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

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 ¹⁵NO₃⁻ labelling technique in three different ways: (a) ¹⁵N pool dilution, (b) organic ¹⁵N recovery and (c) microbial biomass ¹⁵N 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. HIGHLIGHTS: Three methods for estimating microbial NO₃⁻ immobilization were compared The methods generated similar NO₃⁻ immobilization rates when amended with plant residues Higher NO₃⁻ immobilization when measured with ¹⁵N dilution with readily available C input A microbial NO₃⁻ immobilization‐specific index for plant residues was developed