Objectives To explore the impact of soil amendments, as well as their combined use with green manure, on soil fertility and the yield and quality of citrus fruits.

Methods A field experiment was conducted from 2022 to 2024 in Shuitianba Township, Zigui County, Yichang City. The experiment involved 10 treatments: five single applications, including a control (CK1), biochar (BC1), calcium magnesium phosphate fertilizer (CM1), microbial inoculant (MA1), and earthworm castings (EW1); and 5 combined treatments incorporating green manure—with the same amendments (e.g., CK2, BC2, CM2, MA2, and EW2). Soil and fruit samples were collected annually during the citrus harvest season to assess changes in soil pH, nutrient availability, fruit yield, and quality.

Results Results from the three-year field experiment demonstrated that both single application of soil amendments and green manure–amendment combinations alleviated soil acidification to varying degrees, enhanced soil fertility, and improved citrus yield and fruit quality. Compared with CK1, single amendment treatments increased soil pH by an average of 0.17 units after three years, accompanied by overall increases in soil organic matter and available nutrient contents, with the EW1 treatment exhibiting a greater and more stable pH improvement. Relative to CK1, four green manure–amendment treatments increased soil pH by an average of 0.08 units over three years, with the largest increase observed in the MA2 treatment. Across these treatments, soil organic matter, alkali-hydrolyzable nitrogen, available phosphorus, available potassium increased by 33.3%, 9.3%, 35.2%, and 19.1%, respectively. Soil available iron, available manganese, and available zinc increased by an average of 143.7%, 34.6% and 84.4% respectively over the past three years. Compared with the corresponding single amendment treatments, green manure–amendment combinations were more effective in enhancing soil organic matter, alkali-hydrolyzable nitrogen, available phosphorus, available potassium, and available iron; the increase in pH was slightly lower than that of the amendment-only treatment. From 2022 to 2024, the mean yield increase rates under single amendment treatments and green manure–amendment treatments were 16.6% and 14.0%, respectively, both exceeding that of CK1 (7.4%). Fruit quality parameters were improved to varying extents across all treatments. Over the three-year period, the average yield per tree under green manure–amendment treatments (71.0 kg) was generally higher than that under single amendment treatments (64.5 kg). Under the same amendment type in 2024, green manure application increased juice content, soluble solids content, and vitamin C content by averages of 14.7%, 6.1%, and 16.5%, respectively. Correlation analysis indicated that soil available phosphorus and available iron were strongly positively correlated with yield, soil organic matter and available zinc were positively correlated with soluble solids content, alkali-hydrolyzable nitrogen and available potassium were positively correlated with vitamin C content, while available iron was negatively correlated with titratable acidity, identifying these factors as the main environmental drivers of citrus yield and fruit quality.

Conclusions Both the independent and combined use of soil amendments and green manure can mitigate soil acidification, improve fertility, and enhance citrus yield and quality. Notably, treatments combining green manure with amendments showed superior effects on soil fertility improvement and citrus yield and quality. Among all treatments, the green manure–microbial inoculant and green manure–earthworm castings treatments achieved the most comprehensive improvements.