Objective Reducing chemical fertilizer application and improving fertilizer utilization is an effective way to stabilize grain production and mitigate chernozem degradation. The purpose of this study was to clarify the effect of humic acid bio-fertilizer on the improvement of degraded chernozem under maize-legume crop rotation system, so as to provide a reference for the conservation of cultivated land in the black soil area of northeast China.

Method A two-year field trial was conducted on kidney beans with six treatments. The six treatments were: local conventional fertilizer application (CF), conventional fertilizer application reduced by 20% (RF), and conventional fertilizer application reduced by 20% with different amounts of humic acid bio-fertilizer 45.0 kg hm ⁻² (RFH1), 67.5 kg hm ⁻² (RFH2), 90.0 kg hm ⁻² (RFH3), 112.5 kg hm ⁻² (RFH4). The effects of humic acid bio-fertilizer on bulk density, agglomeration, enzyme activities, nutrient contents and microbial diversity indices of tillage soil were determined by studying humic acid biofertilizer on soil physicochemical properties at different fertility periods of kidney bean and post-harvest treatments.

Result The results showed that fertilizer reduction with humic acid bio-fertilizer could reduce soil bulk density, promote the production of soil water-stable agglomerates, increase the proportion of water-stable large agglomerates with particle size > 5 mm, 5-2 mm and 2-1 mm, and further improve soil physical structure. The soil mean weight diameters of RFH2 treatment was increased by 17.6% and 10.6% (P < 0.05) in 2021 and 2022 compared to CF treatment. Soil alkaline phosphatase, sucrase, urease and protease activities were increased by 11.8% - 34.6%, 10.1% - 48.4%, 10.4% - 14.7% and 13.7% - 35.2%, respectively, in the treatment with humic acid biofertilizer compared to the CF control. Analysis of microbial α-diversity and species abundance in soil revealed that a significant increase of 16.6% in bacterial Ace index was observed in the RFH2 treatment, and significant increases of 42.6% and 28.8% in fungal Ace and Shannon indices were observed in the RFH3 treatment compared to the CF. The reduction of chemical fertilizers with humic acid biofertilizer could increase the proportion of bacteria Proteobacteria and fungi Ascomycota. Humic acid biofertilizer application promoted yield formation in kidney beans, with the RFH2 treatment having the highest seed yield in the 2021 and 2022 trials, with significant increases of 24.7% and 21.5%, respectively, compared to the CF treatment.

Conclusion In a comprehensive analysis, the 20% reduction in conventional fertilizer application with 67.5 kg hm ⁻² humic acid bio-fertilizer (RFH2) treatment was the most suitable treatment to improve soil physicochemical properties and microbial diversity in this study.