Characteristics of organic material inputs affect soil microbial NO3- immobilization rates calculated using different methods

摘要

Microbial immobilization of nitrate (NO3-) is considered to be an important process in reducing NO3 (-) accumulation and regulating nitrogen (N) retention capacity in soils. Accurately quantifying the rate of microbial NO3- immobilization is, therefore, necessary to predict its role in lowering NO3- accumulation in soils. We compared microbial NO3- immobilization rates using a (NO3-)-N-15 labelling technique in three different ways: (a) N-15 pool dilution, (b) organic N-15 recovery and (c) microbial biomass N-15 recovery, in a nitrate-rich upland soil with and without amendment with organic materials with differing carbon-to-nitrogen ratios (C/N). The three methods generated similar NO3- immobilization rates, except when the soil was amended with easily decomposable organic materials (glucose and sucrose). We also developed a microbial NO3- immobilization-specific quality index that incorporates the C/N ratio, lignin, cellulose and hemicellulose contents and pH for slowly decomposing organic materials (plant residues). This study provides direct empirical evidence that the results of different methods for calculating soil microbial NO3- immobilization rates are affected by the characteristics of organic materials added to the soil.