Long-term cover cropping improved soil bacterial community and soil multifunctionality in a Carya cathayensis plantation

Chinese hickory (Carya cathayensis Sarg.), a nut tree unique to China, is an important source of high-class nuts and oil. However, the intensive management practices, especially removing all the understory, lead to the soil degradation in Chinese hickory plantation. Thus, cover cropping is applied to improve and maintain soil quality and productivity. Soil multifunctionality (SMF) is a practical index to comprehensively assess different soil environmental factors with complicated interactions. However, the long-term effects of cover cropping on soil bacterial community and SMF in Chinese hickory plantations remain unknown. In this study, a field experiment was conducted in a Chinese hickory plantation in Zhejiang province, China, from 2011 to 2021. The effects of cover crops including oilseed rape (Brassica chinensis, OR), Chinese milk vetch (Astragalus sinicus, CMV), and the mixtures of oilseed rape and Chinese milk vetch (OC) on the soil bacterial community and SMF were compared with no cover crop (NC). We employed high throughput sequencing of 16 S rRNA to study soil bacterial community, and adopted the averaging approach to determine the SMF value based on soil carbon (C) and nitrogen (N) storage, potential enzymatic activity, and active nutrient pool. The results revealed that long-term cover cropping significantly improved soil C and N content, which stimulated the soil potential enzymatic activity. And consequently, soil labile nutrient concentrations were increased, suggesting better soil active nutrient pool. The proliferation of bacteria utilizing easily-decomposed organic matter was therefore promoted. Hence, SMF was significantly improved by cover cropping. And OC effectively combined species-specific functions including nutrient retention and utilization. Consequently, OC showed the best improvement of soil environmental factors including C and N storage, active nutrient pool, and potential enzymatic activity among cover cropping treatments. It is therefore that OC manifested better SMF than sole cover crops strategies. Random forest modeling and partial least squares path modeling (PLS-PM) revealed that soil C storage, pH, and potential enzymatic activity, were dominant predictors of SMF. Additionally, C and N storage, and pH directly influenced bacterial richness and composition positively or negatively. This study provides insight into the effects of different cover crops on the soil bacterial community and SMF in Chinese hickory plantations, and highlighted the great potential of the legume-cruciferae cover cropping strategy on SMF improvement. These findings have implications for the optimization of sustainable forest management practices in degraded forest plantations.