Effects of planting density and foliar fertilization on the phyllosphere microbial community of dual-purpose soybean

Soybean (Glycine max (L.) Merr.) is a dual-purpose crop (grain and forage) with considerable economic and nutritional value in modern agriculture. Despite extensive research on soybean cultivation, the synergistic effects of planting density and foliar fertilization on the phyllosphere microbial community structure and its functional diversity remain poorly understood, particularly in dual-purpose soybean production. This study was conducted as a factorial experiment combining three growing seasons (2022, 2023 and 2024), two planting densities (high: 22 × 10⁴ plants ha^{- 1}; low: 10 × 10⁴ plants ha^{- 1}), four fertilization regimes (LCK, no fertilizer; LWF, standard basal fertilizer; LFN, basal + foliar N; LFP, basal + foliar P) and two maturity stages (R4, pod ³/₄-inch long at one of the four uppermost main-stem nodes; R8: 95% of pods exhibiting mature pod color) on the phyllosphere microbial community and dual-purpose (grain and forage) soybean yield. The results showed that the fresh matter yield of the high-density treatment significantly increased by 29.7% compared with that of the low-density treatment (P < 0.05), and the grain yield increased by 20.2% (P > 0.05). Compared with the no fertilizer control, LWF (standard basal fertilizer) was the most effective fertilization treatment under high-density conditions, with its harvest index increasing significantly by 14.6%. The phyllosphere phosphorus content at the R8 stage (3.19 mg g^{- 1}) was lower than that at the R4 stage (3.88 mg g^{- 1}). Low-density planting increased the relative abundance of pathogens such as Alternaria and Fusariella. In contrast, high-density planting enhanced the relative abundances of potentially beneficial bacteria-including Sphingomonas, Quadrisphaera, Methylobacterium-Methylorubrum, and Pseudomonas- and decreased the relative abundance of the pathogen Pantoea. Considering the dual-purpose (grain and forage) yield, plant health performance, and the risk of late-season leaf phosphorus deficiency, we recommend implementing relatively high-density planting combined with foliar phosphorus fertilization in production systems.