The search for the missing nitrogen: Estimating denitrification in whole watersheds from N2 fluxes through agricultural drainage ditches

Background/Question/Methods

Net anthropogenic nitrogen inputs to watersheds often greatly exceed stream water discharges of reactive nitrogen (nitrate, nitrite, ammonium, and organic nitrogen).  The unaccounted or “missing” anthropogenic nitrogen may accumulate in groundwater, soil, biomass, or detritus storage pools that could eventually saturate.  The missing nitrogen may also be converted to gaseous forms by denitrification and released to the atmosphere, a process that could continue without saturating.  Thus, it is important to ask: what is the fate of the missing nitrogen? Denitrification is difficult to estimate at whole-watershed scales because of its high temporal and spatial variability.  By measuring N₂ fluxes, we estimated denitrification for watersheds of three drainage ditch segments in an agricultural basin on the Delmarva Peninsula in Maryland.  We measured inflow and outflow of water, nitrate, and dissolved N₂ in the ditch segments.  We estimated efflux of N₂ from the ditch water to the atmosphere based on the evasion rate of propane gas injected into the ditch water.  We estimated influx of atmospherically-derived N₂ from emerging groundwater based on groundwater recharge temperature inferred from dissolved Ar concentrations in nearby groundwater. 

Results/Conclusions

There was net release of both nitrate and N₂ from ditch segments, with nitrate accounting for 13-56% of nitrogen released in these two forms.  In the local drainage basin, discharge of reactive nitrogen is about 20% of the net anthropogenic nitrogen input, with nitrate discharge equaling about 14% of the anthropogenic input.  Comparing our estimates of nitrate and N₂ fluxes from the ditch segments with the nitrogen budget for the entire drainage basin suggests that much of the net anthropogenic nitrogen input to the ditch watersheds is denitrified and that much of the resulting N₂ is carried to the drainage ditches in emergent groundwater.  After emerging in the ditches, some of the N₂ produced by denitrification escapes to the atmosphere while some remains dissolved in flowing ditch water.  N₂ from denitrification may also escape directly from soil to the atmosphere without passing through ditch water.