Characteristics of organic material inputs affect soil microbial NO 3 − immobilization rates calculated using different methods

Microbial immobilization of nitrate (NO 3 − ) is considered to be an important process in reducing NO 3 − accumulation and regulating nitrogen (N) retention capacity in soils. Accurately quantifying the rate of microbial NO 3 − immobilization is, therefore, necessary to predict its role in lowering NO 3 − accumulation in soils. We compared microbial NO 3 − immobilization rates using a 15 NO 3 − labelling technique in three different ways: (a) 15 N pool dilution, (b) organic 15 N recovery and (c) microbial biomass 15 N 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 NO 3 − immobilization rates, except when the soil was amended with easily decomposable organic materials (glucose and sucrose). We also developed a microbial NO 3 − 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 NO 3 − immobilization rates are affected by the characteristics of organic materials added to the soil. Highlights Three methods for estimating microbial NO 3 − immobilization were compared The methods generated similar NO 3 − immobilization rates when amended with plant residues Higher NO 3 − immobilization when measured with 15 N dilution with readily available C input A microbial NO 3 − immobilization‐specific index for plant residues was developed