Ecological stoichiometry characteristics of soil extracellular enzymes under different citrus ages and analysis of their driving factors

The ecological stoichiometric characteristics of soil extracellular enzymes can be used to evaluate the acquisition of resources and nutrients by microorganisms, sensitively reflect the metabolic characteristics of soil microorganisms, and are important indicators to evaluate soil fertility and microbial activity. Intensive agriculture is characterized by the long-term application of large amounts of chemical fertilizers, which often leads to changes in the composition and activity of microbial communities in the soil. However, the mechanism by which high-intensity citrus cultivation affects the stoichiometric characteristics of soil extracellular enzymes remains unclear. To investigate the effects of intensive agricultural cultivation on the stoichiometric characteristics of soil enzymes and their responses to planting years, this study focused on citrus orchards with varying planting years in the riparian zones of the Three Gorges Reservoir. Rhizosphere soils were collected to measure extracellular enzyme activity related to carbon, nitrogen, and phosphorus cycling. The ecological stoichiometry of extracellular enzymes was used to assess the demand for nitrogen, phosphorus, and other resources from soil microorganisms. The results indicated that with increasing citrus planting years, the content of available nitrogen and phosphorus in the soil increased, particularly with significant accumulation of soil phosphorus. The soil phosphorus content in 30-year citrus orchards was 2.5 times higher than that in 5-year orchards, far exceeding the threshold required for citrus growth. Additionally, in citrus soils with long citrus planting years, other forms of phosphorus also accumulate in large amounts. In citrus soils with different planting years, enzyme activities related to the acquisition of carbon, nitrogen, and phosphorus was different. Increasing the citrus planting year significantly reduced the activity of soil enzymes related to carbon and phosphorus cycling, while increasing nitrogen acquisition enzyme activity. From a functional gene perspective, the abundance of the phoD gene encoding alkaline phosphatase significantly decreased from 4.84×10⁷ copies·g⁻¹ to 9.24×10⁶ copies·g⁻¹. Decrease in the abundance of soil functional microbial genes directly contribute to the reduction in the alkaline phosphatase activity. The stoichiometric characteristics of soil enzymes also changed with increasing citrus planting duration, and the enzyme vector model revealed that the soil enzyme vector angle decreased from 58.21° to 18.70°, indicating a transition in soil microbial nutrient demand from phosphorus to nitrogen limitation. Soil microbial communities in the 5-year citrus orchards were primarily P-limited, whereas those in the 30-year orchards were primarily N-limited. In the process of high-intensity citrus planting, it is necessary to reduce the application of phosphate fertilizers and increase the input of carbon sources to promote phosphorus utilization through carbon and alleviate microbial nutrient limitations. Alkaline conditioners should be appropriately added to soils with high citrus planting years to alleviate soil acidification. These findings provide a theoretical basis for improving soil quality and sustainable management of orchards under intensive citrus cultivation.