Objective The South China region is abundant in water and heat resources but frequently suffers from seasonal droughts and numerous pests and diseases, which significantly impact the yield and quality of citrus fruits. This study investigated the effects of combined applications of alginate oligosaccharides (AOS), potassium fertilizer (K), and water management on the main citrus varieties cultivated in South China, aiming to provide technical support for high-quality citrus production.

Method Two independent experiments were conducted over two years using 'Jinqiushatangju', 'Seedless orah', and 'Hongjiang orange' as experimental materials. During the fruit expansion and maturation periods, the impacts of root application of AOS and K on citrus yield and quality were examined across four treatment groups: conventional fertilization (CK), root application of K, root application of AOS, and mixed root application of AOS and K (AOS+K). Additionally, the effects of combining AOS+K with water management on fruit yield and quality were studied, with four treatment groups: conventional fertilization (CK), AOS+K with 70%-80% water management (AOS+K+W₈₀), AOS+K with 60%-70% water management (AOS+K+W₇₀), and AOS+K with 50%-60% water management (AOS+K+W₆₀).

Result Compared to conventional fertilization, AOS+K treatment significantly increased the soluble sugar content and per-plant yield of the three citrus varieties by 4.57% to 7.52% and 9.54% to 13.13%, respectively. All indicators were better than those of the single application of AOS or K to the root, indicating a significant synergistic effect between AOS and K. Among the coupling techniques, AOS+K+W₇₀ treatment improved fruit skin color and increased fruit diameter, particularly benefiting 'Jinqiushatangju' and 'Seedless Orah', with respective increases in per-plant yield of 13.69% and 10.65%. This treatment also significantly increased the soluble sugar and vitamin C content of the three citrus fruits, with increase rates ranging from 9.30% to 12.38% and 6.70% to 14.66%, respectively. Electronic nose analysis of volatile substances showed that AOS+K+W₇₀ treatment maximized the relative content of nitrogen oxides, hydrogen sulfide, ethanol, aromatic components, and organic sulfides in citrus, promoting fruit ripening. Moreover, this coupling technique significantly enhanced the economic benefits of citrus cultivation.

Conclusion The combined application of AOS and K through root irrigation with the management of soil moisture content at 60%-70%, is the most effective technique for increasing yield and improving quality in the three citrus varieties.