Black locust (BL, Robinia pseudoacacia) is considered a promising tree species for reforestation due to its great ability to fix nitrogen. However, after two or three coppice-harvesting rotations, the productivity of BL declines. Whether soil microbial communities are affected and how these groups correlate with the nitrogen mineralization process across multi-generation stands remains unclear.

We investigated the composition and structure of free-living nitrogen-fixing microorganisms (diazotrophs) by sequencing the marker gene nifH and compared these results to levels of soil nitrogen mineralization in the bulk soil and rhizosphere in black locust plantations on Mount Tai, China.

The results showed multi-generation BL coppice plantations decreased the total soil nitrogen (N), soil phosphorus (P), soil microbial biomass N (MBN), soil microbial biomass C (MBC), soil nitrification rate (Rn), soil ammonification rate (Ra), and net soil N mineralization rate (Rm), but significantly increased the concentration of soil NH₄⁺-N to maintain sufficient NO₃⁻-N. The dominant species in bulk soil and rhizosphere changed from Rhodopseudomonas (22.62% and 15.76%), unclassified_c_Alphaproteobacteria (22.37% and 29.28%), unclassified_o_Rhizobiales (15.40% and 13.31%), Bradyrhizobium (12.00% and 11.74%) in seedling plantations to Bradyrhizobium (45.95% and 47.86%) and Rhodopseudomonas (43.56% and 41.84%) in coppice plantations, respectively. Mantel test and Redundancy analysis (RDA) revealed that Rn, Ra, and Rm were the most important factors shaping the diazotrophic communities.

Our results suggest that the multi-generation BL coppice plantation can homogenize soil diazotrophic communities, which is mainly regulated by the available N loss caused by nitrogen mineralization. Strengthening the management technology of coppice plantations will provide more beneficial external consumption.