The transformations of the indigenous 13C and the 13C from either uniformly enriched 13C-D-glucose or 13C-glycine added to an organic soil were followed during a 28-day incubation using cross polarization (CP) magic angle spinning (MAS) 13C nuclear magnetic resonance (NMR) spectroscopy and dipolar dephased (DDP) MAS 13C NMR. The C mineralization was determined from 13C remaining by mass spectrometry and from CO2 evolution by gas chromatography. DDP MAS 13C NMR of the unamended soil indicated a transient increase in molecularly mobile 13C in the alkyl- and methyl-C over 5 days, which may be due to redistribution of C in the microbial biomass in response to perturbation. The added glucose-13C remaining declined to 43% after 7 days and 34% after 28 days. After 28 days the amount of added glucose-13C remaining was 6 times greater than the biomass C at the outset, while the microbial activity (CO2 production) was 38% greater, indicating that a significant proportion of added glucose-13C was not in microorganisms. Added glycine-13C declined faster, such that 29% and 8% remained after 7 and 28 days, respectively. After 28 days' incubation with 13C-glucose, the O-alkyl-C, the acetal- and ketal-C, and the methyl- and alkyl-C resonances in CP MAS 13C NMR spectra were all enhanced compared with the unamended soil. The calculated T1ρH values of the O-alkyl-C and the acetal- and ketal-C resonances were less than those of crystalline glucose, indicating that there was no substantial reservoir of unreacted glucose. After 7 days' incubation with 13C-glycine, none of the signals in the CP MAS 13C NMR spectra were enhanced when compared with the unamended soil, indicating that the added 13C remaining was distributed in undetectable quantities in a range of functionalities. The calculated T1ρH values indicated that glycine 13C was in O-alkyl-C, acetal- and ketal-C and carbonyl-C. T1ρH values may be more sensitive to changes in the distribution of 13C when 13C content is low. The DDP MAS 13C NMR spectra of both the 13C-glucose- and the 13C-glycine-amended soil showed that the molecularly mobile alkyl- and methyl-C increased compared with the unamended soil.
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Acknowledgements We are grateful to the U.K. Natural Environment Research Council for financial support.
Copyright 2004 Elsevier Science B.V., Amsterdam. All rights reserved.
- Carbon transformations