Nitrate reduction pathways and interactions with iron in the drainage water infiltration zone of a riparian wetland soil

Abstract

Eutrophication of natural water bodies is moderated by transformation of nitrate (NO3−) in riparian wetlands, which serve as filters of infiltrating drain water from upland agricultural areas. The present study comprised field observations, laboratory experiments and metagenomic studies to describe NO3− removing transformation pathways and interactions with the cycling of iron (Fe) in a temperate riparian wetland soil profile down to 1 m depth. Water samples from piezometers showed a distinct plume of NO3− in the subsurface soil where agricultural drain water was infiltrating. However, within a distance of few meters in the water flow direction, NO3− was depleted from the percolating water. Sampling and analyses of soil from the active zone of the biogeochemical NO3− removal showed that denitrifying enzyme activity was ~ tenfold higher in the upper 0–25 cm than in the lower 25–100 cm. Yet, net transformation of NO3− was substantial also at 25–100 cm when assayed with relatively undisturbed soil samples and by 15N tracer techniques in soil slurries. Transformation pathways of dissimilatory nitrate reduction to ammonium and anaerobic ammonium oxidation were identified, but were quantitatively minor as compared to denitrification. Heterotrophic denitrification and denitrification mediated by oxidation of ferrous iron, Fe(II), were identified as important processes in the wetland soil. The latter was substantiated by geochemical observations, by rates of NO3− depletion in slurry incubations with added FeCl2, and by identification of microorganisms with known capacity of NO3− reduction coupled to Fe(II) oxidation (Acidovorax sp.). The transformation pathway of iron-mediated NO3− reduction could involve biotic and abiotic reactions, and N2O, which is a potent greenhouse gas, was a major product of the process. It remains to be seen under field conditions if N2O emission hotspots are linked to specific sites of dynamic NO3− reduction coupled to Fe(II) oxidation.