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This chapter explores the importance and role of watersheds in the transport of nitrogen. Stream N yields have been assessed in watersheds through detailed process-oriented studies at the local scale and over larger, regional scales using statistical techniques. These approaches have been applied to natural and culturally affected environments in watersheds to elucidate the hydrologic and biogeochemical factors that affect N transport. The biogeochemical processing of N has been studied over a range of spatial and temporal scales in watersheds to enable the interpretation of data trends and development of conceptual and numerical models of N yield. Surface and subsurface hydrology, climate, physiography, and basin size all affect the partitioning of precipitation between infiltration and runoff and subsequent water flow paths. Natural and cultural sources of N and their subsequent transformations influence the amount and mobility of N constituents in soil, plant materials, and water. Watersheds represent a physical coupling of the hydrologic and source components in a continuous, dynamic system. Management of land resources based on principles derived from watershed-scale studies is a key component of ongoing efforts to improve the efficiency of N use and limit adverse water quality impacts from excessive N loadings to surface, subsurface, and marine waters. |
