Edge of Field Nitrate Loss and Oxygen-18 Dynamics in a Dryland Agriculture Setting

Four complete water years of nitrate concentrations and oxygen isotope (δ O¹⁸) ratios have been monitored in a tile drain and adjacent soil-water sampling lysimeters beneath a dryland, agricultural field in the Palouse Region of Washington State, USA. The Palouse is semi-arid and is characterized by undulating loess hills and swales drained by ephemeral to perennial streams. Soil type is dominated by silt-loam Mollisols. Our field is subject to typical farming practices and crop rotations, receiving ammonia fertilizer during fall and spring planting at a rate of ∼70kg of nitrogen per acre. Tile drains play an essential role in environmental nitrogen dynamics in this region as they are widely utilized to drain water from lower fields. Nitrate concentrations in tile drainage fluctuate on a seasonal basis ranging from low concentrations of ∼1mg/ NO₃-N/L in late October/November to higher concentrations of 20-30mg NO₃-N/L in late January/February. Rise in concentration occurs rapidly with the onset of high flow conditions differing from a typical dilution effect. Lysimeter waters show similar nitrate patterns ranging in concentration from ~10mg NO₃-N/L to as high as 120mg NO₃-N/L. In the lysimeters, nitrate concentrations are apparently positively correlated with δ O¹⁸ values. Precipitation collected at the field site shows a 12 per mil seasonal variation in δ O¹⁸. A seasonal pattern is also observed in soil water with a smaller range of about 6 per mil. These seasonal fluctuations are not observed in tile drainage, where values vary only 2 per mil with a mean of -15 per ml. We hypothesize that residence-time and scaling effects can explain the δ O¹⁸ fluctuation differences between soil water and tile drainage. The very large nitrate fluctuations throughout the system may be due in part to the seasonality of soil nitrogen cycling.