Nitrogen Use By Pacific Northwest Dryland Canola (Brassica napus) and Its Effect On Rotational N Balances.

Nitrogen (N) fertility recommendations vary widely within canola production regions including the Pacific Northwest.  Canola has a high N uptake efficiency (unit of total plant N per unit of supplied N) but a low N utilization efficiency (unit of grain per unit of total plant N), leading to an overall low N use efficiency (NUE) (unit of grain produced per unit of N supplied) compared to wheat.  Therefore, canola is able to take up nitrogen from the soil very well, but is poorer at allocating that nitrogen to its seeds. Calculations for estimating the N requirement for canola based upon maximum theoretical yields have proven unsuccessful in our region. Recent research indicates that spring canola can root up to 1.5 m, and efficiently scavenges high levels of residual soil N thereby minimizing responses to N fertilizer. Though rainfall gradient largely determines yield potential of canola in the Pacific Northwest, yields at economically optimum N supply (EONS) are consistently lower than maximum theoretical yields and reached at a relatively lower total N supply. The N requirement of canola at EONS can vary among years, but a single unit N requirement (UNR) of 11 kg N per kg seed was determined by considering multiple years and locations within a rainfall gradient.  In order to better understand the effect of canola fertility on overall N balances, we developed a component analysis of NUE of an entire cropping sequence featuring canola (spring canola-spring pea-winter wheat). This approach provided insight into the propensity of cropping systems to retain and recycle N within a rotation by factoring in crop yields, grain and residue N, fertilizer N, N mineralization estimates, and changes in soil residual inorganic N.. The inclusion of field peas led to positive N balances (N output exceeding N inputs) due biological N fixation.  Interestingly, N balances were also more positive for sequences that received higher rates of N fertilization during its spring canola cropping.  This result suggests elevated N mineralization due to the return of canola residues with higher N concentrations, as well as contributions of fertilizer carry-over to the overall rotational NUE. By tracking changes in soil N supply between crops, the rotational NUE will help us evaluate and adopt alternative cropping systems with the propensity to retain and recycle N within a rotation.