Comparison of soil microbial responses to nitrogen addition between ex-arable grassland and natural grassland

Purpose The return of arable to natural grassland is an important initiative for grassland restoration and has effects on soil microbes. Nitrogen (N) addition has been documented to significantly affect soil microbial biomass, community composition, and functions in grasslands. However, the difference in microbial biomass, community composition, and functions between ex-arable and natural grasslands and whether soil microbes respond to N addition in a similar way across different land use types remain unknown. Methods We conducted a 3-year multi-level N addition experiment (0, 10, 20, 30, 40, and 50 g N m −2 year −1 ) in natural grassland (NG) and ex-arable grassland (EG) of northern China and measured microbial biomass carbon (MBC) and microbial biomass N (MBN), community composition, and soil extracellular enzymes activity (EEA). Results MBC and MBN in NG were about two times greater than in EG due to the higher soil moisture, but they showed no significant response to N addition in two grasslands. Microbial community composition differed between NG and EG. N addition decreased the biomass of arbuscular mycorrhizal fungi by 20.4% and 2.1% but increased the ratio of gram-positive bacteria to gram-negative bacteria by 12.0% and 6.4% in NG and EG, respectively. The variation in the microbial community composition correlated with soil dissolved N concentration. N addition decreased the soil N-acquiring EEA by 60.6% and 45.4% but increased the phosphorus (P)-acquiring EEA by 48.4% and 133.2% in NG and EG, respectively. Variation in soil N-acquiring EEA was driven by N addition with the increasing soil available N concentration, whereas variations in soil C-acquiring and P-acquiring EEAs were attributed to the land use change via affecting the soil moisture. Conclusion Our study found that the differences in soil microbial biomass, community composition, and extracellular enzyme activity occurred between ex-arable and natural grasslands due to their different soil moisture. The responses of soil microbes to N addition varied between two land use types owing to the different soil available N concentration after N addition. Therefore, land use change had direct effects on soil microbes and indirect effects on soil microbes via regulating the effects of N addition. Our findings are meaningful to accurately assess the N addition effects across different land use types and provide the scientific basis for the restoration of disturbed grassland.