Whole-stream response to nitrate loading in three streams draining agricultural landscapes

摘要

Physical, chemical, hydrologic, and biologic factors affecting nitrate (NO(3)(-)) 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 NO(3)(-) sources, NO(3)(-) mass balance, in-stream processing (nitrification, denitrification, and NO(3)(-) uptake), and NO(3)(-) 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 NO(3)(-) input. Streambed processes potentially reduced 45 to 75% of ground water NO(3)(-) before discharge to surface water. In all streams, transient storage was of little importance for surface water NO(3)(-) 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 NO(3)(-) was largely independent of nitrification because both stream and ground water were sources of NO(3)(-). Unamended denitrification rates extrapolated to the reach-scale accounted for <5% of NO(3)(-) exported from the reaches minimally reducing downstream loads. Nitrate retention as a percentage of gross NO(3)(-) 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 NO(3)(-) variation. Biotic processing potentially removed 75% of ground water NO(3)(-) at this site, suggesting an important role for photosynthetic assimilation of ground water NO(3)(-) relative to subsurface denitrification as water passed directly through benthic diatom beds.