How does organic amendment affect soil microbial nitrate immobilization rate?

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.