Winter oilseed rape-quinoa rotation and humic acid mitigate continuous cropping obstacles by reshaping rhizosphere microbial communities and root exudate profiles

Continuous cropping of quinoa leads to soil degradation, reduced yield, and crop failure, underscoring the need for sustainable management practices. This study evaluated the effects of winter oilseed rape–quinoa rotation and humic acid (HA) application on quinoa growth, soil enzyme activity, microbial communities, and rhizosphere health under field conditions in Shanxi Province, China. Crop rotation significantly enhanced soil sucrase and urease activity, improved root morphology, increased dry matter accumulation, and enriched beneficial rhizosphere bacteria (e.g., Actinobacteriota , Acidobacteriota , Gemmatimonadota ,), while suppressing pathogenic fungi such as Fusarium and Aspergillus. Rotation increased single-spike weight, grain weight, and overall yield by 23.38%, 30.72%, and 11.73%, respectively. Under continuous cropping, HA improved enzyme activity, reduced toxic root exudates, promoted beneficial fungi, and increased yield by over 16%. Under continuous cropping, HA application increased grain yield by 16.90–17.29%. Redundancy and correlation analyses revealed that soil enzyme activity positively influenced root development, dry matter accumulation, and yield formation through interactions with microbial communities. Overall, the rotation system, especially when combined with HA, offers a sustainable solution to overcome continuous cropping obstacles and enhance quinoa productivity by restoring a healthy soil-microbe-plant interface.