Thermodynamic Constraints on Nitrogen Transformations and Other Biogeochemical Processes at Soil–stream Interfaces

There is much interest in biogeochemical processes that occur at the interface between soils and streams since, at the scale of landscapes, these habitats may function as control points for fluxes of nitrogen (N) and other nutrients from terrestrial to aquatic ecosystems. Here we examine whether a thermodynamic perspective can enhance our mechanistic and predictive understanding of the biogeochemical function of soil–stream interfaces, by considering how microbial communities interact with variations in supplies of electron donors and acceptors. Over a two-year period we analyzed .1400 individual samples of subsurface waters from networks of sample wells in riparian wetlands along Smith Creek, a first-order stream draining a mixed forested–agricultural landscape in southwestern Michigan, USA. We focused on areas where soil water and ground water emerged into the stream, and where we could characterize subsurface flow paths by measures of hydraulic head and/or by in situ additions of hydrologic tracers. We found strong support for the idea that the biogeochemical function of soil–stream interfaces is a predictable outcome of the interaction between microbial communities and supplies of electron donors and acceptors. Variations in key electron donors and acceptors (NO3, N2O, NH4, SO4, CH4, and dissolved organic carbon [DOC]) closely followed predictions from thermodynamic theory. Transformations of N and other elements resulted from the response of microbial communities to two dominant hydrologic flow paths: (1) horizontal flow of shallow subsurface waters with high levels of electron donors (i.e., DOC, CH4, and NH4), and (2) near-stream vertical upwelling of deep subsurface waters with high levels of energetically favorable electron acceptors (i.e., NO3, N2O, and SO4). Our results support the popular notion that soil–stream interfaces can possess strong potential for removing dissolved N by denitrification. Yet in contrast to prevailing ideas, we found that denitrification did not consume all NO3 that reached the soil–stream interface via subsurface flow paths. Analyses of subsurface N chemistry and natural abundances of d15N in NO3 and NH4 suggested a narrow near-stream region as functionally the most important location for NO3 consumption by denitrification. This region was characterized by high throughput of terrestrially derived water, by accumulation of dissolved NO3 and N2O, and by low levels of DOC. Field experiments supported our hypothesis that the sustained ability for removal of dissolved NO3 and N2O should be limited by supplies of oxidizable carbon via shallow flowpaths. In situ additions of acetate, succinate, and propionate induced rates of NO3 removal (;1.8 g N·m22·d21) that were orders of magnitude greater than typically reported from riparian habitats. We propose that the immediate near-stream region may be especially important for determining the landscape-level function of many riparian wetlands. Management efforts to optimize the removal of NO3 by denitrification ought to consider promoting natural inputs of oxidizable carbon to this near-stream region.