Effects of planting density on nitrogen accumulation, carbon-nitrogen metabolism, and yield in cotton

Cotton yield formation relies on nitrogen (N) uptake and the balance of carbon (C) and nitrogen metabolism. This study aimed to investigate how optimized plant density modulates N allocation toward reproductive organs during the flowering and boll-setting stages to enhance carbon-nitrogen metabolism and subsequent lint yield. Field experiments were conducted in 2020 and 2021 using the cultivar CCRI 425, with five density gradients (7.5, 9.75, 12, 14.25, 16.5 plants m−2). Logistic and beta growth models were applied to quantify the spatiotemporal dynamics and key parameters of nitrogen accumulation. Concurrently, principal component analysis (PCA) was utilized to reduce the dimensionality of enzymatic data, allowing for an objective evaluation of carbon-nitrogen metabolic intensity and balance. Results indicated that optimized planting density (12 plants m−2) maximized total nitrogen (N) accumulation and enhanced N partitioning toward reproductive organs, which positively correlated with seed cotton yield. Regression analyses demonstrated that yield increased positively with the elevation of overall carbon-nitrogen metabolic intensity (PC1). Furthermore, seed cotton yield exhibited a highly significant parabolic relationship with the sucrose-to-protein (C/N) ratio, a parameter quantifying the metabolic balance indicated by PC2, identifying 1.59–1.60 as an optimal C/N range for yield formation under short season high-density cotton system directly seeded after wheat harvest. Path analysis confirmed that N accumulation consistently drove carbon-nitrogen metabolism across treatments; however, density exceeding the 12 plants m−2 threshold exerted a direct negative regulatory effect on yield due to intensified inter-plant competition.