Iron forms in peat soils and their relation to indices for the decomposition degree of peat

Surface peat samples were collected from Sarobetsu peatland, Kiritappu peatland, Bekanbeushi peatland and Tokachi coastal peatland in Hokkaido, Japan. Iron forms in peat soils were determined by using a selective dissolution method, and their relation to the several physical and chemical properties, as well as the indices for the decomposition degree of peat were investigated.Total carbon and nitrogen contents had a significant correlation with the decomposition degree of peat, such as fiber content and the degree of humification. In several samples, an incorporation of inorganic materials, such as volcanic ash and alluvial deposit, rather than the decomposition degree of peat affected ash content of peat. The average total iron content (Fet) for all samples was 16.4 g kg**-1. When the average was calculated with samples which were less affected by the incorporation of mineral materials, they still had a relatively high average of 11.1 g kg**-1 Fet. In these peat samples, more than half of total iron was extracted with 0.1 mol L**-1 sodium pyrophosphate solution. The sodium pyrophosphate extractable iron content (Fep) had little difference from acid oxalate extractable and dithionitecitrate extractable iron contents, which should be higher than Fep, since they have strong abilities to dissolve iron from high crystalline amorphous iron oxides and minerals. These results suggested that a considerable part of the iron was in the form of iron-humus complexes and amorphous iron hydroxides. The positive correlation between Fep and the degree of humification of peat at 340nm also supported these results. Consequently, iron mainly existed in the iron-humus complexes in the peat samples that were less affected by an incorporation of inorganic materials, while highly crystalline iron oxides and primary minerals were the main pools of iron in the samples that were rich in mixed volcanic ash and alluvial deposit.