Determining the effects of nitrogen (N) and phosphorus (P) application on arbuscular mycorrhizal fungi (AMF) communities is important for predicting AMF responses to nutrient deposition. The AMF parameters and soil properties were monitored in karst grassland after 2 years of N and P addition. Then, AMF abundance, diversity, and community composition significantly differed between seasons. AMF abundance was higher in July (summer) than in December (winter), whereas richness and Chao1 estimator values showed the opposite results. The numbers of the genera Funneliformis and Sclerocystis were significantly more abundant in December, but the proportions of Scutellospora, Redeckera, and Diversispora were significantly higher in July. N and NP treatments significantly increased AMF abundance; richness and Chao1 values in the P treatment were significantly higher than those of the control in July. AMF community composition changed substantially between December and July but did not respond to fertilization. AMF abundance was significantly correlated with total N (TN), while AMF richness was also significantly correlated with available P (AP) and pH. pH and nitrate N (NO 3− -N) strongly affected AMF community composition. These results suggested that P became more limiting with N fertilization, AMF investment increased access to more P, and richness was lower when certain AMF taxa (Diversisporales) increased in abundance during the growing season and under more P-limiting conditions. These results also suggested that N and P addition have specifically different effects on AMF abundance and diversity, and consequently potential effect on long-term vegetation composition and productivity.