LIGNIN AND CARBOHYDRATES IN SOILS UNDER SECONDARY FOREST, ALLEY-CROPPING, AND CONTINUOUS FARMING, THAILAND

This study was designed to investigate the impact of three land-use pr actices in Thailand on soil organic matter (SOM) composition. The land uses were continuous farming (till), alley-cropping, and secondary fo rest. Samples, taken from the top 10 cm soil surface, were fractionate d into clay- (< 2 mu m), silt- (2-20 mu m), and sand-sized particles ( 20-2000 mu m) prior to analyses of C, N, lignin-derived phenols and in dividual carbohydrates. As particle size increased, the C/N ratio and the concentration of liganin-derived phenols increased, whereas the de gree of lignin oxidation as well as ratios of microbially derived hexo ses to plant-derived penetoses decreased, Thus, the coarser the partic le size, the less the SOM was altered by microbes. Seven years of alle y-cropping increased SOM levels only little but considerably affected SOM composition by affecting the SOM of the sand fraction. The SOM con centration in the sand fraction increased in the order continuous farm ing < alley-cropping < secondary forest. In the same order, microbial alteration of lignin and carbohydrates of sand-sized SOM decreased. Bo th the different concentrations of SOM in the sand fractions as well a s its different composition contributed, therefore, to different SOM a mong the sites. Increasing lignin-to-carbohydrate ratios for bulk soil s and fractions in the order continuous farming < alley-cropping < sec ondary forest reflected such changes in SOM composition sensitively.