CULTIVATION EFFECTS ON THE NATURE OF ORGANIC-MATTER IN SOILS AND WATER EXTRACTS USING CP MAS C-13 NMR-SPECTROSCOPY/

The objective of this study was to examine the chemical structure of t he organic matter (SOM) of Oxisols soils in slash and burn agriculture , in relation to its biological properties and soil fertility. The CP/ MAS C-13 technique was used to identify the main structural groups in litter and fine roots as SOM precursors; to identify the changes on th e nature of the SOM upon cultivation and the proportion of labile and stable components; and to identify the nature of the organics present in water extracts (DOG). Carbohydrates were the main structural compon ents in litter, whereas components such as carbonyl C, carboxyl C,O-al kyl C and alkyl C were more common in SOM. Phenolic C and the degree o f aromaticity were similar in litter and SOM. Cultivation resulted in a small decrease in the relative proportion of carbohydrates in SOM, L ittle change in the levels of O-alkyl C and carbonyl C, but an increas e in carboxyl C, phenolic C and aromaticity of the SOM. The level of a lkyl C in soil was higher than the level of O-alkyl C, indicating the importance of long-chain aliphatics along with Lignins in the stabiliz ation of the SOM in Oxisols. The SOM of Mollisols from the Canadian Pr airies differed from the OxisoI, with a generally stronger expression of aromatic structures, particularly in a cultivated soil in relation to a native equivalent. Carbohydrate components were the predominant s tructures in the DOG, indicating their importance in nutrient cycling and vertical translocations in the Oxisol.