High soil fertility supports greater soil quality, microbial network complexity, and keystone taxa in arable black soil.

The depletion of soil organic matter (SOM) represents one of the most conspicuous hallmarks of Mollisol degradation in Northeast China. This study focuses on typical black soil with high, medium, and low fertility levels to investigate the responses of soil properties and microbial communities to varying soil fertility. We observed a significant decline in the Soil Quality Index (SQI) and key enzyme activities from high to low fertility levels, which mirrored the synchronous depletion of soil carbon fractions (SOM, POC, and MBC). Our results indicated that while microbial alpha diversity peaked in soils with medium fertility, overarching network complexity was highest in environments with high fertility. Compositionally, this robust community structure was primarily associated with significant increases in the abundance of Actinobacterota and Mortierellomycota. Furthermore, network analysis identified specific keystone taxa, including Gemmatimonadaceae, Roseiflexaceae, Penicillium, and Trichoderma, which are closely associated with efficient nutrient cycling and enhanced disease resistance. This study elucidates the microbial taxa associated with high-fertility soils. And by elucidating these underlying microbial associations, this study provides a solid theoretical foundation for restoring degraded black soils through targeted microbiome regulation. Ultimately, our findings highlight promising avenues for developing novel soil conservation technologies centered on core functional microorganisms.