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

Abstract

Wetland soil oxygen (O 2) is rarely measured, which limits our understanding of a key regulator of nitrogen loss through denitrification. We asked: (1) How does soil [O 2] vary in riparian wetlands? (2) How does this [O 2] variation affect denitrification rates and end products? and (3) How does [O 2] variation and previous exposure to O 2 affect trace gas fluxes? We collected a continuous seven‐month record of [O 2] dynamics in a “wet” and “dry” riparian zone. In April 2009, soil [O 2] ranged from 0 to 13% and consistently increased with increasing distance from the stream. [O 2] gradually declined in all sensors until all sensors went anoxic in early September 2009. In mid‐fall, a dropping water table increased soil [O 2] 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 N 2 production against a helium background. Denitrification rates were significantly higher in the wetter areas, which correlated to lower O 2 conditions. Denitrification rates in the drier areas correlated with [O 2] in the early spring and summer, but significantly decreased in late summer despite decreasing O 2 concentrations. Increasing [O 2] significantly increased core N 2 O production, and therefore may be an important control on nitrous oxide yield. Field N 2 O 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 O 2 dynamics, and O 2 dynamics should be included in models of soil N cycling and trace gas fluxes.