Incorporation of uniformly labeled 13C glucose carbon into the organic fraction of a soil - carbon balance and CP MAS 13C NMR Measurements

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Abstract
The incorporation of uniformly labelled 13C-giucose into soil organic matter was followed using mass spectrometry to make carbon balance measurements, and using solid state CP/MAS 13C NMR (cross polarization/magic angle spinning 13C nuclear magnetic resonance) spectroscopy to determine changes in the chemical structure of the added 13C with time. A fine sandy loam soil was incubated in the presence and absence of the labelled 13C-glucose for up to 34 days at 22°C and a soil water matric potential of -33 kPa. Carbon balance measurements indicated that no priming effect of glucose addition on decomposition of the native organic carbon occurred, and that 65% of the glucose 13C was mineralized during the incubation period. The ability of solid-state CP/MAS 13C NMR to quantitatively detect all of the substrate 13C present in the samples was assessed by comparing the residual substrate 13C contents of the samples analysed with the corresponding CP/MAS 13C NMR signal intensities. Incorporation of the glucose 13C into the soil organic matter resulted in the synthesis of alkyl (26%), O-alkyl (66%), and carboxyl (8%) carbon, but little if any aromatic carbon. The influence of decomposition processes on the chemical characteristics of the soil organic matter is discussed, and the chemical structure of the materials synthesized by the microbial biomass is compared with that of the native soil organic matter.