Effects of Ecosystem Alternation on Soil Carbohydrates
生态系统转换对土壤中碳水化合物的影响

Carbohydrates were the very important and liable soil organic compounds.Their responses to many factors such as cultivations and eco\|environmental changes are much more sensitive than those of total soil organic matter.Several researches suggested that hexoses and pentoses in soils have quite different sources.Hexoses originate from soil microbes,while pentoses mainly come from the decomposition of plant residues.Based on the above knowledge,soil carbohydrate has been proposed as a sensitive indicator to the changes in both content and origin of soil organic matter. To study the effects of ecosystem alternation on the soil carbohydrates,samples of forested and neighboring agricultural soils were collected from karst area in southwestern China,where the forest ecosystem was changing into agricultural ecosystem.After removal of plant residues and stones,soils were analyzed for five major monosaccharides using GC(Shimadzu GC\|9A),and the total carbohydrates content was calculated as the sum of five major monosaccharides.Analysis of data gave us the following conclusions and implications.Total carbohydrates contents in forested soils were much higher than those in agricultural soils,which might be attributed to the fact that the inputs of organic matter to agricultural soils were much smaller than those to forested soils.In the sampling region,few plant residues were incorporated into the agricultural soils because the crop stubble was either taken away from field after harvest,or burned.In both forested and agricultural soils total carbohydrates decreased invariably along soil profile,but decreasing rates of total carbohydrate contents in agricultural soils were significantly slower than those in forested soils.Such phenomena can be explained by the disturbance resulted from agricultural managements.Although five major monosaccharides have similar change trends to the total carbohydrates,there were significant variations in the change magnitude among the different monosaccharides.Such variations have led to the difference in the ratios of hexoses to pentoses.In the three ratios( M/X,(M+G)/(A+X) and M/(A+X)) ,the M/X ratio ranging from 0.81 to 2.46 changes most significantly.Moreover,t\|test showed that M/X ratios of agricultural soils were significantly higher than those of forested soils ( P <0 001, n =20),which could imply that during the ecosystem alternation the carbohydrates and soil organic matter from plant residues decreased,while those from soil microbe activity increased.In conculusion,the alternation from forest ecosystem to agricultural ecosystem has not only decreased the content of total carbohydrates but also changed their composition.