Soil anaerobic microsites mediate land use and precipitation effects on soil organic carbon on a regional scale

Anaerobic microsites promote soil organic carbon (SOC) storage by shielding it from oxidative degradation. However, relationships between anaerobic microsite abundance and SOC storage in upland soils have yet to be explored on a regional scale, where variation in climate and land use co-occur. This study examines the association of a proxy for soil anaerobic volume and SOC concentration across the North American mid-continent's mean annual precipitation (MAP) gradient (478-1040 mm, ∼1.1 mm km^-1) and three common land uses (native prairie, agriculture, and post-agriculture). We analyzed soil samples for the concentration of 0.5 N hydrochloric acid extractable ferrous iron (Fe(II)), which we use as an integrative proxy of antecedent anaerobic conditions. Soil Fe(II) concentration was highest in soils with minimal sand content and increased with MAP in agricultural and post-agricultural soils but not in native prairie soils. Soil Fe(II) concentration was consistently higher near the soil surface (0-5 cm) than at depth (5-15 or 15-30 cm). We found that Fe(II) was a better predictor of SOC across our 20 study locations and three soil depths than was MAP or soil pH or texture. Furthermore, our results highlight how anaerobic microsites in soils can mediate well-established relationships between SOC concentrations and land use and MAP. Land use had a predictable effect on SOC, but these land use effects on SOC were partly attributed to variation in anaerobic microsites. Specifically, 25% (P < 0.05) of the greater SOC in post-agricultural compared to agricultural sites was associated with increased anaerobic microsite concentration. Additionally, half of the total effect of MAP on SOC was explained by variation in soil Fe(II) (52.6%, P < 0.018). Taken together, our findings indicate that soil anaerobic microsites contribute significantly to SOC stabilization across large-scale precipitation gradients in otherwise well aerated soils, and that current agricultural management destabilizes both anaerobic microsites and SOC.