Synergistic effects of green manure and biochar for a win-win in nitrogen reduction and soil health: insights from multiple assessment frameworks

Integrated nutrient and field management is key to farmland soil health, but evaluation frameworks for improving soil quality index (SQI) through combined organic amendments under reduced nitrogen (N) input are still limited. Hence, we conducted a three-period field experiment to examine the effects of green manure (GM) and its synergy with biochar (GB) on SQI under reduced N input. Results were as follows: (1) GM and GB rapidly and persistently improved the SQI by 66.67–116.67% compared to winter fallow, across both conventional and controlled-release fertilizer (CRF) systems. (2) Under 30% CRF reduction, the GB treatment showed the greatest improvements in water retention (0.92), carbon sequestration (0.68), N fixation (0.67), nutrient supply (0.81), and microbial diversity provision function (0.75) indices and the SQI (0.77). However, 45% CRF reduction posed risks of excessive N cutback. (3) Among three constructed assessment frameworks, the soil functional index-based framework was the most precise, the principal component-based minimum dataset (MDS) best balanced efficiency and accuracy, and the network analysis-based MDS maintained reasonable accuracy with only four indicators. (4) Bayesian regression and structural equation modeling identified soil microbial diversity as the key factor enhancing SQI, with GM and biochar acting principally by boosting this function and soil water retention to indirectly enhance carbon sequestration and nutrient supply. Our findings provide a novel, mechanistic blueprint for achieving synergy between yield stability and soil health restoration, offering a quantitative basis for designing sustainable cropping systems.