Submitted to: Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 6/1/2005
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Synthetic fertilizers are routinely applied to more than 90% of the land surface across large dryland-farmed regions of eastern Washington. The environmental partitioning of the nitrogen (N) in these fertilizers is important with respect to the economics of food production, non-point contamination of water resources, and the groundwater-surface water interactions which control the fate of N in runoff. We have monitored more than four water years of nitrate concentrations and stream discharges in nested watersheds of the Missouri Flat Creek basin near Pullman, WA. The drainage areas of these watersheds range from 6 to 6000 ha. Undulating hills and basins of loess and reworked loess dominate the surficial geology; the main soil type is silt-loam Mollisol. Mean annual precipitation is approximately 600 mm, nearly all of which falls from late fall to early spring. Annual nitrate fluxes exiting the watersheds range from 5 to >20 kg (N)/ha, corresponding to approximately 5-20% of typical annualized mean application rates. The ratio of annual nitrate flux to annual runoff increases with annual runoff, i.e. larger flows are generally associated with larger N concentrations which typically reach 15-40 mg (N) /L in late February/March. This pattern is different from those observed in humid regions where most such studies have been conducted. Edge-of-field monitoring of soil water, tile drainage, and ephemeral surface runoff, suggest strong nitrate concentration response to fertilizer application timing in shallow soil and in-field runoff, but not at larger scales. Detailed studies of soil-profile water content dynamics and tile-drain flow rates suggest that high-nitrate soil water is mobilized throughout the profile when water contents near saturation are attained. This process can explain the generation of high-nitrate seepage and drainage to ditches almost immediately at the onset of high flow, once the antecedent profile is sufficiently wet. High flows may also increase contributions of shallow-profile water to runoff by raising water tables. Environmental-tracer (electrical conductivity, oxygen-isotope) dynamics are being studied to test these ideas, which have implications for understanding how management practices affect fertilizer loss and N contamination of runoff.