Abstract

Soil CO₂ efflux (F_soil) represents a significant source of ecosystem CO₂ emissions that is rarely quantified with high-temporal-resolution data in carbon flux studies. F_soil estimates can be obtained by the low-cost gradient method (GM), but the utility of the method is hindered by uncertainties in the application of published models for the diffusion coefficient. Therefore, to address and resolve these uncertainties, we compared F_soil measured by 2 soil CO₂ efflux chambers and F_soil estimated by 16 gas transport models using the GM across 1 year. We used 14 published empirical gas diffusion models and 2 in situ models: (1) a gas transfer model called “Chamber model” obtained using a calibration between the chamber and the gradient method and (2) a diffusion model called “SF₆ model” obtained through an interwell conservative tracer experiment. Most of the published models using the GM underestimated cumulative annual F_soil by 55% to 361%, while the Chamber model closely approximated cumulative F_soil (0.6% error). Surprisingly, the SF₆ model combined with the GM underestimated F_soil by 32%. Differences between in situ models could stem from the Chamber model implicitly accounting for production of soil CO₂, while the conservative tracer model does not. Therefore, we recommend using the GM only after calibration with chamber measurements to generate reliable long-term ecosystem F_soil measurements. Accurate estimates of F_soil will improve our understanding of soil respiration's contribution to ecosystem fluxes.

Citation

Sánchez-Cañete E.P., Scott R.L., van Haren J., and Barron-Gafford G.A. (2017): Improving the accuracy of the gradient method for determining soil carbon dioxide efflux. Journal of Geophysical Research: Biogeosciences 122(1): 50-64. DOI: 10.1002/2016JG003530

This Paper/Book acknowledges NSF CZO grant support.