Effect of post-sowing nitrogen management on co-limitation of nitrogen and water in canola and mustard

In rainfed Mediterranean environments crop yields are limited by nitrogen (N) and water. The concept of water and N co-limitation has been used to describe how these competing resources are allocated during growth, it has been proposed that growth is optimum when water and N are equally limiting. All the published work so far on water and N co-limitation has been done in cereals using single applications of fixed amounts of N. However, delayed and split applications of N at key phenological growth stages can improve N use efficiency and alter the severity of N and water stress during the growth of the crop. The aim of the work reported here was to assess water and N co-limitation in the indeterminate crops canola (Brassica napus) and mustard (B. juncea) under different post-sowing N treatments. Four field experiments were conducted over three years with different cultivars of canola and mustard, under different water regimes, and grown with three N rates (0, 100 and 200 kg N ha−1 as granular urea) applied at different phenological growth stages. The results suggested that yield gaps (Yg; the difference between actual and attainable yield) increased as total stress from water and N (TWN) and the maximum of the two stresses (MWN) increased and declined as the degree of water-N co-limitation increased, whether based on total stress (CTWN) or maximum stress (CMWN). However, seasonal and genotypic variation in the Yg reduction and improvement in water use efficiency (WUE) with the degree of co-limitation were observed for CTWN. Application of N improved the CTWN without having the effect of split N application timing. No relationship was found between N use efficiency for seed yield (NUESY) of canola and mustard and co-limitation indices, which may be due to low N uptake efficiency during the pre-flowering period and low physiological efficiency during the post - flowering period. This study provides the first empirical evidence that yield of canola and mustard is co-limited by water and N under post-sowing N application with seasonal and genotypic variation in response to CTWN. Future studies need to focus on the interaction of pre and post-flowering water and N stresses and their effect on CTWN in devising crop management tools for this environment.