A modeling study of the impact of the δ 18O value of near-surface soil water on the δ 18O value of the soil-surface CO 2 flux

Abstract

Measurements of 18O in atmospheric CO 2 have been used to partition site-level measured net ecosystem CO 2 fluxes into gross fluxes and as a constraint on land surface biophysical processes at regional and global scales. However, these approaches require prediction of the δ 18O value of the net CO 2 flux between the soil and atmosphere (δ F), a quantity that is difficult to measure and accurately predict. δ F depends on the depth-dependent δ 18O value of soil water (δ sw), soil moisture and temperature, soil CO 2 production, and the δ 18O value of above-surface CO 2. I applied numerical model manipulations, regression analysis, a simple estimation method, and an analysis of the characteristic times of relevant processes to study the impacts of these parameters on δ F. The results indicate that ignoring δ sw gradients in the near-surface soil can lead to large errors. In particular, in systems where δ sw gradients exist, generalizing previous experimental observations to infer that a bulk (e.g., 5–10 cm or 5–15 cm depth) estimate of δ sw can be used to estimate δ F is problematic. These results highlight the need for further experiments and argue for the importance of accurately resolving near-surface δ sw in the context of partitioning ecosystem CO 2 fluxes and CO 2 source attribution.