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 µmol CO₂ m² 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.