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

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.