Compound-specific 15N stable isotope probing of N assimilation by the soil microbial biomass: a new methodological paradigm in soil N cycling

A. F. Charteris, T. D. J. Knowles, K. Michaelides, R. P. Evershed
2015 SOIL Discussions  
<p><strong>Abstract.</strong> A compound-specific nitrogen-15 stable isotope probing (<sup>15</sup>N-SIP) technique is described which allows investigation of the fate of inorganic- or organic-N amendments to soils. The technique uses gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) to determine the δ<sup>15</sup>N values of individual amino acids (AAs; determined as <i>N</i>-acetyl, <i>O</i>-isopropyl derivatives) as proxies of biomass protein production. The
more » ... tion. The δ<sup>15</sup>N values are used together with AA concentrations to quantify N assimilation of <sup>15</sup>N-labelled substrates by the soil microbial biomass. The utility of the approach is demonstrated through incubation experiments using inorganic <sup>15</sup>N-labelled substrates ammonium (<sup>15</sup>NH<sub>4</sub><sup>+</sup>) and nitrate (<sup>15</sup>NO<sub>3</sub><sup>-</sup>) and an organic <sup>15</sup>N-labelled substrate, glutamic acid (<sup>15</sup>N-Glu). Assimilation of all the applied substrates was undetectable based on bulk soil properties, i.e. % total N (% TN), bulk soil N isotope composition and AA concentrations, all of which remained relatively constant throughout the incubation experiments. In contrast, compound-specific AA δ<sup>15</sup>N values were highly sensitive to N assimilation, providing qualitative and quantitative insights into the cycling and fate of the applied <sup>15</sup>N-labelled substrates. The utility of this <sup>15</sup>N-AA-SIP technique is considered in relation to other currently available methods for investigating the microbially-mediated assimilation of nitrogenous substrates into the soil organic N pool. This approach will be generally applicable to the study of N cycling in any soil, or indeed, in any complex ecosystem.</p>
doi:10.5194/soild-2-1135-2015 fatcat:hbvhngixajavplroro7l7croju