Effects of nitrogen addition on forage production performance and nutritional quality of perennial alpine cultivated grassland

To investigate the effects of nitrogen form and application rate on forage productivity and nutritional quality of perennial alpine cultivated grassland, a nitrogen addition experiment was carried out in June 2022 using a 4-year-old cultivated grassland. Three types of nitrogen fertilizers, urea (amide nitrogen, U), ammonium sulfate (ammonium nitrogen, A), and calcium nitrate (nitrate nitrogen, N), were used in the experiment, and four nitrogen application levels (0, 22.5, 45, and 67.5 kg·hm⁻²·a⁻¹) were set. Parameters such as aboveground biomass, forage crude protein content, and relative feeding value were measured and analyzed across treatments. Results showed that nitrogen application rates had highly significant effects (P<0.001) on community aboveground biomass and forage crude protein content, and significant effects (P<0.05) on forage crude fat content. Nitrogen form significantly influenced aboveground biomass and exhibited highly significant effects on forage crude protein content and crude fat content. Aboveground biomass, forage crude protein, and crude fat content increased with nitrogen application. At the same nitrogen application rate, the promotion effects on aboveground biomass and forage crude fat content followed the order U>N>A, while the promotion effects on forage crude protein content followed U>A>N. Compared to the control (CK) treatment, aboveground biomass under UT3, AT3, and NT3 treatments increased by 43.22%, 26.54%, and 33.11%, respectively. Comprehensive evaluation indicated that the T3 level was optimal for enhancing forage productivity and nutritional quality, with amide nitrogen fertilizer, applied at 67.5 kg·hm⁻²·a⁻¹, showing the best combined performance of production and quality of perennial cultivated grassland.