Molecular-scale heterogeneity of humic acid in particle-size fractions of two Iowa soils

To explore the hypothesis that enrichment of decomposition-resistant compounds might vary in humic acid (HA) that is associated with silt and clay-size microaggregates, we used infrared spectroscopy, thermal analyses, nuclear magnetic resonance spectroscopy, and pyrolysis techniques to characterize the functional groups and structures of HA isolated from coarse silt and clay fractions of two Iowa Mollisols. Samples from the A horizons of Sparta (a sandy, mixed, mesic Entic Hapludoll) and Zook (a fine, smectitic, mesic Cumulic Vertic Endoaquoll) soils were gently dispersed and divided into silt and clay fractions before HA was extracted by standardized techniques. The FTIR, thermal, and NMR analyses revealed that HA in the sandy, well-drained Sparta soil and size fractions was enriched in aliphatic compounds, readily oxidizable carbohydrates, and lignin compared with HA in the poorly drained, clayey Zook soil. Moreover, pyrolysis spectra demonstrated that fatty acids in the Sparta HAs had longer chain lengths than those of the Zook HAs. Clay-fraction HA contained more readily oxidizable components and aliphatic moieties than did silt-fraction HA. The NMR-derived observation that Zook soil HA contained more charcoal-like moieties than Sparta HA was also consistent with the greater amounts of combustion-resistant material observed in the thermal analyses of Zook HA. Others have shown that “whole” soil organic matter characteristics differ with the size of the aggregates and particles that the organic matter is associated with. For these prairie-derived soils, our study confirms and deepens this observation specifically for HA associated with particle-size fractions.