CARBON ISOTOPE RATIOS IN BELOWGROUND CARBON CYCLE PROCESSES

TLDR

δ13C analyses of soil organic matter and respired CO₂ reveal carbon‑cycle dynamics, identify soil CO₂ efflux components, and quantify historical C3–C4 shifts, yet a global database to test latitudinal differences is still missing. The study aims to distinguish δ13C signals from SOM versus soil CO₂ efflux, addressing disequilibrium effects, and to clarify the mechanism behind progressive SOM enrichment to improve SOM turnover quantification. The authors propose that progressive δ13C enrichment of SOM results from a gradual shift in microbial versus plant contributions to residual SOM, rather than differential degradation or microbial fractionation.

Abstract

Analyses of carbon isotope ratios (δ13C) in soil organic matter (SOM) and soil respired CO2 provide insights into dynamics of the carbon cycle. δ13C analyses do not provide direct measures of soil CO2 efflux rates but are useful as a constraint in carbon cycle models. In many cases, δ13C analyses allow the identification of components of soil CO2 efflux as well as the relative contribution of soil to overall ecosystem CO2 fluxes. δ13C values provide a unique tool for quantifying historical shifts between C3 and C4 ecosystems over decadal to millennial time scales, which are relevant to climate change and land-use change issues. We identify the need to distinguish between δ13C analyses of SOM and those of soil CO2 efflux in carbon cycle studies, because time lags in the turnover rates of different soil carbon components can result in fluxes and stocks that differ in isotopic composition (disequilibrium effect). We suggest that the frequently observed progressive δ13C enrichment of SOM may be related to a gradual shift in the relative contributions of microbial vs. plant components in the residual SOM and not to differential SOM degradation or to microbial fractionation during decomposition. Clarifying this mechanism is critical for applying δ13C analyses to quantification of SOM turnover rates. Across latitudinal gradients, large differences should occur in the δ13C values of CO2 effluxing from soils, but as of yet a global database is lacking with which to test this prediction. Such a global database would be a useful input for global carbon cycle models that rely on δ values to constrain source and sink relations.