Net anthropogenic nitrogen inputs to watersheds and riverine N export to coastal waters: a brief overview

► Anthropogenic N inputs represent the dominant sources of N to most watersheds and their coastal waters globally. ► Net Anthropogenic Nitrogen Inputs (NANI) include fertilizer N, atmospheric N deposition, crop N fixation, and N in net human food and livestock feed. ► Research has demonstrated empirical relationships between NANI and N export to coastal waters. ► Rivers typically export 15–30% of NANI to coastal waters; hydroclimatic factors explain much of the variation of NANI export in Europe, North America and China. In recent years, watershed-scale nutrient accounting methods have been developed which provide a simple yet powerful approach to estimate major anthropogenic sources of nutrients to terrestrial and aquatic ecosystems. For nitrogen (N), ‘anthropogenic sources’ include fertilizer, atmospheric N deposition, N fixation by plants (e.g. legumes), and the net import or export of N in human food and livestock feed, and are collectively referred to as Net Anthropogenic Nitrogen Inputs (NANI). Since the development of industrial N-fixing processes early in the 20th century, anthropogenic N inputs have grown to dominate the global N cycle, and have become the main sources of N in most watersheds affected by humans. It is now clear that riverine N transport from human-influenced watersheds to coastal waters is strongly related to NANI, as well as to hydroclimatic variables (precipitation, discharge, temperature) that can affect the amount of N retained in or removed from watersheds. Potential implications for increased N load from NANI include increased eutrophication, loss of species diversity and habitat, and growth of hypoxic areas (‘dead zones’) in coastal waters.