Temporal Variation of Water and Carbon Influence the Spatial Distribution of Upland Iron Reduction in Soils.

Iron reduction is an important process in soil that influences ecosystem form and function by altering the cycling of carbon, nutrients, and trace elements, and influencing particle (colloid) mobility. Research on flooded soils provides a strong framework for understanding iron biogeochemistry, but this framework needs to be translated to upland soils which experience dynamic redox conditions. We hypothesized that in upland soils, iron reduction intensity and prevalence would be affected by seasonal variations in soil moisture and carbon inputs. We tested this by delineating the spatial and temporal distribution of upland soil iron reduction in a sub-tropical forested ecosystem using in situ, passive probes as field measurements over the course of a year. The passive probes (steel IRIS probes) are rusted steel rods that provide easily reducible iron to a depth of 70 cm. After two weeks, the rods are collected from the soil, and iron removed is quantified with image processing software. We deployed the steel IRIS probes after an extreme rainfall event in October 2015, during the end-of-winter warming in March 2016, and planned for a period of high evapotranspirational demand in June 2016. The probes were deployed across a range of soil conditions determined from electromagnetic induction (EMI) surveys taken the previous year. Soil moisture and water table level were monitored over the course of the study. Our results indicate that iron reduction varied with depth and intensity over the sampling periods. Iron reduction was greatest at depth in response to a large rainfall event that created a perched water table, and it was greatest in shallow soils in late winter during a period of high biological oxygen demand from labile carbon inputs and soil warming. This research integrates data across fields of study in the critical zone sciences to illustrate that variation in upland soil iron reduction is governed at times by either water table location or labile carbon inputs. Our findings suggest the importance of synergistic factors (soil water, soil carbon, low oxygen) in creating soil sites that favor reducing processes in upland soils.