Effects of Biogas Slurry Application on Vegetation Community Restoration in Degraded Grassland

Biogas slurry is rich in nitrogen, phosphorus and bioactive substances, making it an effective material for restoring degraded grasslands. Against this background, we conducted a field experiment in Zhenglan Banner, Xilingol League, Inner Mongolia Autonomous Region, China, from 2024 to 2025, to study the short-term effects of biogas slurry fertilizer on vegetation characteristics and above- and belowground plant traits. The experiment comprised three treatments: a water control (CK), 50% diluted biogas slurry (BS 50% ), and full-strength biogas slurry (BS 100% ). All treatments were applied at a rate of 300 m 3 ·ha −1 , with CK receiving an equivalent volume of water. The biogas slurry contained 0.11% nitrogen (N), 0.07% phosphorus (P 2 O 5 ), and 0.09% potassium (K 2 O). Results showed that, compared with the control, biogas slurry application increased plant height, coverage, and biomass by 8.04–54.00%, 5.48–17.76%, and 18.40–96.01% in the first year, respectively. Plant crude protein and crude fat also increased by 7.33–31.17% and 21.54–30.00%. In the second year, the increases were 26.41–50.22%, 6.16–20.55%, and 13.91–52.42% for plant height, coverage, and biomass and 4.46–28.27% and 14.24–19.89% for crude protein and crude fat, respectively. The carbon, nitrogen and isotope indices of leaves and roots also increased simultaneously. Biogas slurry application altered plant community composition, BS 50% transiently increased plant family richness, BS 100% exerted persistent inhibitory effects, and species diversity across all fertilization treatments showed a recovery trend in the second year. Principal component analysis and redundancy analysis showed that treatment groups were clearly separated in 2024 but overlapped substantially in 2025. Root δ 13 C and root δ 15 N were key indicators distinguishing vegetation community characteristics. The results of this study confirmed that the application of biogas slurry fertilizer could actively improve the vegetation recovery of degraded grasslands. It provided reference support for the resource utilization of biogas slurry fertilizer and the sustainable management of grassland ecosystems.