Fates of C-13 from enriched glucose and glycine in an organic soil determined by solid-state NMR

The transformations of the indigenous C-13 and C-13 from either uniformly enriched C-13-D-glucose or C-13-glycine added to an organic soil were followed during a 28-day incubation using cross polarization (CP) magic angle spinning (MAS) C-13 nuclear magnetic resonance (NMR) spectroscopy and dipolar dephased (DDP) MAS C-13 NMR. The C mineralization was determined from C-13 remaining by mass spectrometry and from CO2 evolution by gas chromatography. DDP MAS C-13 NMR of the unamended soil indicated a transient increase in molecularly mobile C-13 in the alkyl-and methyl-C over 5 days, which may be due to redistribution of C-13 in the microbial biomass in response to perturbation. The added glucose-C-13 remaining declined to 43% after 7 days and 34% after 28 days. After 28 days the amount of added glucose-C-13 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-C-13 was not in microorganisms. Added glycine-C-13 declined faster, such that 29% and 8% remained after 7 and 28 days, respectively. After 28 days' incubation with C-13-glucose, the O-alkyl-C, the acetal-and ketal-C, and the methyl-and alkyl-C resonances in CP MAS C-13 NMR spectra were all enhanced compared with the unamended soil. The calculated T-1 rho H 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 C-13-glycine, none of the signals in the CP MAS C-13 NMR spectra were enhanced when compared with the unamended soil, indicating that the added C-13 remaining was distributed in undetectable quantities in a range of functionalities. The calculated T-1 rho H values indicated that glycine C-13 was in O-alkyl-C, acetal- and ketal-C and carbonyl-C. T1 rho H values may be more sensitive to changes in the distribution of C-13 when C-13 content is low. The DDP MAS C-13 NMR spectra of both the C-13-glucose-and the C-13-glycine-amended soil showed that the molecularly mobile alkyl-and methyl-C increased compared with the unamended soil.