Optimizing water and potassium management enhances yield and water-nutrient use efficiency through improved soil nitrogen availability in citrus orchards

Water and potassium (K) management is an important measure to improve fruit production in citrus orchards, but its effects on rhizosphere microenvironment and soil water-nutrient utilization are still unknown. In this study, two degrees of water deficit (20% and 40%) and three degrees of K fertilizer deficit (20%, 40% and 60%) were set at citrus fruit expansion (stage III) and maturation stages (stage IV), respectively. Results showed the III-HWHK treatment increased soil ammonium availability. This effect was attributed to a higher abundance of nitrogen (N) fixation genes ( nifD and nifK ). Additionally, the treatment suppressed denitrification genes ( nirK and norB ) and denitrifiers ( Ochrobactrum , Rhizobium , and Achromobacter ). The IV-HWMK treatment improved soil nitrate availability by increasing the relative abundance of the amoC gene and ammonia-oxidizing archaeal Nitrosocosmicus . Redundancy analysis (RDA) showed that soil pH, NO 3 - -N and available K (AK) were the main factors affecting rhizosphere nitrifiers and denitrifiers. Moreover, III-HWHK and IV-HWMK treatments significantly increased citrus fruit yield, partial factor productivity of nitrogen (PFP N ), partial factor productivity of potassium (PFP K ), and irrigation water productivity (WP I ) by 7.10%-68.83%, 7.10%-68.83%, 8.95%-51.00% and 8.88%-73.42%, respectively, in 2023 and 2024. The structural equation model (SEM) indicated that optimizing irrigation and K fertilizer could increase citrus yield characteristics by improving soil inorganic N availability. Therefore, III-HWHK and IV-HWMK treatments at fruit key growth stages were the best water-potassium management modes to promote citrus production. These findings provide important guidance for increasing soil water-nutrient utilization and improving the rhizosphere microenvironment in citrus orchards in Southwest China.