Optimizing Nitrogen Management for Sustainable Wheat Production in Semi-Arid Subtropical Environments: Impact on Growth, Physio-Biochemical, and Yield Attributes

Nitrogen fertilization plays a critical role in promoting plant growth, improving physiological and biochemical traits, and enhancing wheat productivity. This study aimed to evaluate the effects of seven nitrogen (N) doses—0, 45, 90, 135, 180, 225, and 270 kg N ha−1—on wheat performance under semi-arid subtropical conditions over three consecutive growing seasons (2015–2018). A randomized complete block design (RCBD) was used to assess a comprehensive set of agronomic, physiological, biochemical, and nutritional parameters. Key factors examined included grain yield, above-ground biomass, nitrogen harvest index, total nitrogen content in grains and straw, and physiological traits such as photosynthetic rate, stomatal conductance, transpiration rate, and the accumulation of soluble proteins, sugars, and amino acids. The study also incorporated multivariate statistical techniques, such as multi-trait genotype–ideotype distance index (MGIDI), principal component analysis (PCA), and descriptive statistics to identify the most effective nitrogen dose. Results indicated that 180 kg N ha−1 (T4) was the most effective treatment for improving wheat growth, physiological efficiency, and grain yield, with 135 kg N ha−1 (T3) also showing favorable outcomes. In contrast, higher doses (225 and 270 kg N ha−1) led to diminished performance, suggesting a threshold beyond which nitrogen becomes counterproductive. These findings support 180 kg N ha−1 as the optimal dose for maximizing yield and biochemical quality while contributing to more sustainable and profitable wheat production.