Microbial immobilization of 15N from legume residues in soils of differing textures: Measurement by persulphate oxidation and ammonia diffusion methods

The immobilization of 15N by the soil microbial biomass was measured in three soils of differing texture (sandy loam, a loamy sand and a sandy clay loam), amended with 15N-labelled legume residues of differing N status. Modifications to a persulphate oxidation and ammonia micro-diffusion method are described to measure total N and 15N contents in 0.5 M K2SO4 soil extracts using an automated CN analyzer in series with a stable isotope ratio mass spectrometer (ANCA-MS). The three soils were amended with 10 mg g−1 of shoot or root material from subterranean clover (Trifolium subterraneum) containing 1.59 and 2.40% N, respectively; lupin leaf (Lupinus angustifolius) containing 1.42% N, or a defined substrate of glucose (10 mg g−1) plus ammonium sulphate (1 mg g−1). The amended soils were incubated at 45% maximum water holding capacity, at 25°C, and measurements taken after 0, 7 and 14 days incubation. Microbial biomass N and 15N were measured by fumigation-extraction, followed by persulphate oxidation, diffusion and ANCA-MS; microbial N was also estimated using the ninhydrin method. There was reasonable agreement between microbial N estimated by both methods (r = 0.74, 0.85 and 0.92, measured 0, 7, and 14 days after amendment, respectively). There was a maximum of a four-fold increase in microbial N after incubation with added substrates. After 14 days about 23, 14 and 18% of the 15N from clover shoots, clover roots, and lupin leaf, respectively, had been incorporated into microbial 15N. Clover root residues had the lowest incorporation into microbial N despite having the highest N content. There was little effect of soil texture on microbial immobilization of legume N, with all three soils showing a similar pattern of incorporation. This was in contrast to ammonium sulphate N, where, in the presence of glucose, 21% of the added N was estimated to be incorporated into the microbial biomass of the sandy loam soil after 14 days, compared to the 28 and 29% microbial incorporation of the two finer-textured soils.