Watershed missing N: Can biogenic N gas fluxes account for it?
N export from watersheds drives eutrophication in downstream waters. This is especially important in the Chesapeake region as the watershed states and counties are attempting to reduce N losses to meet the Chesapeake Total Maximum Daily Load (TMDL). Unfortunately for the TMDL process, we are unable to account for all of the watershed net anthropogenic N inputs (NANI) in terms of stream N export. This “missing N” is typically 8-30% of NANI and is assumed to be stored in soils, groundwater, and biomass within the watershed, or lost to the atmosphere as biogenic N gases. We hypothesize that much of the missing N can be accounted for as gaseous N2 and N2O losses from watershed streams and soils. We have been measuring watershed N concentrations and fluxes in soils, groundwater, and streams in the Choptank watershed on the Delmarva Peninsula for the past 10 years, and we summarize here the evidence supporting this hypothesis. The missing nitrogen in our small watersheds (6-50 km2) varies over a large range of NANI, providing us with a wide variety of watershed conditions in which to investigate the fate of NANI.
Nitrate (NO3-) and biogenic N gases are ubiquitous in agricultural watersheds but small in forested watersheds. NO3- is typically 0.1-1.0 mM in agricultural areas, but 0.1-10 μM in forests. Excess N gases in groundwater ranged over 20-600 μM N2-N and undetectable-75 μM N2O-N, and excess N2 was inversely correlated to O2. Excess N2-N in stream baseflow varied over 2-250 μM N2-N and was inversely correlated with NO3- and O2. In one stream draining a watershed that receives agricultural N, biogenic N2 and N2O gas fluxes were measured using the open channel method. Integrated over stream length and time, the fluxes accounted for 90% of the missing N. Soil N2O-N fluxes from agricultural fields were estimated using the static chamber method, and N2O fluxes responded to fertilizer and manure applications and rainfall events. Integrated over time, N2O-N fluxes were 2-13 kg N ha-1 y-1. No direct measurements of soil N2 fluxes were available, but measurements by capillary inlet mass spectrometry and diffusion-advection modeling of N2/Ar indicated detectable increases in N2/Ar in the vadose zone of soils. Collectively, these measurements provide significant evidence that biogenic N gas fluxes from watersheds can account for large proportions of the missing N.