Rotational Nitrogen Use Efficiency: Beyond Single Season Estimates.

Despite opportunities to improve the agronomics and economics of crop rotations, crop diversification in the inland Pacific NW (iPNW) has lagged behind other semi-arid regions of the world. Canola has been identified as a potential rotation crop for agronomic and market diversification, which has stimulated agronomic research to fit canola into the unique cropping systems of Eastern Washington. The present study was conducted to examine the effects of fertilization of spring canola and available water on rotational nitrogen use efficiency (rNUE) of a three-year cropping sequence of spring canola-spring pea-winter wheat at two site locations. The single-season NUE component analysis was extended over multiple seasons to assess the influence of soil and plant N processes on rotational yields. With an increase in water availability, the cropping sequence was more efficient at retaining N in the rooting zone and utilizing N to produce grain. Compared to a single season of winter wheat, the rNUE at optimal yields was slightly lower when spring crops were rotated with winter wheat. The inclusion spring pea in the cropping system resulted in greater levels of residual N and enhanced uptake of N by the subsequent wheat crop by increasing the N supply. Over three years, the rNUE at economically optimal yields varied with water availability and cropping sequence. Therefore, reassessing N fertility is important for integrating spring canola into wheat-based cropping systems due to the differences in nutrient dynamics and residual effects of N fertility of spring canola.