Maize//Soybean Intercropping Enhances Enzyme Activity and Promotes Carbon, Nitrogen, and Phosphorus Stoichiometric Stability in Red Soil

Red soils suffer from nutrient imbalances and low-phosphorus availability. Rational intercropping plays an important role for increasing crop yield and improving nutrient use efficiency, while its long-term effects on biogeochemical cycles and ecological stoichiometric stability are poorly understood. Based on a 7-year continuous field experiment in low-phosphorus red soil, the soil enzyme activity, soil carbon (C), nitrogen (N), phosphorus (P) and C:N:P content, soil microbial biomass (MBC, MBN, MBP), and their ecological stoichiometric characteristics in maize monoculture (MM) and maize//soybean intercropping (MI) under four phosphate fertilization gradients (0, 60, 90, 120 kg P2O5 hm−2) were investigated. The impacts of continuous MI on soil CNP ecological stoichiometric stability in red soil were studied. The results showed that intercropping significantly elevated the content of soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and microbial biomass (MBC, MBN, MBP). Compared to maize monoculture, the contents of SOC, TN, and TP in intercropping soils increased by an average of 26.01%, 12.08%, and 7.58%, respectively, and soil MBC, MBN, and MBP increased by an average of 40.87%, 29.50%, and 38.34%, respectively, across different phosphate application gradients. Intercropping also significantly enhanced the activities of key C-, N-, and P-cycling enzymes (β-glucosidase, urease, acid phosphatase), increased by an average of 33.47%, 14.69%, and 60.15%, respectively. Most importantly, intercropping substantially improved the stoichiometric homeostasis of the microbial biomass and decreased the homeostasis index 1/H of MBC, MBN, MBP. Continuous intercropping shifted MBN from a sensitive to a strongly homeostatic state, MBP to homeostatic and the MBC/MBP ratio from weakly to strongly homeostatic in red soil. In conclusion, continuous MI in low-P red soil demonstrably increases soil nutrient content, enhances soil enzyme activity, and promotes ecological stoichiometric stability. This system represents one of the optimized cropping models for the synergistic enhancing of soil ecological stability in red soil regions.