Potential for Iron Reduction Increases with Rainfall in Montane Basaltic Soils of Hawaii

Abstract

Core Ideas In upland soils, the potential for iron reduction to occur increases with rainfall. Bulk soil anoxia is not a requirement for soil iron reduction. Soil iron reduction likely occurs at most sites that experience periodically high soil moisture. Microbe‐mediated Fe reduction modulates the role of Fe‐bearing minerals, and can occur without saturation, in upland soils. Quantifying this Fe reduction is difficult, but critical for identifying climates in which Fe reduction plays a role in soil biogeochemistry. We measured potential for Fe reduction in upland soils along a rainfall gradient in Maui, Hawaii (2200 to 4400 mm yr–1 mean annual precipitation [MAP]), hypothesizing that potential for Fe reduction correlates with MAP. We determined the potential for Fe reduction by removal of Fe coating from (a) Fe oxide‐coated polyvinyl chloride (PVC) tubes (Indicator of Reduction in Soils; “PVC IRIS”) and (b) uniformly rusted steel rods (“Steel IRIS probes”) at 7, 11, and 14 d after installation. We measured soil redox potential (Eh) and pH at each site. Some coating was removed from all PVC and Steel IRIS probes, and fraction of Fe removed from Steel IRIS at 14 d (0.13 to 0.67) correlated with MAP (r2 = 0.66, p = 0.016). However, bulk soil Eh remained high (∼900 to 650 mV), except at the subsurface (45 cm) depth of the 4200 mm MAP site, suggesting overall oxidizing conditions. We also conducted in‐laboratory experiments to constrain the conditions of Fe removal. These laboratory experiments indicated (i) Fe reduction drives Fe coating removal, (ii) Fe coating removal (Fe reduction) initiates in unsaturated soils (iii) coating removal increases with increasing soil moisture. Our findings demonstrate that rainfall increases the likelihood for Fe reduction in otherwise oxic soils, and suggests that Fe redox influences the biogeochemistry of many upland soils.