A nitrogen fertilizer strategy for simultaneously increasing wheat grain yield and protein content: Mixed application of controlled-release urea and normal urea

Increasing grain yield and protein content simultaneously in wheat (Triticum aestivum L.) is challenging. Efficient nitrogen (N) management strategies, such as mixed application of controlled-release urea and normal urea (CRUNU), has been considered as a potential method for overcoming this challenge. Here, we tested CRUNU application in the winter wheat variety, Xiaoyan22, during three growing seasons (2017–2020) to examine its effects on photosynthetic characteristics, grain filling, dry matter, and N accumulation and remobilization. We aimed to determine the effect of CRUNU application on wheat grain yield and protein content and its underlying mechanisms. The results from the three wheat-growing seasons showed that CRUNU application significantly increased grain yield and protein content at three N application levels, with an average increase of 3.89% in grain yield and 6.35% in protein content compared with the results obtained using normal urea (NU). The main determinant for the increased grain yield was the increase in apical grain weight, which was mainly due to the improvement in grain filling. At the three N application levels, increased N accumulation was observed in vegetative organs (pre-anthesis) following CRUNU application compared with NU application; however, CRUNU application did not improve the contribution rate of pre-anthesis N accumulation in grains. The increase in grain N accumulation and protein content were mainly attributed to elevated post-anthesis N accumulation and its improved contribution to grain N content. Because CRUNU application improved wheat grain yield and protein content without significant variations between the medium and high N application levels, we suggest that CRUNU application, in combination with optimum N application levels, can be considered an effective agronomic management strategy to improve grain yield and protein content in winter wheat.