Submitted to: European Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 24, 2003
Publication Date: June 2, 2004
Citation: Smernik, R.J., Olk, D.C., Mahieu, N. 2004. Quantitative solid-state 13c NMR spectroscopy of organic matter fractions in lowland rice soils. European Journal of Soil Science. 55(2):367-379.
Interpretive Summary: Soil organic matter plays a central role in several soil processes, including nutrient cycling, water retention, soil structure, and chemical adsorption. Yet its behavior is not fully predictable, partly because technology constraints have precluded a proper understanding of its chemical nature. Recent improvements have made nuclear magnetic resonance (NMR) spectroscopy a prime method for studying organic matter chemistry. It estimates the proportions that the element in question, usually carbon, is found in different chemical groups. However, newer NMR techniques suggest inaccuracies in previous measurements. In this study, two new NMR techniques were compared with the older, most commonly used technique on well-characterized samples of organic matter. A new, highly quantitative NMR technique was found to estimate greater proportions of two common carbon forms than did the old method. A second new NMR technique was used to identify the chemical processes responsible for inaccurate estimates of carbon proportions by the older method. Results provide a process-level explanation for inaccuracies with the older method and illustrate the significance of using newer techniques when studying the chemical nature of soil organic matter. Results will benefit scientists who use NMR or interpret NMR results for investigating organic matter chemistry.
Spin counting on solid-state **13C cross-polarization (CP) nuclear magnetic resonance (NMR) spectra of two humic fractions isolated from tropical lowland soils showed that only 32-81% of potential **13C NMR signal was detected. The observability of **13C NMR signal (Cobs) was higher in the mobile humic acid (MHA) than the calcium humate (CaHA) fraction, and increased with increasing intensity of irrigated rice cropping. NMR observability appeared to be related to the nature of the organic carbon, with phenol- and methoxyl-rich samples having the higher values of Cobs. The Bloch decay (BD) technique provided more quantitatively reliable **13C NMR spectra, as evidenced by values of Cobs in the range 91-100% for seven of the eight humic fractions studied. The BD spectra contained considerably more aryl and carbonyl signal, and less O-alkyl and alkyl signal, with the greatest differences between CP and BD spectra observed for the samples with low Cobs(CP). The causes of low CP observability were investigated using the spectral editing technique RESTORE (REstoration of Spectra via TCH and T One Rho (T1pH) Editing). Rapid T1pH relaxation was found to be primarily responsible for the under-representation of carbonyl carbon, whereas inefficient cross polarization was primarily responsible for the under-representation of aryl carbon in CP spectra. Proton NMR relaxation rates T1H and T1pH were found to correlate with other NMR properties and also with cropping management. Non-uniform rates of T1H relaxation in two of the CaHA fractions enabled the generation of proton spin relaxation editing (PSRE) subspectra.