Field-Scale Cropping System N Use Efficiency after 10 Years of Continuous No-Tillage.

Nitrogen use efficiency (NUE) is often determined on a single crop for the year it was produced. These NUE calculations usually do not incorporate nitrogen (N) that is internally cycled within soils and residues of the cropping system. Consequently, evaluating NUE for a longer time period that represents the cropping system may provide an improved assessment of NUE. In addition, cropping system NUE may vary spatially across heterogeneous landscapes and soils. Our overall objective was to use an N mass balance approach to better understand how terrain, no-tillage, and the implementation of multiple crop rotations influence cropping system NUE. A multi-year cropping systems study under no-tillage was initiated in 1999 on a 37-ha field of the Washington State University Cook Agronomy Farm near Pullman, WA. In 1999, soil samples were collected from 183 geo-referenced locations in a systematic non-aligned grid across the entire field. Samples were divided in to 10-cm increments for the top 30-cm of the soil and by horizon to a depth of 153-cm. Laboratory analysis included total soil organic N and carbon (C). Crop rotations initiated in the fall of 2000 and the spring of 2001 consisted of six different 3-year rotations of spring wheat – winter wheat – alternative crop (spring or winter plantings of barley, canola, lentil, or pea). In 2008, another set of soil samples were collected from the same geo-referenced locations to a depth of 153-cm and analyzed using the previously described technique. All N inputs from fertilizer applications and N output from harvested grain were monitored at each of the geo-referenced locations. Site-specific, field-scale assessments of NUE for each cropping system will be presented.