Reconciling Negative Soil CO2 Fluxes: Insights from a Large-Scale Experimental Hillslope

Soil fluxes of CO2 (F-s) have long been considered unidirectional, reflecting the predominant roles of metabolic activity by microbes and roots in ecosystem carbon cycling. Nonetheless, there is a growing body of evidence that non-biological processes in soils can outcompete biological ones, pivoting soils from a net source to sink of CO2, as evident mainly in hot and cold deserts with alkaline soils. Widespread reporting of unidirectional fluxes may lead to misrepresentation of F-s in process-based models and lead to errors in estimates of local to global carbon balances. In this study, we investigate the variability and environmental controls of F-s in a large-scale, vegetation-free, and highly instrumented hillslope located within the Biosphere 2 facility, where the main carbon sink is driven by carbonate weathering. We found that the hillslope soils were persistent sinks of CO2 comparable to natural desert shrublands, with an average rate of -0.15 +/- 0.06 mu mol CO2 m(2) s(-1) and annual sink of -56.8 +/- 22.7 g C m(-2) y(-1). Furthermore, higher uptake rates (more negative F-s) were observed at night, coinciding with strong soil-air temperature gradients and [CO2] inversions in the soil profile, consistent with carbonate weathering. Our results confirm previous studies that reported negative values of F-s in hot and cold deserts around the globe and suggest that negative F-s are more common than previously assumed. This is particularly important as negative F-s may occur widely in arid and semiarid ecosystems, which play a dominant role in the interannual variability of the terrestrial carbon cycle. This study contributes to the growing recognition of the prevalence of negative F-s as an important yet, often overlooked component of ecosystem C cycling.