Plant litter quality regulates soil eco-enzymatic stoichiometry and microbial nutrient limitation in a citrus orchard

The impacts associated with litter decomposition in intensive cropping on eco-enzymatic stoichiometry and microbial nutrient limitation are still under debate. To evaluate the dynamics of carbon (C), nitrogen (N), and phosphorus (P) in response to different litter additions, we investigated the variations in soil nutrients (nitrate N, ammonium N, available P), carbon mineralization rate, soil microbial biomass, and extracellular eco-enzymatic activities (alkaline phosphatase enzyme, AP; β-1,4-glucosidase, BG; β-1,4-N-acetyl-glucosaminidase, NAG; l-leucine aminopeptidase, LAP) of soils inoculated with different orchard litters (citrus leaf and alfalfa leaf) and their mixtures in a microcosm experiment. Litter addition improved soil nutrients and microbial biomass. During the plant litter decomposition process, soil BG and AP continuously increased in the first 7 days and then decreased, while NAG and ALP reached a peak at the 14th day and then continuously reduced. Litter type was the main factor controlling eco-enzymatic activities and the ratios of eco-enzymatic activities (stoichiometry). The additions of alfalfa leaf enhanced soil nutrients and eco-enzymatic activities more than citrus leaf alone. Enzyme vector analysis revealed that soil microbial communities were co-limited by C and P during litter decomposition. Enzyme vector analysis showed that all vector angles were > 45°, suggesting that soil microorganisms were more limited by P than by N during the decomposition process. Random forest model revealed that litter type, cumulative mineralized C, and microbial biomass were the best predictor of enzyme vector angle. The present study indicates that soil microbial activity (microbial biomass and enzyme activities) was sensitive to litter decomposition in orchards, and litter additions would relieve microbial nutrient limitations in orchards. Alfalfa litter played positive roles in improving soil nutrients and regulating nutrient limitation in citrus orchards, which would help manage highly intensive cropping and develop organic orchards for smart farming.