Plant growth improvement and microbial community reorganization are decoupled during restoration of trampled urban forest soil

A key question in trampled urban forests is whether improved understory plant growth is accompanied by corresponding changes in the belowground microbial community. We compared ploughing, Bacillus thuringiensis NL-11 inoculation, biochar addition, and NL-11 plus biochar co-application to determine whether improved plant growth was accompanied by parallel bacterial and fungal responses in a trampled Ophiopogon japonicus system. At the one-year sampling, NL-11 and co-application both showed improved plant growth, but their soil and microbial responses differed. NL-11 increased nitrate nitrogen and urease activity. Co-application increased sucrase and urease activities but reduced available phosphorus, available potassium, and average root diameter, showing that improved plant growth under co-application was not accompanied by uniformly greater soil nutrient availability. Relative to ploughing, NL-11 and co-application reduced bacterial richness and Shannon diversity, whereas fungal ACE, Chao1, and Shannon indices did not differ significantly among treatments. Bacterial and fungal community composition differed overall among treatments. Network and assembly analyses showed clear bacterial-fungal asymmetry. Among the restoration treatments, the NL-11 bacterial network descriptively had the largest number of edges and the highest average degree and density, and the NL-11 fungal network descriptively had the largest numbers of nodes and edges. Relative to the ploughing procedural control, NL-11 showed higher bacterial and fungal positive cohesion, whereas biochar showed lower bacterial positive cohesion. iCAMP showed that homogeneous selection was the largest observed bacterial assembly component across all five groups, whereas drift or undominated processes were the largest fungal component; the focused NL-11-versus-ploughing bacterial contrasts did not remain significant after multiple-testing correction. NL-11 and co-application improved O. japonicus growth one year after transplanting, with distinct changes in soil nutrient availability and bacterial and fungal communities.