The Mixed Addition of Biochar and Nitrogen Improves Soil Properties and Microbial Structure of Moderate–Severe Degraded Alpine Grassland in Qinghai-Tibet Plateau

The degradation of grassland system has severely threatened the safety of ecological environment and animal husbandry. The supplement of key substances lost due to degradation is widely used to accelerate the restoration of degraded grassland ecosystem. In this study, we investigated the effects of biochar and nitrogen addition on soil properties and microorganisms of degraded alpine grassland. The experimental treatments consisted of the control without any addition, only nitrogen addition (10 g N m-2), only biochar addition (4.00 kg m-2 biochar), and the mixed addition of biochar and nitrogen (4.00 kg m-2 biochar and 10 g N m-2), respectively. Adding N alone did not significantly change soil pH, total organic carbon (TOC), total nitrogen (TN), microbial biomass (MB) and the composition proportion of microbes, but increased the contents of soil water content (SWC), NH4+-N, NO3--N, available phosphorous (AP), the biomass of bacteria and fungi. The addition of biochar or the mixture of biochar and nitrogen increased the contents of pH, TOC, TN, MB, SWC, NH4+-N, NO3--N, AP, bacteria and fungi in soil, and changed the structure of soil microbial community. The increasing intensity of AP, bacteria and fungi under the addition of biochar or the mixture of biochar and nitrogen was significantly greater than that under N addition alone. These results indicated that the separated addition of nitrogen and biochar and the mixed addition of biochar and nitrogen all improved soil condition of moderate-severe degraded alpine grassland, but the mixed addition of biochar and nitrogen could be a better strategy to remediate the degraded alpine grassland.