Search

Recent studies show sulfur redox processes in terrestrial settings are more important than previously considered, but much remains uncertain about how these processes respond to dynamic hydrologic conditions in natural field settings. We used field observations from a sulfate‐impacted wetland and stream in the mining region of Minnesota (USA) to calibrate a reactive transport model and evaluate sulfur and coupled geochemical processes under contrasting hydrogeochemical scenarios. Simulations of different hydrological conditions showed that flux and chemistry differences between surface water and deeper groundwater strongly control hyporheic zone geochemical profiles. However, model results for the stream channel versus wetlands indicate sediment organic carbon content to be the more important driver of sulfate reduction rates. A complex nonlinear relationship between sulfate reduction rates and geochemical conditions is apparent from the model's higher sensitivity to sulfate concentrations in settings with higher organic content. Across all scenarios, simulated e−balance results unexpectedly showed that sulfate reduction dominates iron reduction, which is contrary to the traditional thermodynamic ladder but corroborates recent experimental findings by Hansel et al. (2015) that “cryptic” sulfur cycling could drive sulfate reduction in preference over iron reduction. Following the thermodynamic ladder, our models shows that high surface water sulfate slows methanogenesis in shallow sediments, but field observations suggest that sulfate reduction may not entirely suppress methane. Overall, our results show that sulfate reduction may serve as a major component making up and influencing terrestrial redox processes, with dynamic hyporheic fluxes controlling sulfate concentrations and reaction rates, especially in high organic content settings. Unlike in oceans, sulfur reactions have not been considered to play a prominent role in the biogeochemistry of terrestrial environments because of much lower concentrations, but recent studies have been showing terrestrial sulfur reactions to be more important than previously thought. These reactions often take place in wetland, stream, and lake sediments, which can contain a mix of both surface water and underlying groundwater. Our study investigated how water fluxes through these sediments can affect sulfur reactions when the surface water and groundwater chemistry differ significantly, such as through influxes of surface water sulfur from mining activities. Using field measurements and a computer model that simulates chemical reactions and water flow, we found that water flux drives water chemistry in wetland and stream sediments. This affects how rapidly sulfur reactions occur, especially in organic‐rich wetland sediments. The model showed sulfur reactions to dominate over iron reactions, which contradicts classic chemical thermodynamics. Also, when water fluxes carry high sulfur concentrations into wetland sediments, sulfur reactions out compete production of methane, a strong greenhouse gas. Our results indicate that sulfur reactions can play a prominent role in terrestrial biogeochemistry, and the degree to which it does is affected by hydrological fluxes. Dynamic water fluxes affect sulfate reduction rates in hyporheic zones, especially where organic content is highField data and reactive transport modeling suggest sulfate reduction can dominate Fe reduction in stream and wetland sedimentsHigh sulfate concentrations slow methanogenesis in wetlands but do not entirely suppress methane

This study reexamines the common expectations that in freshwater systems, sulfur plays a minor role in carbon cycling, and aerobic processes dominate methane oxidation. In anoxic sediments of a sulfate‐impacted wetland‐stream system in Minnesota (USA), a reactive transport model calibrated to geochemical observations predicted sulfate reduction to be the major terminal electron accepting process, and it showed that anaerobic oxidation of methane predominantly coupled with sulfate reduction attenuated methane concentrations near the sediment‐water interface. Consistent with model results, 16S rRNA microbiome analysis revealed a high relative abundance of taxa capable of dissimilatory sulfate reduction. It further supported the conclusion that high simulated sulfate reduction rates could be maintained by a “cryptic” sulfur cycle coupled to iron and methane. Low relative abundance of known iron reducing bacteria raised the possibility of abiotic ferric iron (Fe) reduction driving sulfide reoxidation to intermediate‐valence sulfur forms; widespread potential for microbially mediated disproportionation, oxidation, and reduction of sulfur intermediates provided mechanisms for completing redox cycles; and archaea comprising up to 25% of the microbial community could include consortia capable of anaerobic oxidation of methane. These biogeochemical processes were found to be controlled by hyporheic fluxes. Lower‐magnitude fluxes in wetland compared to channel sediments created sharper geochemical gradients that generated greater heterogeneity in microbial distributions and reaction rates. Changes in upward flux caused fluctuations in sulfate concentrations that led to alternating simulations of methane production and transport. Our work supports the importance of hyporheic flux‐driven iron‐sulfur‐methane cycling in sulfate‐impacted wetlands and prompts further investigations under freshwater conditions. Reactive transport model and microbiome analysis quantify and describe hyporheic flux‐driven Fe‐S‐CH 4processes in sulfate‐impacted wetlandsCryptic S cycling, coupled to Fe reduction and anaerobic CH 4oxidation and involving S intermediates, may drive high sulfate reduction ratesDynamic hyporheic fluxes control spatial distributions of microbial compositions and spatiotemporal distributions of reaction rates