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

Abstract

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.

Macro and microscopic visual imaging tools to investigate metal reducing bacteria in soils

“…The Upland deep zone did not see significant paint removal. In the first 22 d there was 6% removal, but following a rain event, which saturated the deep zone, paint removal increased to 27% in the subsequent 6 d. In all cases, paint removal rates were higher during periods of saturation, consistent with other IRIS studies that show rainfall events can induce iron reduction (Hodges et al, 2018) and soil saturation is the main factor affecting iron-oxide paint removal (Bryant, 2010). One other factor, necessary for microbial activity responsible for paint removal, is a labile C source.…”

“…Rainfall changed both the soil temperature (Supplemental Figure S6) and the degree of soil water saturation in the target zone (0-30 cm bgs; Figure 4). Metal-oxide paint removal rates fit the expected pattern based on soil saturation (Hodges et al, 2018); higher saturation resulted in higher removal rates (Table 1). Iron-oxide tubes (r 2 range: .71-.94) and manganese-oxide tubes (r 2 range: .48-.84) fit a linear model (Table 1).…”

“…The Indicator of Reduction in Soils (IRIS) is one of three U.S.‐approved techniques (NRCS, 2015) used to demonstrate soils are reducing (Berkowitz & Sallee, 2011; Castañeda, Luna, & Rabenhorst, 2017; Hodges, King, Pett‐Ridge, & Thompson, 2018). Iron‐oxide IRIS rely on iron reduction (Jenkinson & Franzmeier, 2006; Castenson & Rabenhorst, 2006).…”

Soil CO2 and O2 Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations

“…Under shifting climate regimes, intense storms that saturate soil are anticipated to become more frequent (Greve et al, 2014), and this increased precipitation has been demonstrated to stimulate Fe-reducing conditions, even under bulk oxidizing conditions in soils (Hodges et al, 2018). Therefore, it is likely that under these future rainfall predictions the proportion of anaerobic respiration contributing to total soil C flux will increase (Keiluweit et al, 2016(Keiluweit et al, , 2017.…”

Hot Spots and Hot Moments of Soil Moisture Explain Fluctuations in Iron and Carbon Cycling in a Humid Tropical Forest Soil

“…This suggests that rainfall does not have an immediate impact on Fe and C biogeochemistry, but rather it is the accumulation of water in the soil (soil moisture) that has a rapid impact on these parameters; reinforcing that soil moisture drives biogeochemical cycling, not rainfall. Additionally, elevated soil moisture content had a strong relationship with the potential for Fe reduction as measured by uniformly oxidized steel rods (i.e., Indicator of Reduction in Soils-IRIS) deployed across a topographic gradient in volcanic soils from Hawaii [76], and also in upland Fe-rich and weathered soils from the United States (USA) southeastern piedmont [77]. In these studies, Fe reduction was likely stimulated by exogenous C inputs (such as DOC).…”