Regional and Land-Use Variations in Watershed Nitrogen Budgets

The fate of anthropogenic nitrogen varies dramatically across watersheds, influenced by a complex interplay of land use, climate, and hydrology. This results in wide disparities in the proportion of nitrogen inputs that are exported by rivers versus retained or lost as "missing N." For instance, a distinct latitudinal pattern is observed on the U.S. East Coast. Watersheds in the cooler Northeast export a significantly higher percentage of their nitrogen inputs via rivers compared to the warmer Southeast. Research suggests a critical breakpoint around 38°N latitude, corresponding to an average temperature of 10-12°C. In the warmer southern watersheds, higher rates of denitrification are believed to convert a larger fraction of nitrogen inputs into gaseous forms, leaving less to be transported by streams—a finding with profound implications for managing nitrogen under a warming climate.

On the U.S. West Coast, the relationship is dominated by hydrologic variability. Studies there have found riverine nitrogen output ranging from a minimal 2% to over 115% of inputs. The highest export rates occurred in small, frequently logged watersheds in Oregon with very high rainfall runoff. When these outliers are excluded, export rates fall below 20% of inputs, similar to Northeastern watersheds. Unlike the East, streamflow volume was the primary predictor of nitrogen export on the West Coast, highlighting that in these hydrologically variable systems, water availability can be a greater limiting factor for nitrogen transport than the input load itself. This underscores the critical role of regional hydrology in shaping nitrogen budgets.

The Dominant Role of Agricultural Management. Agricultural watersheds demonstrate how human-engineered water transport directly controls nitrogen fate. In heavily tile-drained landscapes of the U.S. Midwest, studies have shown that nearly 100% of net nitrogen inputs can be exported to streams. This occurs because tile drains provide a rapid, subsurface pathway that bypasses the soil matrix, short-circuiting natural removal processes like denitrification. Conversely, in a rice paddy-dominated system in China receiving high nitrogen inputs, stream export was a mere 1.5%. The prolonged flooding of paddies creates ideal anaerobic conditions for denitrification, effectively removing nitrogen within the landscape itself. This stark contrast between tile-drained and flood-based agriculture illustrates how management practices can drastically alter the nitrogen cycle.

Nitrogen in Forested and Urban Systems. Even forested watersheds with relatively low nitrogen inputs exhibit significant nitrogen retention. In a classic aggrading forest at Hubbard Brook, New Hampshire, over 80% of nitrogen inputs were unaccounted for in streamflow, likely stored in growing biomass and soil organic matter. Studies in other forested catchments have confirmed that a large fraction of deposited nitrogen is retained, though accurately measuring all inputs, such as dry deposition, remains a challenge. Similarly, urban and suburban watersheds show substantial retention. Research from the Baltimore Ecosystem Study found that stream nitrogen output represented only 25% of inputs in suburban areas, compared to 56% in agricultural and 5% in forested watersheds. This indicates that engineered landscapes, like their natural counterparts, can store or transform a significant portion of anthropogenic nitrogen.

In conclusion, the "missing N" in watershed budgets is not truly lost but is primarily accounted for by two processes: long-term storage in soils and groundwater, and gaseous loss as N2 and N2O via microbial activity. The balance between these fates is determined by local conditions. For example, deep soil storage has been shown to close the budget in agricultural fields, while groundwater carrying dissolved gases can account for missing nitrogen in mixed-use rural watersheds. Ultimately, a combination of storage and gas loss likely explains the discrepancy in most systems, though the specific conditions favoring one pathway over the other remain an active and critical area of research for improving water quality predictions and management.