The relationship between soil CO2 efflux and its carbon isotopic composition under non-steady-state conditions

Soil CO2 efflux and its carbon isotopic composition are undoubtedly important for estimating ecosystem carbon budgets and for partitioning respiration sources at various spatial and temporal scales. Under natural conditions, non-steady processes will strongly influence the diffusive fluxes of 13CO2 and 12CO2 between soil and the atmosphere, which results in variations of effluxed soil δ13CO2 and will lead to bias in respiration source partitioning. In this study, we present a set of quantitative relationships between soil CO2 efflux and its δ13C by solving the diffusion equation. The results showed that the effluxed δ13CO2 converged toward the respiratory δ13CO2 with an increasing efflux rate but that the values greatly differed at low efflux rates. Both our own experiments and data from the literature verified this convergence pattern of the effluxed δ13CO2, which implies that most of the variations in the δ13C of soil effluxed CO2 may derive from diffusive fractionation rather than from biological causes. Our results explain the isotopic flux patterns of CO2 under natural environmental variations and are vitally important for isotope-based modeling of ecosystem carbon exchange under changing climatic regimes.