Mechanisms of mycorrhiza-mediated phosphorus activation in red soil under maize//soybean intercropping

Mycorrhizae play an important role in driving soil phosphorus (P) transformation, while intercropping or phosphate application influences rhizosphere mycorrhizal traits and enzyme activity. However, the mycorrhizal-mediated mechanism through which intercropping promotes P activation in red soil remains poorly understood. Based on a 7-year field experiment, we analyzed the effects of maize//soybean intercropping on maize yield, soil P fractions, mycorrhizal colonization, and the rhizosphere alkaline phosphatase activity (APA) with two plantation models, maize monoculture and maize//soybean intercropping under four phosphate application rates (0, 26.2, 39.3, and 52.4 kg P·hm^-2). We further explored the mycorrhizal-mediated role of intercropping on promoting phosphorus activation. The results showed that maize//soybean intercropping significantly increased maize yield, the proportion and content of liable P pools, P activation, and mycorrhizal colonization. Under the four different P application levels, intercropping increased maize yield by 21.1%, 60.0%, 58.5%, and 44.3%, respectively. The proportion of liable phosphorus pools under intercropping increased significantly by 27.3%, 18.2%, 10.6%, and 9.2%, respectively. The Resin-P content increased by 13.7%, 31.3%, 22.9% and 18.4%. NaHCO₃-Pi content increased by 15.9%, 28.8%, 16.1% and 6.9%. NaHCO₃-Po content increased by 23.8%, 19.5%, 11.8% and 2.6%. The P activation coefficient (PAC) increased by 36.7%, 51.4%, 19.8% and 14.1%. Intercropping increased the colonization rate by 35.2%, 42.9%, 28.8% and 25.9%, hyphal density by 21.7%, 67.5%, 27.5% and 6.0%, spore density by 30.8%, 35.7%, 28.2% and 21.9%, total glomalin by 8.3%, 30.2%, 25.1%, and 17.3%, and rhizosphere APA by 20.6%, 24.6%, 16.8%, and 13.8%, respectively. Random forest analysis indicated that the liable phosphorus pools, Resin-P, NaHCO₃-Po, NaHCO₃-Pi, were the most important factors driving soil P activation. Key factors influencing P fractions, in descending order of importance, were total glomalin, alkaline phosphatase, hyphal density, mycorrhizal colonization rate, and spore density. Structural equation modeling further demonstrated that maize//soybean intercropping promoted P activation primarily by enhancing mycorrhizal colonization and rhizosphere alkaline phosphatase activity, which in turn increased the content and proportion of both liable organic and inorganic phosphorus.