There is little knowledge about the link between the fungal community and soil nutrient availability in the conversion of alkaline grasslands from abandoned farmland to grassland in northeastern China. In this paper, the diversity, composition, and function of fungi, as well as basic soil properties and inorganic phosphorus fractions, were investigated under land-use conversion from maize cropland to alfalfa (AG), Lyemus chinensis (LG), and restored grasslands (RG). The results show that the relationships between Olsen-P and the Pi fractions of Ca ₈-P, Fe-P, Ca ₂-P, and Ca ₁₀-P were close. Significantly elevated available phosphorus (Olsen-P) and available nitrogen (AN) values were found in the 0-10 cm and 10-20 cm soil layers, respectively, in the AG treatment, relative to LG and RG treatments ( P < 0.05). The O-P content at 0-10 cm and Al-P content at 10-20 cm in the RG treatment were the highest. The RG treatment increased the Shannon index of fungi and the abundances of Mortierellomycota, Glomeromycota, and Mortierella, while decreasing the abundance of Cladosporium. The RG treatment greatly reduced plant pathogens and enhanced mycorrhizal and ectomycorrhizal growth. Olsen-P and AN were both positively correlated with plant pathogen abundance ( P < 0.01). Mycorrhizal fungi had negative effects on Ca ₈-P, Fe-P, Olsen-P, and AN ( P < 0.01). In conclusion, land-use conversion from maize cropland to restored grassland could reduce the abundance of some plant pathogens and enhance useful fungi by decreasing the availability of P and N.