Carbon isotope ratios in belowground carbon cycle processes

Analyses of carbon isotope ratios (delta(13)C) in soil organic matter (SOM) and soil respired CO2 provide insights into dynamics of the carbon cycle. delta(13)C analyses do not provide direct measures of soil CO2 efflux rates but are useful as a constraint in carbon cycle models. In many cases, delta(13)C analyses allow the identification of components of soil CO2 efflux as well as the relative contribution of soil to overall ecosystem CO2 fluxes. delta(13)C values provide a unique tool for quantifying historical shifts between C-3 and C-4 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 delta(13)C 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 delta(13)C 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 delta(13)C analyses to quantification of SOM turnover rates. Across latitudinal gradients, large differences should occur in the delta(13)C 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 delta values to constrain source and sink relations.