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Quantitative solid‐state 13 C NMR spectroscopy of organic matter fractions in lowland rice soils
Author(s) -
Smernik R. J.,
Olk D. C.,
Mahieu N.
Publication year - 2004
Publication title -
european journal of soil science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.244
H-Index - 111
eISSN - 1365-2389
pISSN - 1351-0754
DOI - 10.1111/j.1365-2389.2004.00600.x
Subject(s) - chemistry , humic acid , nmr spectra database , carbon 13 nmr , nuclear magnetic resonance spectroscopy , aryl , analytical chemistry (journal) , alkyl , spectral line , proton nmr , nuclear chemistry , stereochemistry , organic chemistry , physics , fertilizer , astronomy
Summary Spin counting on solid‐state 13 C cross‐polarization (CP) nuclear magnetic resonance (NMR) spectra of two humic fractions isolated from tropical lowland soils showed that only 32–81% of potential 13 C NMR signal was detected. The observability of 13 C NMR signal (C obs ) was higher in the mobile humic acid (MHA) than in 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 C obs . The Bloch decay (BD) technique provided more quantitatively reliable 13 C NMR spectra, as evidenced by values of C obs 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 C obs (CP). The causes of low CP observability were investigated using the spectral editing technique RESTORE ( RE storation of S pectra via T CH and T O ne R ho ( T 1 ρ H ) E diting). Rapid T 1 ρ H 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 T 1H and T 1 ρ H were found to correlate with other NMR properties and also with cropping management. Non‐uniform rates of T 1H relaxation in two of the CaHA fractions enabled the generation of proton spin relaxation editing subspectra.