Stable isotopic constraints on global soil organic carbon turnover

Carbon dioxide release during soil organic carbon (SOC) turnover is a pivotal component of atmospheric CO₂ concentrations and global climate change. However, reliably measuring SOC turnover rates on large spatial and temporal scales remains challenging. Here we use a natural carbon isotope approach, defined as beta (<i>β</i>), which was quantified from the <i>δ</i>¹³C of vegetation and soil reported in the literature (176 separate soil profiles), to examine large-scale controls of climate, soil physical properties and nutrients over patterns of SOC turnover across terrestrial biomes worldwide. We report a significant relationship between <i>β</i> and calculated soil C turnover rates (<i>k</i>), which were estimated by dividing soil heterotrophic respiration rates by SOC pools. ln( − <i>β</i>) exhibits a significant linear relationship with mean annual temperature, but a more complex polynomial relationship with mean annual precipitation, implying strong-feedbacks of SOC turnover to climate changes. Soil nitrogen (N) and clay content correlate strongly and positively with ln( − <i>β</i>), revealing the additional influence of nutrients and physical soil properties on SOC decomposition rates. Furthermore, a strong (<i>R</i>² = 0.76; <i>p</i> &lt; 0.001) linear relationship between ln( − <i>β</i>) and estimates of litter and root decomposition rates suggests similar controls over rates of organic matter decay among the generalized soil C stocks. Overall, these findings demonstrate the utility of soil <i>δ</i>¹³C for independently benchmarking global models of soil C turnover and thereby improving predictions of multiple global change influences over terrestrial C-climate feedback