The basic mechanism of acidification on acid sulfate soils (ASS) was studied, but the relation between matric potential (soil moisture) and the behaviour of microorganisms in ASS was still unclear. In this paper, the effects of drying on the mechanism of acidification were identified, especially in relation to different microbial populations and soil respiration through soil incubation experiments. Six soil moisture conditions were set by drying in the incubator at 30°C. Soil pH, soil respiration and total bacterial population, Thiobacillus ferrooxidans (T. ferrooxidans) that had a big role as a catalyst on the acidification of ASS and other gram-negative bacteria were analysed. It was determined that within the matric potential range of —35〜220 kPa (pF 2.55〜3.35),soil respiration was active. Specifically at —35 kPa (pF 2.55), the main bacterial population in the soil was T. ferrooxidans, while population of gram-positive bacteria decreased, though, total bacterial population had a significant relation to soil respiration. Soil acidification was effected not because of the population of T. ferrooxidans but mainly due to the speed in their increase. Carbon dioxide fixation by T. ferrooxidans (per cell) and aerobic respiration (per cell) were active in high matric potential range, but the high population of these bacterium were more effective in increasing the amount of carbon dioxide fixed and aerobic respiration.
