Physical, chemical, hydrologic, and biologic factors affecting nitrate (NO3-) removal were evaluated in three agricultural streams draining orchard/dairy and row crop settings. Using 3-d "snapshots" during biotically active periods, we estimated reach-level NO3- sources, NO3- mass balance, in-stream processing (nitrification, denitrification, and NO3- uptake), and NO3- retention potential associated with surface water transport and ground water discharge. Ground water contributed 5 to 11% to stream discharge along the study reaches and 8 to 42% of gross NO3- input. Streambed processes potentially reduced 45 to 75% of ground water NO3- before discharge to surface water. In all streams, transient storage was of little importance for surface water NO3- retention. Estimated nitrification (1.6-4.4 mg N m-2 h-1) and unamended denitrification rates (2.0-16.3 mg N m-2 h-1) in sediment slurries were high relative to pristine streams. Denitrification of NO3- was largely independent of nitrification because both stream and ground water were sources of NO3-. Unamended denitrification rates extrapolated to the reach-scale accounted for <5% of NO3- exported from the reaches minimally reducing downstream loads. Nitrate retention as a percentage of gross NO3- inputs was >30% in an organic-poor, autotrophic stream with the lowest denitrification potentials and highest benthic chlorophyll a, photosynthesis/respiration ratio, pH, dissolved oxygen, and diurnal NO3- variation. Biotic processing potentially removed 75% of ground water NO3- at this site, suggesting an important role for photosynthetic assimilation of ground water NO3- relative to subsurface denitrification as water passed directly through benthic diatom beds. Copyright ?? 2008 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved.
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Whole-stream response to nitrate loading in three streams draining agricultural landscapes