Iron Redox Reactions Can Drive Microtopographic Variation in Upland Soil Carbon Dioxide and Nitrous Oxide Emissions

Abstract

Topographic depressions in upland soils experience anaerobic conditions conducive for iron (Fe) reduction following heavy rainfall. These depressional areas can also accumulate reactive Fe compounds, carbon (C), and nitrate, creating potential hot spots of Fe-mediated carbon dioxide (CO2) and nitrous oxide (N2O) production. While there are multiple mechanisms by which Fe redox reactions can facilitate CO2 and N2O production, it is unclear what their cumulative effect is on CO2 and N2O emissions in depressional soils under dynamic redox. We hypothesized that Fe reduction and oxidation facilitate greater CO2 and N2O emissions in depressional compared to upslope soils in response to flooding. To test this, we amended upslope and depressional soils with Fe(II), Fe(III), or labile C and measured CO2 and N2O emissions in response to flooding. We found that depressional soils have greater Fe reduction potential, which can contribute to soil CO2 emissions during flooded conditions when C is not limiting. Additionally, Fe(II) addition stimulated N2O production, suggesting that chemodenitrification may be an important pathway of N2O production in depressions that accumulate Fe(II). As rainfall intensification results in more frequent flooding of depressional upland soils, Fe-mediated CO2 and N2O production may become increasingly important pathways of soil greenhouse gas emissions.

Fe(II) Redox Chemistry in the Environment

“…Indeed, tropical soils have revealed some amazing findings in relation to the impact of Fe redox chemistry on soil carbon cycling. Fluctuating redox conditions encourage the oxidation of soil carbon to CO 2 due to the generation of ROS following abiotic Fe­(II) oxidation in conjunction with anaerobic metabolism, , and indeed, Fe-rich soils are often reported to have high rates of CO 2 and NO x emissions …”

“…Fluctuating redox conditions encourage the oxidation of soil carbon to CO 2 due to the generation of ROS following abiotic Fe(II) oxidation in conjunction with anaerobic metabolism, 613,614 and indeed, Fe-rich soils are often reported to have high rates of CO 2 and NO x emissions. 615 Nevertheless, over time, iron oxide crystallinity increases, associated with Fe(II)-catalyzed transformation processes, 614,616,617 and this makes the Fe (oxyhydr)oxides within these soils less prone to reductive dissolution, providing a degree of protection against soil carbon mineralization to CO 2 . Similarly, the formation of Fe−soil organic matter (SOM) aggregates with soil minerals such as clays, again aided by the presence of Fe(II) (through a process of Fe(II) oxidation and SOM coprecipitation), is known to make soil organic carbon more resistant to oxidation and degradation (i.e., more stable) over time.…”

Coprecipitation with Ferrihydrite Inhibits Mineralization of Glucuronic Acid in an Anoxic Soil

“…This finding contrasts with the results of similar anoxic incubation studies, which report increased CO 2 production in soils following the addition of pure ferrihydrite or goethite, where it is suggested that Fe(III) in these minerals acted as electron acceptors. 39,40 However, in the soil used here, easily reducible native iron was abundant (7.3 wt % Fe T , Table S1, ∼31.7 mg g −1 Fe O…”

“…12,17 However, the impact of iron mineral additions on CO 2 production in anoxic soils is less clear, with increases, decreases, and no effect on CO 2 production reported in various mineral-enriched soils. 39,40,57 This may be explained by the . Total CO 2 produced, which includes SOM-CO 2 and 13 GluC-CO 2 ; panel (c).…”

“…22,38 Thus, under reducing soil conditions, mineralization of SOM can even be stimulated by the addition of Fe(III) minerals acting as electron acceptors. 39,40 In anoxic soils, microbial Fe reduction may account for up to 44% of anaerobic OC mineralization. 41 Yet, as demonstrated in the millennial-scale age of Fe-bound OC found buried in marine sediments 42 and through abundant examples of very long turnover times for mineral-bound SOM in subsurface soil horizons, 10,43 evidence overwhelmingly agrees that significant stores of Fe-bound OC exist in soil and sediment environments which experience continual or recurring reducing conditions.…”