit a SOC decrease and δ 13C increase with depth. The regressed gradient, termed β, of δ 13 C and the logarithm of SOC with depth in the soil column ranged from À1.09 to À1.65 for the measured soils. Twenty-four soils from 11 published studies across a range of cool temperate to tropical forest soils are used to show that β is dependent upon mean annual temperature (MAT) alone as well as mean annual temperature, mean annual precipitation, and soil texture, thus connecting the natural (nonlabeled) carbon isotope signature to the soil factors controlling soil decomposition and physical mixing. Carbon elemental and isotopic mass balance modeling of multiple SOC pools and multiple soil depths suggest that rates of decomposition and mixing are of the same order of magnitude for turnover in the studied forest soils. The results support the hypothesis that a pronounced negative, regressed β is indicative of isotopic fractionation during decomposition and physical mixing processes that occurs during soil turnover, and other hypotheses posed in the literature are marginalized using modeling and discussion. We discuss integration of the isotope method with existing SOC turnover models as a future research avenue.Citation: Acton, P., J. Fox, E. Campbell, H. Rowe, and M. Wilkinson (2013), Carbon isotopes for estimating soil decomposition and physical mixing in well-drained forest soils,

Stable isotopic constraints on global soil organic carbon turnover

“…Larger ln(-β) translated to faster SOC decomposition rates (Acton et al, 2013;Powers and Schlesinger, 2002). Soil ln(-β) was analyzed and across different biome types.…”

“…For sites with reasonably stable vegetation stocks, measures of vertical soil-profile δ 13 C can provide constraints on SOC turnover rates in ecosystems (Acton et al, 2013;Garten et al, 2000;Wynn et al, 2006). Soil δ 13 C generally increases from shallow to deep mineral soils in relatively welldrained systems, concomitant with decreasing SOC concentrations (Fig.…”

“…Therefore, SOC δ 13 C tends to increase with depth along vertical soil profiles until it reaches a maximum value at which point a steady-state is achieved (Kohl et al, 2015;Accoe et al, 2002;Brunn et al, 2014;Garten et al, 2000;Wynn et al, 2006;Brunn et al, 2016). These trends result in a negative linear relationship between the log-transformed SOC concentration and soil δ 13 C (Acton et al, 2013;Garten et al, 2000;Garten and Hanson, 2006;Powers and Schlesinger, 2002). The slope of the linear regression between soil δ 13 C and the log-transformed SOC concentration is defined as 60 beta (β), which has been proposed as a proxy for SOC turnover rate in a select number of sites (Acton et al, 2013;Garten et al, 2000;Powers and Schlesinger, 2002).…”

“…The slope of the linear regression between soil δ 13 C and the log-transformed SOC concentration is defined as 60 beta (β), which has been proposed as a proxy for SOC turnover rate in a select number of sites (Acton et al, 2013;Garten et al, 2000;Powers and Schlesinger, 2002). β has also been assessed in set of regional scale analyses (Acton et al, 2013;Brunn et al, 2014); however, whether β values can be used to constrain rates and controls on SOC turnover is yet to be explored at the global scale.…”

“…S1). The slope of the this linear regression is defined as beta (β) value (Acton et al, 2013;Garten et al, 2000;Powers and Schlesinger, 2002).…”