Nitrogen Use Efficiency and Nitrogen Uptake of juncea Canola under Diverse Environments

Nitrogen accounts for the largest energy input in oilseed production. Understanding N use characteristics of oilseed crops will help improve N use efficiency and minimize production costs. This study determined nitrogen use efficiency (NUE, defined as seed yield produced per unit of N supply), nitrogen fertilizer use efficiency (NFUE, defined as seed yield produced per unit of fertilizer N), and crop N uptake for oilseed crops under diverse environments. Five oilseed crops, namely Brassica juncea, B. napus, and B. rapa canolas, and B. juncea and Sinapis alba mustards, were grown at seven rates of N fertilizer (0, 25, 50, 100, 150, 200, and 250 kg N ha⁻¹), at 11 sites (year × location combinations) in Saskatchewan from 2003 to 2005. At sites with low soil N supply or low rainfall, alba mustard, juncea canola, and rapa canola had lower NUE and NFUE than juncea mustard and napus canola. At sites with high soil N supply or rainfall, napus canola had the greatest NUE and was the most sensitive to the gradient of productivity among the five oilseeds. All oilseed species responded to N fertilizer rates in a similar manner; both NUE and NFUE decreased as N fertilizer rate increased. The minimum NUE and NFUE were obtained with N fertilizer rate greater than 150 kg N ha⁻¹ At sites with low soil N supply or rainfall, alba mustard had the least NUE or NFUE response to increasing N fertilizer rates and napus canola the greatest. At sites with high soil N supply or rainfall, juncea mustard had the least NUE and NFUE response to increasing N fertilizer rates and rapa canola the greatest. On average, seed N uptake was greatest for juncea canola and juncea mustard and least for alba and rapa canola. The five oilseed species had similar response patterns of seed N uptake to N fertilizer rates, while the magnitude of response varied among species. Improving NUE in oilseed production systems requires optimizing rates of N fertilizer which vary depending on environmental conditions, and soil N supply and rainfall during the critical growth period of the oilseed crops play an important role in affecting NUE.