The quality and quantity of exogenous organic carbon input control microbial NO3− immobilization: A meta-analysis

The accumulation of NO3− in the soil has resulted in increased nitrogen (N) loss through runoff, leaching, and gaseous emissions. Microbial immobilization of NO3− as an important process in reducing soil NO3− accumulation has been for a long time neglected due to the predominant viewpoint that microbes preferentially immobilize NH4+-N. Microbial NO3− immobilization is generally carbon (C)-limited, and thus exogenous organic C input may enhance microbial NO3− immobilization. However, the effect of the quality and quantity of exogenous organic C input on soil microbial NO3− immobilization is poorly understood, and a synthetic assessment on such an effect is lacking. We thus assessed the impact of exogenous organic C type, the application rate of simple organic C (glucose and acetate), complex organic C type (animal manure, plant residue) and the C/N ratio of complex organic C on soil microbial NO3− immobilization rate using a meta-analysis. We found that the quality and quantity of exogenous C input affect soil microbial NO3− immobilization: microbial NO3− immobilization was enhanced with the addition of simple organic C at rates >500 mg C kg−1, or complex organic C with C/N ratios >18. Furthermore, a positive relationship between the natural log of response ratio of soil microbial NO3− and NH4+ immobilization indicates the simultaneous utilization of NH4+ and NO3− under elevated C availability. We conclude that specific exogenous organic C input at a high rate or with a high C/N ratio can enhance microbial NO3− immobilization and reduce soil NO3− accumulation.