Studying soil organic matter using 13C CP-MAS NMR: the effect of soil chemical pre-treatments on spectra quality and representativity.

(13)C CP-MAS NMR spectroscopy is a technique that has proved to be useful in studying soil organic matter (SOM). Nevertheless, NMR spectra exhibit a weak signal and have very low resolution due to: the low natural abundance of (13)C (1.1% of C) in SOM, the generally low SOM content of soils, and the presence of paramagnetic impurities. This paper studies the effects of soil chemical pre-treatments on (13)CP-MAS NMR spectra quality and spectra representativity i.e. soil C mass balance. After chemical pre-treatment to increase total organic carbon (TOC) content and C/Fe ratio, eight soils characterized by different levels of organic carbon content and C/Fe ratios were studied using (13)CP-MAS NMR. Moreover, where chemical treatments were not applicable due to high carbon losses, the number of (13)CP-MAS NMR scans was increased in order to obtain satisfactory spectra. Results show that chemical pre-treatment of soils with C/Fe>1 caused high C losses. Bulk soils were therefore studied by increasing the number of (13)CP-MAS NMR scans. Acceptable spectra were obtained from 8K scans (1K=1024 transient). On the other hand, even when a large number of scan (32K) are used, soil with C/Fe<1 cannot be studied. As these soils are characterized by low C losses after HCl treatments (range of 2.9-25.4%), a pre-treatment of at least 1.39 mol l(-1) HCl removes excess Fe and at the same time increases C/Fe ratio resulting in 32K scans providing good spectra.