Seasonality in flux of nitrogen to streams and atmosphere in a semi-arid and dryland agriculture dominated watershed

Background/Question/Methods

Understanding how anthropogenic nitrogen sources are cycled during transport from agricultural systems to aquatic and atmospheric systems is essential to complete nitrogen mass balance approaches and in improving nitrogen management. This project is part of a longer-term effort to quantify the fate of nitrogen soil-water-atmosphere components of an agriculture to surface water continuum and the role streams play in mitigating nitrate loading in a semi-arid region characterized by deep soils, wet winters and dry summers. Previous work at the Washington State University Cook Agronomy Farm and adjacent watershed has shown large seasonal variation in nitrate concentrations (0.5 – 45 mg/L NO₃^--N), as well as reductions in nitrate concentrations as water moves from smaller to larger catchments in the watershed. Here we address the mass flux of DIN from these watersheds, seasonal variability of this flux and the relative contribution from land use types in the basin. We also compare the dissolved fraction of exported DIN with gas flux from water to the atmosphere.

Results/Conclusions

A 12 year record of DIN mass discharge shows DIN export from the watershed occurs primarily during the winter season and annual averages are strongly influenced by infrequent, but large-impact, storm events. As much as 5% - 20% of the 150-200 kg N ha^-1 applied as fertilizer is exported from the terrestrial system as nitrate leaching to streams. This export occurs largely during cold months and at high discharge when physical controls on biological activity make in-stream retention least likely. The timing of nitrous oxide export from tile drain, or deep-soil, water occurred later in the season than DIN export, peaking in late spring and tile drain N₂O concentrations exceeded in-channel N₂O. This may be an indication that upland soil thawing changes water flow path to include deeper soils. We hypothesize that N₂O production in soil during winter months and exported to the stream may be a larger contributor of aquatic-atmospheric N₂O exchange than in-channel production.