THERMODYNAMIC CONSTRAINTS ON NITROGEN TRANSFORMATIONS AND OTHER BIOGEOCHEMICAL PROCESSES AT SOIL-STREAM INTERFACES

There is much interest in biogeochemical processes that occur at the i nterface 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 mechanist ic 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 accepters. Over a t wo-year period we analyzed >1400 individual samples of subsurface wate rs from networks of sample wells in riparian wetlands along Smith Cree k, a first-order stream draining a mixed forested-agricultural landsca pe in southwestern Michigan, USA. We focused on areas where soil water and ground water emerged into the stream, and where we could characte rize 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 p redictable outcome of the interaction between microbial communities an d supplies of electron donors and accepters. Variations in key electro n donors and accepters (NO3-, N2O, NH4+, SO42-, CH4, and dissolved org anic carbon [DOC]) closely followed predictions from thermodynamic the ory. Transformations of N and other elements resulted from the respons e of microbial communities to two dominant hydrologic flow paths: (1) horizontal flow of shallow subsurface waters with high levels of elect ron donors (i.e., DOG, CH4, and NH4+), and (2) near-stream vertical up welling of deep subsurface waters with high levels of energetically fa vorable electron accepters (i.e., NO3-, N2O, and SO42-). Our results s upport the popular notion that soil-stream interfaces can possess stro ng potential for removing dissolved N by denitrification. Yet in contr ast to prevailing ideas, we found that denitrification did not consume all NO3- that reached the soil-stream interface via subsurface flow p aths. Analyses of subsurface N chemistry and natural abundances of del ta(15)N in NO3- and NH4+ suggested a narrow near-stream region as func tionally the most important location for NO3- consumption by denitrifi cation. This region was characterized by high throughput of terrestria lly derived water, by accumulation of dissolved NO3- and N2O, and by l ow levels of DOG. Field experiments supported our hypothesis that the sustained ability for removal of dissolved NO3- and N2O should be limi ted by supplies of oxidizable carbon via shallow flowpaths. In situ ad ditions of acetate, succinate, and propionate induced rates of NO3- re moval (similar to 1.8 g N.m(-2).d(-1)) that were orders of magnitude g reater than typically reported from riparian habitats. We propose that the immediate near-stream region may be especially important for dete rmining the landscape-level function of many riparian wetlands, Manage ment efforts to optimize the removal of NO3- by denitrification ought to consider promoting natural inputs of oxidizable carbon to this near -stream region.