USING C-13 CPMAS NMR TO ASSESS EFFECTS OF CULTIVATION ON THE ORGANIC-MATTER OF PARTICLE-SIZE FRACTIONS IN A GRASSLAND SOIL

We examined particle size fractions from the surface horizons of a nat ive untilled grassland site and an adjacent field cultivated for 65 ye ars in the Peace River region of British Columbia, Canada. While culti vation resulted in the loss of approximately 50% of the C and N in the soil, there was little change in the distribution of particle sizes, in their C/N ratios, and in the fraction of C found as carbohydrate. C hanges in the nature of the organic C were generally small, but obviou s differences were seen in the C-13 CPMAS NMR spectra of two of the si ze fractions. Compared with the native grassland, the sand fraction fr om the cultivated field had a lower proportion of O-alkyl C and a loss of resolution. The clay fraction from the cultivated field had lower O-alkyl and higher alkyl C than its grassland counterpart. Subtraction of linear combinations of spectra of the whole soils from the two sit es showed a 10% increase with cultivation in the proportion of humifie d to decomposable organic C. All of these trends are consistent with a greater degree of decomposition in the cultivated site. The changes a re small and do not suggest a serious degradation in the quality of th e soil organic matter, compared with the large change in quantity. We also examined factors that might affect the quality and quantitative r eliability of the C-13 CPMAS NMR spectre. Dithionite treatment was not very effective in improving resolution or fraction of observable C fo r these samples. The latter was in the range of 10-30%, largely a resu lt of the association of organic matter with paramagnetic iron. For th is reason, comparison of relative areas must be interpreted with cauti on and confined to samples with reasonably similar contents of C and i ron.