Physicochemical Perturbation Increases Nitrous Oxide Production in Soils and Sediments

Abstract. Atmospheric concentrations of nitrous oxide (N₂O), a potent greenhouse gas that is also responsible for significant stratospheric ozone depletion, have increased in response to intensified use of agricultural fertilizers and other human activities that have accelerated nitrogen cycling processes. Microbial denitrification in soils and sediments is a major source of N₂O, produced as an intermediate during the reduction of oxidized forms of nitrogen to dinitrogen gas (N₂). Substrate availability (nitrate and organic matter) and environmental factors such as oxygen levels, temperature, moisture, and pH influence rates of denitrification and N₂O production. Here we describe the role of physicochemical perturbation (defined here as a change from the ambient environmental conditions) on denitrification and N₂O production. Changes in salinity, temperature, moisture, pH, and zinc in agricultural soils induced a short-term perturbation response characterized by lower rates of total denitrification and higher rates of net N₂O production. The N₂O to total denitrification ratio (N₂O : DNF) increased strongly with physicochemical perturbation. A salinity press experiment on tidal freshwater marsh soils revealed that increased N₂O production was likely driven by transcriptional inhibition of the nitrous oxide reductase (nos) gene, and that the microbial community adapted to altered salinity over a relatively short (within one month) timeframe. Perturbation appeared to confer resilience to subsequent disturbance, and denitrifiers from an environment without salinity fluctuations (tidal freshwater estuarine sediments) demonstrated a stronger N₂O perturbation response than denitrifiers from environments with more variable salinity (oligohaline and mesohaline estuarine sediments), suggesting that the denitrifying community from physicochemically stable environments may have a stronger perturbation response. These findings provide a framework for improving our understanding of the dynamic nature of N₂O production in soils and sediments, in which changes in physical and/or chemical conditions initiate a short-term perturbation response that promotes N₂O production that moderates over time and with subsequent physicochemical perturbation.