How does organic amendment affect soil microbial nitrate immobilization rate?

Microbial immobilization of nitrate (NO 3 − -N, I NO3 ), which represents a major nitrogen (N) retention mechanism, is carbon (C)-limited in soils. Our previous study has demonstrated that soil I NO3 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 I NO3 remains elusive. We hypothesized that soil I NO3 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 15 N 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 I NO3 rate increased linearly with an increase in organic C application rate across different organic C types that can stimulate I NO3 . In addition, soil I NO3 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 I NO3 rate by influencing bacterial abundance at the highest organic C application rate. The results could enhance our capacity to predict soil I NO3 trends. • Soil I NO3 increased with increasing carbon application rate. • Soil I NO3 increased with increasing holocellulose content in organic amendment. • Holocellulose drove I NO3 via influencing soil bacterial abundance.