Spring Snowmelt: a 'Hot Moment’ for Soil Denitrification in Riparian Areas?

Biological soil denitrification (i.e. the microbial reduction of nitrate or nitrite resulting in gaseous N2, or an oxide of N), provides a valuable ecosystem service by reducing the amount of nitrate (NO3-) that is mobilized from the soil to adjacent streams and rivers. In the Northeast United States, the spring snowmelt period is a highly dynamic time for the soil environment, triggering fluctuations in soil conditions (e.g. soil temperature, moisture, oxygen and substrate availability) that could support denitrification. As such, while riparian areas (i.e. the interface of the terrestrial and aquatic environment) represent ‘hot spots’ for this process, the spring snowmelt period could be a ‘hot moment’ for soil denitrification (Groffman & Gold, 1998). For example, elevated soil moisture, soil temperature, and substrate availability throughout the snowmelt period could lead to increased denitrification rates. However, the connections between variations in soil conditions and denitrification rates remain poorly understood, especially during spring snowmelt (Brooks et al., 1998). A better, more mechanistic understanding of this process is needed in order to evaluate the capacity of riparian soils to denitrify during this critical period for nitrogen (N) mobilization.

To address this knowledge gap, we measured denitrification rates from riparian soils of different adjacent land use (agricultural and forested) and landscape position (upland and wetland) in Vermont, USA. We sampled throughout the spring snowmelt period, as well as during intermittent snowmelt events in the winter months. We paired data on denitrification rates with surface nitrous oxide (N2O) fluxes, soil and soil water carbon (C) and N concentrations, and high frequency data from an in-situ soil sensor network that provide continuous data on CO2, O2, oxidation/reduction potential, volumetric water content, electrical conductivity, and temperature. These data will be used to assess linkages between soil conditions and the capacity of the riparian soils to denitrify. Preliminary results indicate higher denitrification rates in the agricultural riparian area, compared to the forested site, suggesting that substrate availability is a major driver of denitrification rates during the snowmelt period.