Grassland degradation affects the response of soil bacterial and plant but not fungal diversity to nitrogen addition

Abstract

1. Atmospheric nitrogen (N) deposition poses serious threats to plant and soil microbial communities in natural grasslands, which can have consequences on ecosystem functioning. However, the effects of atmospheric N deposition can be modulated by grassland degradation, defined here as soil erosion, soil nutrient decline and loss of plant diversity, which has become a global threat to the functioning of these ecosystems. It remains poorly understood whether the responses of plant and soil microbial communities to N deposition vary with grassland degradation. 2. We conducted a 9-year multilevel (0, 10, 20, 30, 40, 50 g N m−2 year−1) N enrichment experiment at four sites in Inner Mongolia, China. We evaluated grassland degradation using a compound index (grassland degradation index) and classified grassland sites into four categories: non-degraded, moderately, severely and extremely degraded grasslands. One site was selected for each category, and then three replicates were set up within each site. N was added in the form of urea. 3. We first observed that the response of soil bacterial diversity to N addition was stronger in degraded grasslands than in non-degraded grasslands, whereas the response of soil fungal diversity was not affected by degradation. Meanwhile, N enrichment had a weaker impact on plant diversity in degraded grasslands compared to non-degraded grasslands. The controlling factors that affected the responses of soil microbial diversity and community composition to N addition were inconsistent among grasslands with different degradation status. Soil pH was the main driver in non-degraded grasslands, and plant community composition became an important driver in degraded grasslands. 4. Synthesis. Our findings highlighted that grassland degradation affected the response of bacterial and plant communities to N deposition and provided new information for the management and conservation of biodiversity in degraded grasslands.