ミミズの土壌形成に伴う元素組成の変化について

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ミミズの土壌形成に伴う元素組成の変化を対照土壌とミミズの糞を比較して調べた.ミミズの糞はバングラデシュ,シャムタ村および山陰地域の3カ所から採集した.シャムタ村のミミズの糞は筒状で長さ8cm,直径3cmに達する.淡灰色のシルトからなり乾燥すると堅固である.これに対して日本のミミズの糞は団粒を形成し,柔らかである.ミミズの糞とその下位の対照土壌を主元素と微量元素を蛍光X線分析装置(XRF)により分析した.シャムタ村の場合はCa,As,Pb,CuおよびZnなどが対照土壌に比べて糞で濃縮率が低下している.本邦の場合はAs,PbおよびCuは同様に低下するがZnは大きな変化を示さない.本邦の2例については糞がCaに富む場合があり,酸性土壌化を防ぐのに役立っている可能性がある.これらの結果は,ミミズが糞形成に伴ってAsやPbを土壌中から浄化していることを示す.

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Changes of geochemical composition of mold by activity of earthworms have been examined by comparison of soils and fecal pellets of earthworms. They were collected from the Samta village, Bangladesh and three localities in San'in district, southwest Japan. The castings of the Samta earthworms are one of the largest pellets of the world, consisting of thick-walled tube shapes with more than 8cm length and about 3cm in diameter. The wall is composed of light brownish grey silty mud gradually become hardened when they are dried. In general fecal pellets of the Japanese earthworms are dark grey color, irregular round shaped with 2-5mm in diameter. They are relatively fragile, and softer than those of the Bangladesh. Major and trace elements were analyzed using XRF, and geochemical compositions of casting and pellets of earthworms were compared with those of the underlying soils at each site. The Samta earthworm casting shows depletion in Ca, As, Pb, Cu and Zn, but there are no significant anomalies in other elements compared to composition of soils. Earthworm pellets of the two examples in Japan show depletion in As, Pb and Cu relative to in situ soil composition, but Zn does not show significant variation from the soils in all examples. These earthworm pellets show enrichments in Ca in two examples suggesting an effect of preventing from acid soil formation. These compositional variations between earthworm products and soils suggest significant remediation of As and Pb for mold by earthworm activities.