Nitrogen–phosphorus stoichiometry shapes seedling growth and yield performance of perennial cool-season forage grasses in saline–alkali soil

Grass production faces increasing challenges due to excessive fertilization, which contributes to greenhouse gas emissions and poor soil quality, especially in saline-alkali regions, threatening long-term sustainability. Most previous studies have focused on short-term effects of nitrogen (N) or phosphorus (P) applied individually, with limited understanding of their interactive and long-term impacts. This study investigated the combined effects of N and P fertilization on the seed germination and yield production of perennial cool-season forage grasses under saline-alkali soil conditions. Over three consecutive years, large-scale orthogonal field experiments were conducted with 18 N levels (0-480 kg N ha⁻¹) and 16 P levels (0-240 kg P₂O₅ ha⁻¹) across six grass species to determine the optimal N:P ratio for seed germination, seed yield and yield components. Results showed that P had a stronger impact on early seedling vigor, while N primarily influenced reproductive growth and yield. Species varied in their nutrient responses, with shallow-rooted species favoring low nutrients and deep-rooted species benefiting from moderate to high fertilization. Optimal N: P ratios were around 2:1 for seed germination and 2.86:1 for yield. Excessive or imbalanced N and P reduced growth and yield due to nutrient antagonism and stress. These findings emphasize balanced fertilization strategies to improve nutrient use efficiency and grass productivity in saline-alkali environments.