Hydrologic control on redox and nitrogen dynamics in a peatland soil

Soils are a dominant source of nitrous oxide (N2O), a potent greenhouse gas. However, the complexity of the drivers of N2O production and emissions has hindered our ability to predict the magnitude and spatial dynamics of N2O fluxes. Soil moisture can be considered a key driver because it influences oxygen (O2) supply, which feeds back on N2O sources (nitrification versus denitrification) and sinks (reduction to dinitrogen). Soil water content is directly linked to O2 and redox potential, which regulate microbial metabolism and chemical transformations in the environment. Despite its importance, only a few laboratory studies have addressed the effects of hydrological transient dynamics on nitrogen (N) cycling in the vadose zone. To further investigate these aspects, we performed a long term experiment in a 1.5 m depth soil column supplemented by chamber experiments. With this experiment, we aimed to investigate how soil moisture dynamics influence redox sensitive N cycling in a peatland soil. As expected, increased soil moisture lowered O2 concentrations and redox potential in the soil. The decline was more severe for prolonged saturated conditions than for short events and at deep than at the soil surface. Gaseous and dissolved N2O, dissolved nitrate (NO3−) and ammonium (NH4+) changed considerably along the soil column profile following trends in soil O2 and redox potential. Hot spots of N2O concentrations corresponded to high variability in soil O2 in the upper and lower parts of the column. Results from chamber experiments confirmed high NO3− reduction potential in soils, particularly from the bottom of the column. Under our experimental conditions, we identified a close coupling of soil O2 and N2O dynamics, both of which lagged behind soil moisture changes. These results highlight the relationship among soil hydrologic properties, redox potential and N cycling, and suggest that models working at a daily scale need to consider soil O2 dynamics in addition to soil moisture dynamics to accurately predict patterns in N2O fluxes.

► Water dynamics strongly influence mineral N concentrations and N2O emissions through effects on soil O2 and redox potential. ► Soil moisture may not be good enough to describe the aerobic/anaerobic conditions of the soil and to predict N2O emission. ► Saturated zone and the vadose zone are characterized by different inorganic N pools and N2O fluxes. ► Periodic flooding and NO3- addition through irrigation practices have a potential impact on N2O emissions in N-rich soils.