酸性硫酸塩土壌の生成過程における化学性および硫黄酸化細菌・鉄酸化細菌数の経時的変化

海成の還元型硫黄化合物、主にパイライト(FeS2)を含むと思われる新潟県佐渡市の農業用水路脇の水田跡地で深層土壌(深度1.0～1.5m)を採取し、その土壌を好気的条件で培養し、化学性および硫黄酸化細菌・鉄酸化細菌数の経時的変化を測定し、酸性硫酸塩土壌の生成過程におけるこれら細菌の作用機構について検討した。その結果、pH (H2O)、pH (KC1)は0～2週にかけて低下し、その後はほぼ一定の値で推移した。ECは0～2週にかけて急激に増加し、それ以降は緩やかな増加を続けた。可酸化性硫黄は0～4週にかけて急激に減少したのに対して、水溶性硫酸イオンは同時期に急激に増加し、それ以降は双方ともほぼ一定の値で推移した。Fe2+は0～2週にかけて減少し、その後はほぼ一定の値で推移した。Fe3+は0～4週にかけて増加し、その後も緩やかに増加した。以上のことから、供試土壌は還元型硫黄化合物の硫酸への酸化によって酸性化したことが確認された。また2週目以降pH (H2O)が3.80～3.88と4.0以下で推移したことから、2週目までで酸性硫酸塩土壌が生成されたことが明らかになった。硫黄酸化細菌は培養期間を通じて培地pHに関わらず107～8 cells g-1で推移したのに対して、鉄酸化細菌は2週目まで検出されず、4週目以降104 cells g-1で推移した。以上のことから、供試土壌の酸性硫酸塩土壌の生成過程初期には、還元型硫黄化合物の化学的酸化と硫黄酸化細菌による生物的酸化が大きく作用していて、鉄酸化細菌は関与していないことが示唆された。We sampled the subsoil at 1.0-1.5m depth in the bank area of agriculture waterway where marine reduced sulfur compounds such as pyrite was considered to be contained in buried horizon on Sado Island in Niigata. To evaluate the function of sulfur-oxidizing bacteria and iron-oxidizing bacteria in the process of acid sulfate soil genesis, the changes in the chemical properties and the numbers of sulfur- and iron-oxidizing bacteria of the sampled soil at aerobic condition were studied. The pH (H2O) and pH (KC1) of the sampled soil lowered from 0 to 2 weeks after experiment started and changed at almost same value after 2 weeks. The value of electric conductivity (EC) increased rapidly from 0 to 2 weeks and increased slowly after 2 weeks. The concentration of oxidizable sulfur decreased from 0 to 4 weeks and changed at the same level after 4 weeks. On the other hand, the concentration of water-soluble sulfate ion increased from 0 to 4 weeks and changed at the same level after 4 weeks. The concentration of ferrous ion (Fe2+) decreased from 0 to 2 weeks and changed at the same level after 2 weeks. The concentration of ferric ion (Fe3+) increased from 0 to 4 weeks and increased slowly after 4 weeks. These results suggested that the soil sample was acidified as a result of the oxidation of reduced sulfur compounds in its soil. Furthermore, the soil sample was generated to acid sulfate soil within 2 weeks, because its pH (H2O) was less than 4.0 after 2 weeks. The number of sulfur-oxidizing bacteria changed at 107～8 cells g-1 regardless of the pH of medium during the whole incubation period. On the other hand, no iron-oxidizing bacteria were detected from 0 to 2 weeks and its number was changed at 104 cells g-1 after 4 weeks. Above results shows that oxidization of reduced sulfur compounds by chemically oxidization and sulfur-oxidizing bacteria took an important part of the initial process of acid sulfate soil genesis and iron-oxidizing bacteria took no part of it.