Quantifying Watershed Nitrogen Budgets: Inputs, Outputs, and Storage Dynamics

Watershed N budgets are invaluable tools for comparing and balancing the complex array of nitrogen processes within an ecosystem. These budgets are systematically defined by their boundaries, inputs, internal storage pools, and output terms, as illustrated in Figure 2. The defined boundaries establish the spatial system for analysis, and its properties—such as area, depth, and volume—enable the quantification of all nitrogen fluxes. Inputs represent nitrogen moving across a boundary into the system, while outputs constitute nitrogen leaving the system; internal storage quantifies the mass of nitrogen held within various ecosystem components. While internal transformations, such as soil nitrification, can be measured, they may not directly impact the overall watershed budget if they do not result in a flux across its boundaries.

Figure 2. Conceptual model of a watershed with lateral boundaries defined by the watershed drainage area and the vertical boundaries defined as the maximum plant height and the first and second aquicludes. The airshed, however, is an area larger than the watershed boundary shaped by prevailing winds and which contributes to wet (precipitation) and dry (dust) deposition within the watershed. Emissions from the watershed also enter the airshed and may deposit within (recycling) or outside the watershed (output)

A fundamental challenge in constructing these mass balances is that not all budget terms can be measured directly. However, the direct quantification of certain key fluxes often allows for the estimation of others through mass balance principles. Typically, watershed N budgets are calculated on an annual timescale, expressed as the difference between the sum of all input terms (ΣI, in kg N ha−1 yr−1) and the sum of all output terms (ΣO, in kg N ha−1 yr−1). This relationship is captured by the central mass balance equation:

dS∕dt = ΣI − ΣO (1)

where S represents the total nitrogen storage within the watershed (kg N ha⁻¹) and t is time in years. This equation provides a mathematical framework for assessing the system's nitrogen status.

The term dS/dt is a critical indicator of the system's biogeochemical status. This conceptual framework is analogous to familiar everyday budgets, such as a financial bank balance or a caloric balance. For instance, if an individual's caloric intake consistently exceeds their energy expenditure, they will gain weight, indicating a positive energy balance. Conversely, if they consume fewer calories than they burn, they will lose weight, indicating a negative balance. A stable weight results from a balance between input and output, where the "storage" of energy remains constant. This same logic applies directly to the nitrogen cycle within a watershed.

Applying this approach, we can interpret watershed nitrogen dynamics clearly. Values of dS/dt greater than zero signify that nitrogen inputs exceed outputs, leading to an accumulation of nitrogen within the watershed's storage pools. In contrast, dS/dt values less than zero indicate that outputs surpass inputs, meaning the watershed is experiencing a net loss of nitrogen and its internal stores are depleting. An example of such depletion can be found in reference [25]. Furthermore, the dominant land use within a watershed exerts a powerful influence on both dS/dt and the magnitude of individual input and output terms. For example, agriculturally dominated watersheds export large quantities of nitrogen in the form of food products to urban centers. Consequently, food-associated nitrogen constitutes a major net output term for agricultural watersheds and a significant net input term for the urbanized watersheds that import the food.