Microbial immobilization of nitrate (NO3−-N, INO3), which represents a major nitrogen (N) retention mechanism, is carbon (C)-limited in soils. Our previous study has demonstrated that soil INO3 can be enhanced with the addition of simple organic C at rates >500 mg C kg−1, or complex organic C with C/N ratios >18. However, whether the quantity and quality of organic amendments can be used to quantify soil INO3 remains elusive. We hypothesized that soil INO3 rate increases with an increase in both organic C application rate and amount of a particular organic C component of added organic materials. Using the organic 15N recovery method, following the application of seven organic C sources at a gradient of 0, 1, 2, and 5 g C kg−1, we demonstrated that soil INO3 rate increased linearly with an increase in organic C application rate across different organic C types that can stimulate INO3. In addition, soil INO3 rates, expressed as mg N per g C applied increased logarithmically with an increase in holocellulose (cellulose + hemicellulose) content in the organic amendments. Our structural equation modeling, for the first time, suggest that holocellulose drives soil INO3 rate by influencing bacterial abundance at the highest organic C application rate. The results could enhance our capacity to predict soil INO3 trends. •Soil INO3 increased with increasing carbon application rate.•Soil INO3 increased with increasing holocellulose content in organic amendment.•Holocellulose drove INO3 via influencing soil bacterial abundance.