Organic fertilizer substitution synergistically improves the fruit-soil-bacteria system in Newhall navel orange under long-term management

• Long-term substitution of chemical fertilizers with organic fertilizers can improve the quality of citrus fruits. • 50 % organic nitrogen substitution is the appropriate proportion for promotion in Newhall navel orange production. • Long-term organic substitution promotes greater contribution of rhizosphere microorganisms to active SOC. • Interactions between active SOC fractions and bacterial communities in rhizosphere soils are key contributors. The Newhall navel orange ( Citrus sinensis Osbeck cv. Newhall) is one of the most popular citrus varieties globally. Substituting chemical fertilizers with organic fertilizers is regarded as a key agronomic measure to promote the green cultivation of citrus. However, insufficient data on relevant field effects hindered the optimization of the organic substitution model in Newhall navel orange cultivation. Here, based on an 8-year field experiment, we used fermented liquid rapeseed cake fertilizer as the type of organic fertilizer to explore the long-term effects of substituting chemical fertilizers with organic fertilizers of different nitrogen contents on the fruit-soil-bacteria system of Newhall navel oranges, aiming to evaluate the interrelated responses of the rhizosphere microecology and fruit quality of Newhall navel orange to the fertilizer substitution strategy. The results showed that rhizosphere soil properties (pH, available nutrients, organic carbon fractions, and enzyme activities), microbial characteristics (bacterial diversity and richness), and fruit quality (soluble solids, vitamin C, and sugar content) exhibited positive responses to long-term organic substitution. Microbial biomass carbon and dissolved organic carbon showed significant positive effects on fruit soluble solids, vitamin C, and total sugar content. Long-term organic substitution significantly enhanced the activity of rhizosphere carbohydrate-active enzymes, with Proteobacteria played a positive role in regulating carbon metabolism. Comprehensive analysis of all indicators indicated that 50 % organic nitrogen substitution is an appropriate proportion for promotion in Newhall navel orange production. These observations provide a basis for establishing an effective fertilization model for Newhall navel oranges.