Degradation-dependent shifts in pathways linking straw incorporation and cover crops to soil quality in Mollisols

Soil degradation alters the effectiveness of agricultural management practices, yet how it reshapes the pathways linking management to soil quality remains poorly understood. We evaluated the effects of straw incorporation, cover crops, and their combined application on soil quality and associated pathways across a reconstructed gradient representing slight, moderate, and severe degradation in Mollisols. A soil quality index (SQI) was developed by integrating key indicators representing structural, chemical, and biological attributes using a minimum data set derived from principal component analysis. Partial least squares path modeling was employed to identify the pathways linking soil functional dimensions to SQI across degradation levels. SQI responded significantly to degradation level, management practices, and their interactions. Under slight degradation, straw incorporation, cover crops, and their combinations increased SQI by 21.3–46.1% relative to the control, while aggregate stability was the dominant dimension associated with SQI ( β = 0.88), indicating structural predominance. Under moderate degradation, individual practices did not significantly affect SQI, whereas straw–cover crop combinations increased SQI by 35.5–90.9% relative to the control. Aggregate stability, organic-matter status, and the microbial dimension were positively associated with SQI ( β = 0.40, 0.24, and 0.67, respectively), reflecting a multidimensional response. Under severe degradation, however, neither individual nor combined practices significantly improved SQI. These findings reveal degradation-dependent shifts in the modeled pathways linking management to soil quality. By showing that degradation altered both management effectiveness and the soil dimensions contributing to SQI, this pathway-based perspective clarifies why the same practices perform differently across degradation levels.