Field-Scale Evaluation of Enhanced Agricultural Management Practices Using a Novel Unsaturated Zone Nitrate Mass Load Approach
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The monitoring of nitrate mass load through the unsaturated zone below agricultural land was proposed as a novel technique to assess the effect of agricultural best management practices (BMPs). The objectives of the study were to: develop field techniques and apply computational models for the quantification of unsaturated zone nitrate mass flux; scale the point mass flux results to a nitrate mass load across an agricultural parcel; and assess the resulting nitrate mass load measurements as indicators to evaluate the effect of a BMP. At several locations across the study site, groundwater quality and profiles of soil water content and temperature were regularly monitored, and several rounds of geologic cores were collected for analysis of bulk soil nitrate and an applied bromide tracer. The field data were applied in several analytical techniques for estimating recharge, and in two unsaturated zone numerical models used to refine the recharge estimates. The recharge rate at each measurement location was then combined with unsaturated zone nitrate data to quantify nitrate mass flux. Upscaling of the flux values to field-scale mass load was based mainly on topography, geology and field observations. The calculation of stored nitrate mass in the shallow subsurface showed some correlation to changes in surface nitrogen application, with the greatest decreases in stored mass observed at locations underlain by sand where there was a switch to a grass crop. In contrast, the calculation of nitrate mass load suggested that the post-BMP value (4.1 t NO3-N/yr) was greater than the pre-BMP value (2.2 t NO3-N/yr). However, the calculation of nitrate mass load was limited by several factors, including a lack of nitrate concentration data from the deep unsaturated zone and an above-average annual precipitation rate; as a result, the findings suggesting an increase in nitrate mass load in response to decreasing nutrient inputs should be interpreted with caution. Continued monitoring of nitrate mass load and stored nitrate mass in the unsaturated zone is recommended to determine whether further benefits from the BMPs are observed as the measurement period lengthens and the unsaturated zone is progressively flushed.