The effect of water content on solid-state 13C NMR quantitation and relaxation rates of soil organic matter

Two hydrofluoric acid‐treated soils were prepared with water contents ranging up to 22% by exposing them to a range of atmospheric humidities. There was no effect of water content on the chemical shift distribution of nuclear magnetic resonance (NMR) signal in ¹³C cross‐polarization (CP) NMR spectra. The sensitivity of the ¹³C CP NMR spectra decreased slightly with increasing water content. Much of this decrease could be attributed to decreases in T_1ρH relaxation rates, caused by enhanced molecular mobility of the organic matter in the presence of absorbed water. Rates of T_1H relaxation were very sensitive to water content, and average T_1H relaxation rates decreased four‐ to five‐fold from the smallest to the largest water content. Rates of T_1H relaxation were non‐uniform, and were better modelled by two‐T_1H component fits than one‐T_1H component fits. The ratio of rapidly to slowly relaxing components increased with increasing water content. Proton spin relaxation editing (PSRE) subspectra revealed substantial changes in the nature of these two components with increasing water content. These results indicate the presence of an organic matter component that is very sensitive to water content, transforming from slowly relaxing at a small water content to rapidly relaxing at a greater water content. This component was shown to be rich in O–alkyl and carbonyl C, and may be hemicellulosic root exudates and microbial mucilages. The slowly relaxing PSRE component was a mixture of ligno‐cellulose and alkyl biopolymers, whereas the rapidly relaxing component was primarily charcoal for one of the soils, and was reminiscent of dissolved organic carbon for the other soil. These findings show that care must be taken in controlling water contents when using PSRE to study organic matter.