Nitrogen Storage, Legacy Effects, and the Impact of Land Use on Watershed Budgets

The discrepancy between Net Anthropogenic Nitrogen Inputs (NANI) and the nitrogen measured in stream exports represents anthropogenic nitrogen that is either stored within the watershed or lost via unquantified pathways. Key storage pools for this nitrogen include living plant biomass, soil organic matter, and groundwater. These pools can be substantial, often containing between 1,000 and 10,000 kg N per hectare. When nitrogen availability increases, meta-analyses show that both plant biomass and tissue nitrogen content can rise significantly, leading to an expansion of this biological storage pool. However, this storage is often temporary, as nitrogen in plant tissue is eventually returned to the soil through decomposition and litterfall.

Soil represents the largest nitrogen reservoir in most ecosystems. The amount of soil organic N is a function of both the soil mass and its nitrogen concentration. While assuming a steady state may be valid for pristine systems over long timescales, this is not the case in landscapes receiving large anthropogenic nitrogen subsidies. Evidence indicates that soil nitrogen pools are increasing due to human inputs, and this accumulation can account for a major fraction of the "missing" nitrogen in budget calculations. This creates a legacy nitrogen problem; even after anthropogenic inputs are reduced, the slow release of stored soil nitrogen can cause elevated stream concentrations for decades, complicating restoration efforts.

Groundwater is another critical, long-term storage pool for nitrogen, primarily in the form of nitrate (NO3-). The total nitrogen stored in an aquifer depends on both the water volume and its concentration. Today, a complex dynamic exists: groundwater volumes are being depleted in many regions due to irrigation, while nitrate concentrations are simultaneously rising. The net effect on storage is unclear, but the implications are profound. If storage is increasing, it creates a significant time lag—often years to decades—between the implementation of conservation policies and observable improvements in surface water quality, as it takes time for the contaminated groundwater to discharge.

How Land Use and Watershed Scale Define Nitrogen Fluxes. The specific sources and sinks of nitrogen in a budget are heavily influenced by the dominant land use. In urban watersheds, the major fluxes are typically the import of food and the export of sewage. Agricultural watersheds dominated by livestock see significant imports of feed, exports of animal products, and substantial ammonia volatilization from manure. In cropland areas, the primary inputs are industrial fertilizers, while the main outputs are harvested crops. Often, the loss of nitrogen to stream discharge is smaller than these large anthropogenic fluxes of fertilizer, food, and feed, highlighting the importance of human transport in the modern nitrogen cycle.

The scale of a watershed fundamentally determines how nitrogen fluxes are classified. For instance, the application of manure is considered an external input only if it is transported from outside the watershed's boundaries. If manure from livestock within the watershed is applied to its fields, it is merely an internal transformation; the original nitrogen source was likely imported feed or fertilizer. Because manure is rarely transported long distances, it is typically not a net input for large watersheds. This underscores the necessity of understanding local processes to create accurate nitrogen budgets.

Atmospheric deposition is also scale-dependent. For larger watersheds, the volatilization of ammonia from internal sources like agriculture may balance the ammonia deposited from the atmosphere, resulting in a net flux near zero. In contrast, smaller watersheds without significant livestock operations but located downwind of agricultural areas are more likely to experience ammonia deposition as a net input. Conversely, deposition of nitrogen oxides is almost always a net input, as these gases are formed during combustion and can be transported long distances before being deposited far from their original source.