Soil microorganisms are involved in the turnover of soil nitrogen and other soil nutrients.. Nitrogen and carbon, two basic elements utilized by soil microbial communities, directly influence the structure and function of these communities. However, few studies have focused on the combined effects of nitrogen application and soil organic carbon level on soil microbial communities. In this study, we conducted field experiments with four different nitrogen applications (control, CK): 0 kg/hm²; low application nitrogen level, LN:84.2 kg/hm²; middle application nitrogen level, MN: 166.8 kg/hm²; high application nitrogen level, HN: 333.7 kg/hm²) in two Eucalyptus plantation sites with significantly different soil organic carbon levels (low organic carbon site, LC and high organic carbon site, HC) in Guangxi, southern China. We identified the compound effects of nitrogen application level and soil organic carbon level on soil microbial function by using the BIOLOG method. The results showed that (1) carbon metabolism of soil microbial communities responds positively to soil organic carbon levels. Carbon metabolism intensity and richness of soil microbial communities in HC sites were significantly higher than those in LC sites (P< 0.01). (2) Carbon metabolism of soil microbial communities was significantly influenced by the nitrogen application level (P< 0.05). With the increase in nitrogen level, carbon metabolism intensity and richness of soil microbial communities increased gradually and then decreased gradually. However, carbon metabolism intensity and richness of soil microbial communities in Eucalyptus plantations with different soil organic carbon levels had significantly different responses to nitrogen application. The maximum carbon metabolism intensity and richness of soil microbial communities were observed in the HC site with MN application; this was also observed in the LC site with the LN level. The interactive effects of nitrogen application level and soil organic carbon level on soil carbon metabolism is significant (P <0.01). (3) The main metabolized carbon sources of soil microbial communities, which differentiated the responses of soil microbial carbon function according to nitrogen application level, were carbohydrates, amino acids, and carboxylic acids. Carbon in the soil microbial biomass was positively correlated with carbon metabolism intensity and richness index of soil microbial communities. Our results suggest that not only nitrogen levels but also soil organic carbon levels influence soil microorganism carbon source utilization, and soil organic carbon level cannot be neglected in analyzing the effects of nitrogen application level on soil microorganism community metabolism.