Water and nitrogen movement through a semiarid dryland agricultural catchment: Seasonal and decadal trends

Water and nutrient budgets in dryland agroecosystems are difficult to manage for efficiency and water quality. This is particularly true where complex terrain and soilscapes interact with pronounced hydrologic seasonality. The purpose of this research was to understand water and hydrologic nitrogen (N) export from a hillslope dryland agroecosystem in a semiarid region where most precipitation occurs outside the growing season. We studied 13 years (2001–2013) of records of water and N inputs and outputs from a 12 ha no‐till artificially drained catchment in the semiarid Palouse Basin of eastern Washington State, USA. Fall‐ and winter‐dominated annual precipitation averaged 462 mm. About 350 mm went to evapotranspiration; crops used ~160 mm from stored soil water during the summer dry‐down season. Soil water replenishment after crop senescence, during the fall wet‐up season, delayed the threshold onset of the high‐discharge season until December. Winter‐dominated drainage fluxes averaged 111 mm or 24% of annual precipitation. Nitrate export in drainage averaged 15 kg·N·ha^?1·year^?1, which was about 10 times the average rate of dissolved organic N export and 15% of the average rate of N application in chemical fertilizer. Fertilizer applications to the catchment were reduced, due to cropping changes, by 1/3 during the last 5 years of the study; however, no corresponding reduction was observed in the nitrate export flux. This lack of change could not be attributed to mineralization of the soil‐organic N legacy of fertilization nor to hydrologic lag of the catchment. Likeliest explanations are (a) despite the reduction, N application continued to exceed crop uptake and accumulation in organic matter; (b) seasonal and interannual variability of catchment connectivity resulted in year‐to‐year field‐scale nitrate storage and carryover. Water and N use efficiencies observed here may be near maximum obtainable for existing crops in this climate. Substantial improvements that would also address multiple environmental issues associated with the N cascade may involve shifts to perennial systems and/or rotations in which N is fixed biologically.