Comparison of humus composition in the subsoil of Japanese paddy and upland fields

Characteristics of the humus composition in the subsoil of paddy fields were examined in two studies. Humus composition of the subsoil samples collected from paddy (n=31) and upland (n=20) fields in Aichi and Gifu Prefectures, central Japan, was compared statistically (Study I). Humification degree of HAs was compared between the profiles of paddy fields and adjacent upland fields on two diluvial soils (Study II). Humus was extracted with 0.1 M NaOH and 0.1 M Na4P2O7 successively, and fractionated into humic acid (HA) and fulvic acid (FA) fractions. The HAs were classified into Types A, B, P, and Rp based on their spectrophotometric properties. Forty one out of 51 Na4P2O7-extractable HA samples from the subsoil of paddy and upland fields belonged to Type A. Type B or Type Rp HAs were often found in the subsoil of Strong Gley Lowland soils and diluvial soils. Although no difference in the spectrophotometric properties was found between HAs from alluvial paddy (n=24) and alluvial upland (n=17) soils, the amount of Na4P2O7-extractable HAs in the subsoil was significantly larger in paddy fields than in upland fields, The other characteristics of the humus composition in the subsoil of paddy fields compared with those of upland fields were the lower proportion of extractable humus in total carbon (C), mainly due to a lower amount of NaOH-extractable FAs, and a higher proportion of Na4P2O7-extractable HAs in total HAs and in total C. Humification degree of the Na4P2O7-extractable HAs in the subsoil of diluvial upland fields decreased with depth, and most of them belonged to Type B. On the contrary, the humification degree of the same fraction from the adjacent paddy fields was high even in deeper layers, and all the samples except for one were classified into Type A (Study II). Therefore, it was concluded that the accumulation of Na4P2O7-extractable HAs belonging to Type A is a characteristic of humus in the subsoil of well drained paddy fields.