[Response of Microbiological Properties to Short-term Nitrogen Addition in Perennial Alpine Cultivated Grassland]

Removing nitrogen limitation is necessary to increase plant productivity in alpine grasslands. A short-term nitrogen addition experiment was conducted to understand the effects of nitrogen addition on the soil physicochemical properties and microbial community structure of perennial alpine cultivated grassland in the region around Qinghai Lake. From June to August 2022, four N application gradients （T0： 0 kg·hm^-2·a^-1, T1： 22.5 kg·hm^-2·a^-1, T2： 45 kg·hm^-2·a^-1, T3： 67.5 kg·hm^-2·a^-1） were set up in a four-age perennial cultivated grassland, and soil samples were collected from the 0-20 cm soil layer. The soil physical and chemical properties and microbial community structure were determined under the different treatments. The results showed that soil water content （SWC）, soil organic carbon （SOC）, ammonium nitrogen （NH₄⁺-N）, nitrate nitrogen （NO₃^--N）, available nitrogen （AN）, available phosphorus （AP）, total phosphorus （TP）, total nitrogen （TN）, and total carbon （TC） tended to increase with the increase in nitrogen application level and the T3 treatment significantly decreased the soil pH. Nitrogen addition did not significantly alter the Alpha diversity of soil microbial communities and different levels of nitrogen application affected the structure of soil microbial communities to different degrees. The T1 and T2 treatments increased the number of soil bacteria and fungi and T3 treatment decreased the number of soil bacteria and fungi. Nitrogen addition increased the relative abundance of the dominant flora of Actinobacteriota, Zygomycota, Blastococcus, and Gibberella and decreased the relative abundance of the dominant flora of Proteobacteria, Bacteroidota, Sphingomonas, and Claroideoglomus. RDA analysis showed that SOC and soil electrical conductivity （EC） were the key soil physicochemical factors affecting changes in soil bacterial communities, and soil pH was the main factor affecting the distribution of soil fungal communities. Collectively, 67.5 kg·hm^-2·a^-1 may be the optimal level of N application to improve the soil environment of perennial alpine cultivated grasslands.