The effect of fine root traits on soil nutrient availability is mediated by microorganisms or by direct interaction between fine roots and soil. However, the former has received a lot of attention, while the latter has rarely been reported. In this study, six mixed tree species (3 resource-acquisitive and 3 resource-conservative) in a subtropical Chinese fir-broadleaf mixed forest were used as the research object. The direct, indirect, and overall effects of fine roots on soil nitrogen and phosphorus availability and the relationship between fine root traits and them were studied by an in-situ net bag test. The results showed that fine root direct contact with soil could improve the availability of soil N and P under both resource utilization strategies. The effect of acquisitive tree species on soil phosphorus was more significant, showing stronger direct, indirect, and overall effects, while the indirect effect of conservative tree species on soil nitrogen was stronger. Moreover, the C/N ratio of fine roots is the primary factor regulating its effect on soil N and P availability. The direct effect of resource utilization strategies on nitrogen availability mainly depends on the strength of nutrient uptake, and the indirect effect is dominated by organic nitrogen mineralization. Fine roots with a low C/N ratio directly promote P availability by producing more phosphatases or indirectly promote P availability by reducing soil pH. This study enriched the regulatory mechanism of fine root traits on soil nutrient availability and provided theoretical support for the selection of associated tree species in mixed forests.