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Foliar nitrogen (N) application is an effective strategy to improve protein content and quality in wheat kernels, but the specific effects of N forms remain unclear. In a two-year field study, foliar application of various N forms (NO3-, urea, NH4+) at anthesis was performed to measure their effects on wheat grain protein accumulation, quality formation, and the underlying mechanisms. Foliar application of three N forms showed varying effects in improving grain gluten proteins and quality traits. Under NH4+ application, there was more post-anthesis N uptake for grain filling, with relatively strong increase in enzyme activities and gene expression associated with N metabolism in flag leaves at 8–20 days after anthesis (DAA), whereas its promotion of grain N metabolism became weaker after 20 DAA than those under NO3- and urea treatments. More N was remobilized from source organs to grain under treatment with foliar NO3- and urea. Genes controlling the synthesis of gluten protein and disulfide bonds were upregulated by NO3- and urea at 20–28 DAA, contributing to increased grain protein content and quality. Overall, foliar applications of NO3- and urea were more effective than those of NH4+ in increasing grain N filling. These findings show that manipulating the source–sink relationship by reinforcing grain N metabolism and N remobilization is critical for optimizing grain protein accumulation and quality formation.