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Publication - Dr Timothy Knowles

    Compound-specific amino acid 15N stable isotope probing of nitrogen assimilation by the soil microbial biomass using gas chromatography/combustion/isotope ratio mass spectrometry

    Citation

    Charteris, AF, Knowles, TDJ, Michaelides, K & Evershed, RP, 2016, ‘Compound-specific amino acid 15N stable isotope probing of nitrogen assimilation by the soil microbial biomass using gas chromatography/combustion/isotope ratio mass spectrometry’. Rapid Communications in Mass Spectrometry, vol 30., pp. 1846-1856

    Abstract

    RATIONALE: Organic nitrogen (N) greatly exceeds inorganic N in soils, but the complexity and heterogeneity of this important soil N pool make investigations into the fate of N-containing additions and soil organic N cycling challenging. This paper details a novel conceptual approach to investigate the fate of applied N in soils, generating quantitative measures of microbial assimilation and of newly synthesized soil protein.

    METHODS:
    Laboratory incubation experiments applying 15N-ammonium, 15N-nitrate and 15N-glutamate were carried out and the high sensitivity and selectivity of gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) exploited for compound-specific nitrogen-15 stable isotope probing (15N-SIP) of extracted incubation soil amino acids (AAs; as N-acetyl, O-isopropyl derivatives). We then describe the interpretation of these data to obtain a measure of the assimilation of the applied 15N-labelled substrate by the soil microbial biomass and an estimate of newly synthesized soil protein.

    RESULTS: The cycling of agriculturally relevant N additions is undetectable via bulk soil N content and δ15N measurements and AA concentrations. The assimilation pathways of the three substrates were revealed via patterns in AA δ15N values with time, reflecting known biosynthetic pathways (e.g. ammonium uptake occurs first via glutamate) and these data were used to expose differences in the rates and fluxes of the applied N substrates into the soil protein pool (glutamate > ammonium > nitrate).

    CONCLUSIONS: Our compound-specific 15N-SIP approach using GC/C/IRMS offers a number of insights, inaccessible via existing techniques, into the fate of applied N in soils and is more widely applicable to the study of N cycling in any soil, or indeed, in any complex ecosystem.

    Full details in the University publications repository