Challenges in Closing the Nitrogen Budget: Measuring Gaseous Losses and Storage

Accurately estimating all components of a nitrogen budget remains a significant scientific challenge. While some terms, like stream nitrogen export, can be measured with reasonable accuracy through frequent discharge and chemistry monitoring, other fluxes are elusive. Inputs such as food and feed can be approximated using county-level agricultural data, and atmospheric deposition is monitored by networks like the National Atmospheric Deposition Program (NADP). However, major uncertainties persist in quantifying two critical fates of nitrogen: biogenic gas fluxes and changes in watershed storage. The inability to measure these terms reliably is the primary reason many watershed budgets fail to balance, leading to the phenomenon of "missing N"—where inputs far exceed the sum of measured outputs.

The Elusive Nature of Gaseous Nitrogen Losses. A primary candidate for the missing nitrogen is its conversion to gaseous forms—dinitrogen (N2) and nitrous oxide (N2O)—via microbial processes like denitrification and anammox. Measuring these gases at the watershed scale is notoriously difficult. A major obstacle is quantifying N2 flux from soils, as it is impossible to detect small microbial productions against the massive 78% background of N2 in the atmosphere. While methods like the Gas-Flow Core Method, which uses a helium atmosphere, can measure N2 from soil cores, they are not easily scaled. The high spatial and temporal variability of denitrification further complicates extrapolation, leading to its characterization as a "miserable process to measure."

Several techniques have been developed to overcome these hurdles, each with limitations. The acetylene inhibition method is a common but flawed approach that blocks the conversion of N2O to N2, allowing for N2O measurement. However, it can underestimate denitrification in saturated soils and may alter microbial activity. The use of 15N-labeled nitrate is a powerful tracer technique but is expensive and difficult to apply representatively across a whole watershed. Analysis of natural isotope abundances in nitrate (15N and 18O) can indicate denitrification has occurred, but it provides an integrated signal that is hard to translate into a quantitative flux for the entire basin.

A more promising approach for aquatic systems involves Membrane-Inlet Mass Spectrometry (MIMS), which precisely measures the N2:Ar ratio to quantify biogenic N2 production. When combined with gaseous tracers like 222Radon to account for groundwater inputs, this open-channel method can effectively measure N2 evading from streams. This measurement integrates denitrification occurring throughout the groundwater flow path and the hyporheic zone, providing a more comprehensive watershed-scale estimate. Despite this advance, a method for directly measuring soil-to-atmosphere N2 fluxes over large areas is still needed to fully close the budget.

Quantifying Nitrogen Storage in Large Pools. The other major fate of missing nitrogen is long-term storage within watershed pools like soil, groundwater, and biomass. Detecting changes in these reservoirs is exceptionally challenging because it requires measuring small concentration changes against very large background pools. For example, a minor change in the average soil organic N concentration, potentially within the margin of analytical error, can represent a massive nitrogen stock when multiplied by the entire soil mass of a watershed. High spatial heterogeneity necessitates intensive sampling to obtain a reliable average.

The assumption that these pools are at steady state is often invalid in human-impacted landscapes. Studies that resample soil cores after decades, like the work of Van Meter et al., have demonstrated that agricultural soils can accumulate significant nitrogen, accounting for a substantial portion of inputs. Similarly, groundwater acts as a vast, slow-moving storage pool, where rising nitrate concentrations can represent a large nitrogen sink. Changes in living biomass also contribute to storage fluctuations. Resolving these changes requires repeated, long-term monitoring of both pool sizes and nitrogen concentrations—a resource-intensive endeavor that explains the current scarcity of comprehensive watershed-scale storage data. Until methods for measuring both gaseous losses and storage changes improve, the fate of a significant fraction of anthropogenic nitrogen will remain uncertain, presenting a critical frontier for environmental research.