Influence of Soil Nitrogen and Water Supply on Canola Nitrogen Use Efficiency

Nitrogen fertilizer requirements for economic optimization of spring canola (Brassica napus L.) production in eastern Washington varies with yield potential. Recent research has revealed that more N is needed per unit of grain (UNR) as yield potential decreases. Because UNR is the inverse of N use efficiency (NUE) at optimal yield, the implication of this research is that canola becomes less efficient at using N as yield potential decreases. Our research goal was to identify the NUE components that contribute to higher potential yields with more available water. In both years of a two-location experiment, grain yield (Gw), grain N (Ng), and N supply (Ns) were significantly greater with increasing available water supply. The NUE component analysis indicated that differences in water-enhanced yields were associated with higher N uptake (plant N [Nt]/Ns) and utilization (Gw/Nt) efficiencies, which in turn were attributed to a higher grain N utilization efficiency (Gw/Ng) component, followed by higher N retention (available N [Nav]/Ns). Differences in grain N accumulation due to a greater availability of water was mostly attributed to greater N retention efficiency. With increasing available water and fertilization, spring canola became more efficient at accumulating (i) grain biomass per unit grain N and (ii) grain N per unit of available N supply. These results emphasize the need to develop breeding and management strategies to improve water use efficiency and to select canola cultivars capable of coping with water stress that limits grain biomass production per unit plant N accumulation.