Kinetics of nitrous oxide mass transfer from porewater into root aerenchyma of wetland plants

Abstract

The creation and/or restoration of wetlands is an important strategy for controlling the release of reactive nitrogen (N) via denitrification, but there can be tradeoffs between enhanced denitrification and the production of nitrous oxide (N 2 O), a potent greenhouse gas. A knowledge gap in current understanding of belowground wetland N dynamics is the role of gas transfer through the root aerenchyma system of wetland plants as a shortcut emission pathway for N 2 O in denitrifying wetland soils. This investigation evaluates the significance of mass transfer into gas-filled root aerenchyma for the N 2 O budget in wetland mesocosms planted with Sagittaria latifolia Willd. and Schoenoplectus acutus (Muhl. ex Bigelow) Á. Löve & D. Löve. Dissolved gas tracer push-pull tests with N 2 O and the nonreactive gas tracers helium, sulfur hexafluoride, and ethane were used to estimate first-order rate constants for gas transfer into roots and microbial N 2 O reduction and thereby disentangle the effects of root-mediated gas transport from microbial metabolism on N 2 O balances in saturated soils. Root-mediated gas transport was estimated to account for up to 37% of overall N 2 O removal from the wetland soils. Rates of microbial N 2 O reduction varied widely based on the organic matter content of the soil media and served as a key control on the fraction of N 2 O that transferred into roots. This research identifies transport through root aerenchyma as a potential shortcut pathway for N 2 O emission from wetland soils and sediments and indicates that this process should be considered in both measurements and mechanistic modeling of belowground wetland N dynamics.