Riparian forest effects on nitrogen export to an agricultural stream inferred from experimental data and a model

The effects of riparian vegetation on the reduction of agricultural nitrogen export to streams have been well described experimentally, but a clear understanding of process-level hydrological and biogeochemical controls can be difficult to ascertain from data alone. We apply a new model, Visualizing Ecosystems for Land Management Assessments (VELMA), to further elucidate how riparian forest buffers reduce stream inputs of dissolved nitrogen in runoff from upland agricultural practices in an intensively studied catchment in the Rhode River Watershed along the western shore of Chesapeake Bay, USA. VELMA is a spatially distributed eco-hydrology model that links hydrological and biogeochemical processes within watersheds. Simulated and observed daily stream flow and export of nitrogen (ammonium, nitrate and organic nitrogen) are in generally good agreement for the period of record (2000-2003) for which complete daily stream flow and chemistry data were available. A sensitivity analysis of the model demonstrates its potential for isolating specific hydrologic and biogeochemical controls on attenuation of dissolved nitrogen within the riparian forest zone, and for identifying upland and riparian best management practices concerning water quality. The results indicate that subsurface flow pathways, forest buffer design (width), and management (stand age) have a significant influence on stream nitrogen loads. However, forest buffers alone may be insufficient for achieving water quality standards, particularly where upland fertilization rates approach those often used for intensive agriculture. These insights include a quantitative description of the trade-off between agronomic production and nitrogen export to surface waters, and the relative importance of denitrification in reducing nitrogen export under different upland and riparian management scenarios.