Quantifying in-Situ Nitrate and Ammonium Production in Response to Site-Specific Setting in No-till Dryland Wheat Agroecosystems of the Pacific Northwest.

Laboratory studies of mineralization potential do not reflect in situ processes occurring in soils. Reduction of agricultural nutrient pollution will require process level understanding of the seasonal dynamics of reactive nitrogen across heterogeneous landscapes. Improving our understanding of environmental and biotic controls on nitrogen transformations is imperative to formulating successful mitigation strategies.

This study investigates in-situ ammonification/nitrification rates utilizing mineralization cores at multiple soil depths as part of a five-year Site-Specific Climate-Friendly Farming (SCF) project. In-depth accounting of nitrate and ammonium production and flux was possible via a six bag mixed-bed ion exchange resin system. Soil cores (7.5 cm diameter by 15 cm deep) were isolated from the surrounding soil by three resin bags sealed in the top and bottom of individual plastic cylinders. Twenty-four locations were selected based on statistical clustering of primary and secondary topographic variables. Surface soil-resin cores were installed in twelve locations in two commercial direct-seed wheat fields immediately after spring planting and removed before harvest. Subsurface soil-resin cores were installed at three of the locations in addition to the surface cores and left in the field for a full water year to assess nitrate leaching beyond the rooting zone.  Crop yield, soil moisture, and additional environmental variables (pH, remote sensing and terrain indices, electromagnetic induction and proximal soil sensing data) will be correlated with C and N availability and site-specific mineralization rates. This information will be used to evaluate the usefulness of current nutrient testing and management strategies.