Sources of Watershed Nitrogen Inputs: Natural vs. Anthropogenic Fluxes

Watersheds receive nitrogen inputs from both natural background sources and human activities. Natural sources occur independently of human influence and include biological N fixation by non-cultivated legumes and free-living microbes, atmospheric deposition of nitrogen oxides generated by lightning, and the slow release of nitrogen from rock weathering. In contrast, anthropogenic N inputs are a direct consequence of human industry and agriculture. These include the application of synthetically fixed nitrogen fertilizers from the Haber-Bosch process, atmospheric deposition of combustion byproducts from fossil fuels, cultivation of N-fixing crops like soybeans, and the net transport of food, feed, and waste between regions. In watersheds with significant agricultural or urban land use, anthropogenic inputs typically dominate the nitrogen budget, vastly exceeding natural background levels.

Natural Nitrogen Inputs to Watersheds. Background nitrogen sources sustain ecosystems in the absence of human disturbance. The largest natural source by far is biological N fixation by uncultivated plants and microbes in terrestrial and marine environments. The second most significant natural input is atmospheric deposition, which includes both wet deposition in rainfall and dry deposition of gases and aerosols, with typical global rates of 2-3 kg N ha⁻¹ yr⁻¹. The production mechanisms for this deposited nitrogen include fixation by lightning and the re-emission and subsequent redeposition of volatile organic compounds from natural ecosystems like wetlands. A more recently quantified natural source is rock weathering, which liberates biologically available nitrogen from geological formations. While generally minor (1-3 kg N ha⁻¹ yr⁻¹), this flux can be significantly higher in mountainous regions with nitrogen-rich bedrock, such as the Alps and Himalayas, where rates can approach 10 kg N ha⁻¹ yr⁻¹.

The Rise of Anthropogenic Nitrogen Inputs. Anthropogenic nitrogen fixation has grown to rival, and in many areas surpass, all natural sources combined, effectively doubling the global flux of reactive nitrogen. For most of Earth's history, the nitrogen cycle was driven solely by natural processes. This changed dramatically in the 20th century with the advent of three key human-driven processes: industrial fertilizer production via the Haber-Bosch process, the intensified cultivation of leguminous crops that host symbiotic bacteria, and the fixation of nitrogen during fossil fuel combustion. The Haber-Bosch process, which converts inert atmospheric N₂ into ammonia (NH₃) under high temperature and pressure, was a transformative innovation. It now supports an estimated 50% of the global food supply and has more than doubled world agricultural production since the 1960s.

The scale of anthropogenic inputs is particularly evident at the watershed level. Fertilizer application and N fixation by cultivated legumes on farmland can range from 100 to 300 kg N ha⁻¹ yr⁻¹. Atmospheric deposition, a mix of natural and human-caused nitrogen, varies regionally, with rates of 5-15 kg N ha⁻¹ yr⁻¹ in North America and up to 44 kg N ha⁻¹ yr⁻¹ in parts of Europe. Furthermore, the transport of food and animal feed creates significant net imbalances; agricultural regions like the upper Mississippi basin can experience a net export of -37 kg N ha⁻¹ yr⁻¹, while urbanized areas like the northeastern U.S. see a net import of up to +10 kg N ha⁻¹ yr⁻¹. This creates a fundamental spatial disconnect, where food is produced in rural watersheds but consumed in urban ones, concentrating nitrogen waste in population centers.

Urban Nitrogen Sources and Pathways to Waterways. Anthropogenic nitrogen is not confined to farmland. Urban and suburban landscapes contribute substantially through the fertilization of managed lawns, golf courses, and parks, with application rates ranging from 2 to 100 kg N ha⁻¹ yr⁻¹. Additionally, combustion in power plants and vehicles fixes atmospheric nitrogen, which is then deposited downwind, linking the airshed of one region to the watershed of another. Regulations like the U.S. Clean Air Act have successfully reduced these emissions, demonstrating their significance through subsequent improvements in water quality, as seen in systems like the Potomac River. Once within a watershed, this anthropogenic nitrogen is ultimately discharged to rivers and streams as either point sources or diffuse sources.

Figure 3. Conceptual model of anthropogenic N processes and their potential variations as input or output depending on land use and other watershed properties

Point source pollution originates from discrete, regulated locations like discharge pipes from wastewater treatment plants. In contrast, diffuse pollution (or nonpoint source pollution) originates over broad areas from sources like agricultural runoff and urban stormwater. These diffuse inputs are closely correlated with land use patterns and human population density, making them more difficult to quantify and control. Watersheds with strong anthropogenic influences typically exhibit elevated nitrate (NO₃⁻) concentrations in groundwater and streams. This occurs because added nitrogen undergoes nitrification in the soil, converting it into the highly mobile nitrate ion, which easily leaches into aquatic ecosystems, leading to eutrophication and other water quality issues.