Pathways of Nitrogen Export and Calculating Net Anthropogenic Inputs (NANI)

Nitrogen is exported from watersheds through three primary pathways: human transport, atmospheric release as gases, and aquatic export in flowing water. Human transport involves the movement of N in agricultural products, as previously discussed, but also includes the disposal of waste products like municipal trash or sewage sludge destined for use as fertilizer. The second pathway, the release of nitrogen gases to the atmosphere, is one of the least quantified output processes. These gases—including ammonia (NH3), dinitrogen (N2), nitrous oxide (N2O), and nitric oxide (NO)—are emitted from various sources such as fertilized fields, waste processing plants, and groundwater that degasses upon entering streams. Despite numerous studies on processes like denitrification, deriving accurate, watershed-scale estimates for these gaseous losses remains a significant challenge.

Aquatic Export of Nitrogen. The export of nitrogen in flowing water is a fundamental output implied by the very definition of a "watershed." The most well-characterized aquatic flux is N loss in streamflow, quantified as the product of stream discharge and nitrogen concentration. Streamflow consists of baseflow, sustained by groundwater inflow, and stormflow, generated during precipitation events. Stormflow includes overland runoff and shallow subsurface flow, which can alter the composition and concentration of nitrogen. During storms, the total nitrogen load often increases due to elevated dissolved organic N (DON) and particulate N (PN), or its composition may shift from nitrate (NO3-) to organic forms as groundwater is diluted and stream bank erosion occurs.

Beyond surface water, nitrogen can also be exported via direct groundwater pathways. This occurs when water from an unconfined aquifer flows into deeper, confined aquifers that bypass surface streams entirely. While typically a small fraction of total flow, this pathway can be significant if the groundwater has high nitrate concentrations. Another often unmeasured pathway is groundwater underflow, where water passes beneath a stream gauging station, making this flux an artifact of measurement. Studies of submarine groundwater discharge into coastal waters have highlighted the potential importance of these subsurface pathways for delivering nitrogen directly to sensitive aquatic ecosystems.

Quantifying Human Influence: Net Anthropogenic Nitrogen Inputs (NANI). To comprehensively assess the human impact on the nitrogen cycle, scientists use the Net Anthropogenic Nitrogen Input (NANI) framework. This metric calculates the net anthropogenic nitrogen input to a watershed by combining major sources and subtracting the export of N in food and feed. The standard NANI equation is:

NANI = (N_fert + N_atm.dep + N_fix + N_{food_in} + N_{feed_in}) - (N_{food_out} + N_{feed_out})

Here, N_fert represents fertilizer application, N_{atm_dep} is atmospheric deposition, N_fix is nitrogen fixation by agricultural crops, and the food/feed terms account for the net import or export of nitrogen in agricultural products.

This calculation does not explicitly include ammonia volatilization, as this process is embedded within the atmospheric deposition and feed input terms. Livestock farming is a major driver of NH3 volatilization, and emissions from operations can exceed the nitrogen contained in exported animal products. Consequently, regions with high livestock density often experience significant atmospheric NH3 pollution, which is then redeposited elsewhere. For practical application, a toolbox has been developed for U.S. watersheds that calculates NANI using GIS and census data, incorporating information on human population, agricultural productivity, fertilizer use, and deposition estimates, allowing for widespread analysis of anthropogenic nitrogen forcing.