Comparison of chemical characteristics of Type A humic acids extracted from subsoils of paddy fields and surface ando soils

Abstract

Chemical characteristics of highly humified humic acids (Type A HAs) from the subsoil of paddy fields (paddy soil HAs) and from the surface layer of ando soils (ando soil HAs) were compared based on the analyses of elemental composition, molecular size distribution, and 13C CPMAS NMR spectra. Humic acids were extracted successively with 0.1 M NaOH and 0.1 M Na 4P 2O 7. The results showed that paddy soil HAs were less oxidized or more decarboxylated than the ando soil HAs. Except O/C value, all the average values for the properties (e.g. C%, M w, and alkyl C%) did not differ significantly between paddy soil HAs and ando soil HAs. Almost all the chemical characteristics were correlated with RF value, which was an index of degree of humification, based on their spectrophotometric properties. The slopes of derived regression lines obtained from RF (independent variables) vs. C%, H%, H/C value, unsaturated degree, weight average molecular weight, and percentage of each C species (dependent variables) differed significantly between paddy soil HAs and ando soil HAs. Therefore, their chemical structures were considered to differ from each other. Type A HAs with RF=100 from ando soils consisted of both the highly and lowly humified HA molecules in blended and/or combined forms, while those from paddy soils were less heterogeneous. Chemical properties of Type A HAs with RF=180 from paddy soils were rather similar to those from ando soils, although the carbonyl C content was noticeably lower than that of ando soil HAs. Ando soils have lost their non-humic components during the advance in the degree of humification, while conjugated electron transfer system of humic substances developed as well as the loss of non-humic components in paddy soil HAs.