Journal of Geophysical Research: Biogeosciences

Soil O2 controls denitrification rates and N2O yield in a riparian wetland

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Abstract

[1] Wetland soil oxygen (O2) is rarely measured, which limits our understanding of a key regulator of nitrogen loss through denitrification. We asked: (1) How does soil [O2] vary in riparian wetlands? (2) How does this [O2] variation affect denitrification rates and end products? and (3) How does [O2] variation and previous exposure to O2affect trace gas fluxes? We collected a continuous seven-month record of [O2] dynamics in a “wet” and “dry” riparian zone. In April 2009, soil [O2] ranged from 0 to 13% and consistently increased with increasing distance from the stream. [O2] gradually declined in all sensors until all sensors went anoxic in early September 2009. In mid-fall, a dropping water table increased soil [O2] to 15–20% within a 2–3 day period. We measured denitrification using the Nitrogen-Free Air Recirculation Method (N-FARM), a direct measurement of N2 production against a helium background. Denitrification rates were significantly higher in the wetter areas, which correlated to lower O2 conditions. Denitrification rates in the drier areas correlated with [O2] in the early spring and summer, but significantly decreased in late summer despite decreasing O2 concentrations. Increasing [O2] significantly increased core N2O production, and therefore may be an important control on nitrous oxide yield. Field N2O fluxes, however, were highly variable, ranging from 0 to 800 ug N m−2 hr−1 with no differences between the wet and dry sites. Future research should focus on understanding the biotic and abiotic controls on O2 dynamics, and O2 dynamics should be included in models of soil N cycling and trace gas fluxes.

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