Long-term organic fertilizer and biofertilizer application strengthens the associations between soil quality index, network complexity, and walnut yield

Although various combinations of organic-inorganic fertilizers can enhance plant yields, the effects of different fertilization regimes on the associations between rhizosphere soil microorganisms and plant productivity are poorly understood. In this study, we investigated the rhizosphere soil microbial composition and structure and the relationships among rhizospheric microorganisms, soil properties, and yield in a walnut (Juglans regia L.) orchard after 7 years of applying different fertilization regimes. The combinations of organic and inorganic fertilizers were found to enhance walnut yield, soil quality index (SQI), and soil biodiversity (bacterial and fungal) (P < 0.05). The soil microbial community composition and co-occurrence networks of rhizosphere soil (RS) and bulk soil (BS) showed different responses to fertilization. Bacillus and Pseudomonas were mainly enriched in RS, whereas Fusarium was significantly more abundant in BS (P < 0.05) under a combined chemical fertilizer, organic fertilizer, and biofertilizer treatment (F + O + BO). Network analysis revealed that organic fertilizer and biofertilizer reduced the competition and network complexity among rhizosphere microbial communities (edges: RS = 388, BS = 967; modularity: RS = 0.790, BS = 0.834). Furthermore, SQI, microbial diversity, and network complexity were significantly correlated with walnut yield (P < 0.01), among which SQI explained the highest proportion of the variation in yield (SOI: RS = 78.02%; BS = 81.18%). In addition, both yields and soil microbial community composition were significantly affected by pH, SOM, TN, and SMBN. Collectively, these findings have important implications for our understanding of the relationship between woody plants and rhizospheric microorganisms.