Modified Fly Ash Regulates Enzyme Stoichiometry and Alleviates Microbial carbon-phosphorus Limitation in saline-alkali Soils

Microbial nutrient limitation can be reflected by the stoichiometry of soil extracellular enzymes; however, the regulatory mechanisms by which modified industrial waste fly ash influences microbial metabolic processes in saline-alkali environments remain unclear. This study aimed to clarify how citric acid-lanthanum co-modified fly ash (CLFA) regulates soil properties, microbial communities, and nutrient limitation patterns. A 100-day pot experiment was conducted to investigate the effects of CLFA at varying application rates (5%, 10%, 15%) on soil physicochemical properties, microbial communities, and enzyme activities. Application of 10% CLFA reduced the soil pH and total salt content by 11.56% and 17.37%, respectively, while simultaneously enhancing organic carbon (20.56%), available nitrogen (10.56%), and microbial biomass carbon, nitrogen, and phosphorus content (129.17%, 4.55%, and 191%, respectively). Vector analysis of enzyme activities revealed that application of 10% CLFA shifted microbial resource limitation patterns from phosphorus limitation (control vector angle: 62.14°) to carbon–phosphorus co-limitation (57.51°–62.87°). A 32.69% increase in vector length indicated enhanced microbial nutrient demand and overall increase in metabolic activity. Structural equation modeling demonstrated that CLFA regulated β-glucosidase and phosphatase activities by optimizing carbon-to-phosphorus ratios and enhancing the dissolved organic carbon content. We identified 10% CLFA as the optimal application rate for ameliorating saline-alkali soils. We provide a theoretical foundation and practical guidance for industrial waste utilization and sustainable amelioration of saline-alkali lands.