•Forest conversion significantly increased the ammonification rate.•Forest conversion decreased the nitrification and mineralization rates.•Forest conversion altered the function of the ectomycorrhizal community.•The ectomycorrhizal community is clearly linked to the altered N dynamics.•Some taxa of ectomycorrhizas are actively driving ecosystem processes. The conversion of natural forests to plantations affects soil carbon (C) and nitrogen (N) dynamics. However, the underlying microbial mechanisms of C and N dynamics caused by forest conversion, particularly the functional role of ectomycorrhizal (ECM) fungi, remain largely unknown. Here, we investigated the soil and root-associated fungal communities, soil and ECM root enzyme activities, and C and N mineralization rates in natural forests and plantations in the western Sichuan subalpine coniferous forest. Soil fungal and root ECM fungal communities were determined by high-throughput and Sanger sequencing, respectively. ECM root surface enzymes were used to assess fungal function, while soil enzymes, C and N mineralization, were used to evaluate soil function. Our results showed that clearing natural forests and converting them to plantations led to lower soil organic carbon (SOC), total nitrogen (TN), and pH, which drove changes in ECM and saprophytic (SAP) fungal communities. After forest conversion, the main difference in the fungal community was an increase in the ratio of ECM to SAP fungi. The most apparent change in the soil ECM fungal community is the shift of the dominant genera from Russula to Cortinarius and Piloderma. Subsequently, the function of the ECM fungal community was altered. The results indicated that the conversion of the natural forest to the plantation reduced ECM community β-glucosidase (βG), β-glucuronidase (βLU), N-acetyl-β-D-glucosaminidase (NAG), and acid phosphatase (AP) activities, and soil βG, NAG, and leucine aminopeptidase (LAP) activities. Among them, the activities of βG, βLU, and NAG in the ECM fungal community were significantly correlated with the activities of βG, βLU, and NAG in soil, respectively. Finally, we show that converting natural forests to plantations significantly increased the ammonification rate while decreasing the nitrification and mineralization rates. The close relationship between the relative abundance and diversity of the ECM fungal community, ECM communities and soil NAG, and N processes indicated that the changes in soil N dynamic after forest conversion were directly related to the changes in the ECM fungal community. Our results provide insight into soil C and N dynamics mechanisms resulting from forest conversion.