Abstract
In order to analyze the microbial biomass and community structure in soil, we investigated the amounts and Composition of phospholipid fatty acids which had been annually applied with rice straw compost (0, 20, 40, 80 Mg ha^<-1>) and extracted from the upland light colored andosol of experimental plots. The phospholipid content increased with increasing compost application levels. The major fatty acids were 16 : 0, 18 : 0, 18 : 1 ω 7, 18 :1 ω 9, 18 : 1 ω 5, 16 : 1 ω 7, 16 : 1 ω 5, 16 : 1 ω 9, i17 : 1 ω 8, p11-19 : 1, 20 : 1, 18 : 2 ω 6, 20 : 4, 17 : 0cyc, 19 : 0cyc, p10 17 : 0, i15 : 0, a15 : 0, i16 : 0, i17 : 0, a17 : 0, p10-19 : 0 and24 : 0 2OH. The phospholipid fatty acid composition indicated that the main origin of the soil phospholipids was bacterial membrane iipids. The proportion of the branched chain fatty acids heightened as amount of applied compost was increased. Their proportions in solis in March and April were lower than in October and December, however, the proportions of cyclopropane fatty acids, 16 : 1 ω 7 and 18 : 1 ω 7, in soils in March and April were higher than in October and December. These results suggest that the ratio of Gram-positive bacteria to Gram-negative bacteria increased under cultivation of wheat. The phospholipid fatty acid content was closely correlated with thc available N ; the amount of N mineralized during four weeks of incubation at 25 ℃. Correlation coefficients between available N and soil content of fatty acids were 0.94 for i15 : 0, 0.93 for p10-17 : 0, 0.90 for p10-19 : 0, 0.76 for 16 : 0, 0.70 for 18 : 2 ω 6 and 0.64 for 18 : 1 ω 7. There were also significant correlations belween the counts of bacteria by microscopical method and phospholipid fatty acid content. Thc proportion derived from soil microbial biomass seemed to be high in soil phospholipid because 23-41% of the Soil phospholipid content declined after chloroform fumigation. The size of biomass based on phosptolipid fatty acid analyses was higher than the size based on microscopical counts and lower or higher than the size based on fumigation-incubation, depending on the conversion factors.
