Characterization of DAX-8 adsorbed soil fulvic acid fractions by various types of analyses

We investigated the optical absorption properties (A(400)/C and A(600)/C values), elemental composition, weight-averaged molecular weight (Mw), and liquid-state carbon-13 (C-13) nuclear magnetic resonance (NMR) spectroscopy of DAX-8 adsorbed fulvic acid fractions (DAX-8 FAs) in various soils (n = 36) to establish the general trends in their chemical characteristics. We also compared our results with those of humic acids (HAs) obtained from the same soils used in this study and other published data to discuss the differences between them. Our results clearly indicate that DAX-8 FAs with high carboxyl C content have small Mw and low hydrogen to carbon (H/C) ratio. In addition, DAX-8 FAs with high carbonyl C and alkyl C content have high A(400)/C values and H/C ratios, respectively. These results strongly suggest that DAX-8 FAs with high aliphatic chemical properties have low carboxyl C content, large Mw and high H/C ratio. There are significant differences in chemical characteristics among the various soil types in almost all data; e. g., DAX-8 FAs from Podzols showed significantly higher A(400)/C values, larger Mw, higher carbonyl C and O-aryl C content than those obtained from Andosols. However, these two C functional groups are relatively low values in the total C content among all soil types, suggesting that DAX-8 FAs may exhibit only small differences in chemical properties among the various soil types. The comparison with published data regarding soil HAs shows that DAX-8 FAs are characterized by smaller Mw; lower C, H, and nitrogen (N); higher oxygen (O) content; higher O/C; lower O/H ratios; higher carboxyl C content; and lower aryl C content in total C.