Heterocyclic N in the highly humified humic acids extracted from the subsoil of paddy fields and surface ando soils

Nitrogen composition of highly humified humic acids (HAs) yielded from the subsoil of paddy fields (paddy soil-Type A HAs) was compared with that of surface ando soils (ando soil-Type A HAs) using N-15 CPMAS NMR spectroscopy. A significant amount of heterocyclic nitrogen was detected in the Type A HAs extracted from both environments, consisting up to 37% of total nitrogen. The relative abundance of heterocyclic N increased concomitantly with the increase of the specific visible absorbance at 600 nm of HAs in alkaline solution. Most paddy fields are reclaimed on alluvial soils in Japan, therefore ando soil-Type A HAs might have been incorporated into the parent materials of the subsoil of paddy fields during the alluviation processes. Since a major vegetation of ando soils is composed of C4 plants, Japanese pampas grasses (Miscanthus sinensis A.), the delta(13)C value of Type A HAs originating from ando soils was expected to differ from that of HAs formed under continuous cultivation of rice plants (Oryza sativa L., C3 plant). Thus, we investigated the origin of highly humified HAs in the subsoil of paddy fields by comparing the delta(13)C values of paddy soil-Type A HAs with those of ando soil-Type A HAs. The delta(13)C values of ando- and paddy soil-Type A HAs were -21.9 +/- 0.6 and -23.4 +/- 2.1%., respectively, which suggests that the paddy soil-Type A HAs consist of a mixture of autochthonous (rice plant origin) and allochthonous (C4 plant) sources. The autochthonous source of paddy soil- Type A HAs in the subsoil will include fulvic acids (FAs) leached from the plow layer, which was also shown to contain a high concentration of heterocyclic nitrogen. (c) 2005 Elsevier Ltd. All rights reserved.