Characterization of two humic acid fractions from a calcareous vermiculitic soil: implications for the humification process uri icon

abstract

  • Increased soil organic matter (SOM) is associated with a reduction in K fixation by a calcareous vermiculitic soil in California. To better understand the mechanism for this effect, humic acids (HA) were extracted from this soil with and without decalcification. The HA extracted without decalcification - the mobile HA (MHA) pool - was dated as modern by C-14-dating, while the radiocarbon age of the HA bound to Ca (CaHA) was estimated at 290 yr. Compared to the MHA, the CaHA had increased aromaticity and C condensation, increased substituent carboxyl groups, and decreased substituent amino acids. Whereas MHA was evenly distributed through a 0-60 cm soil profile, the CaHA was concentrated in the 0-20 cm surface layer. The CaHA pool is probably protected from chemical and biological degradation through development of bonds to exchangeable Ca and possibly structural Fe. The MHA may be bound to the soil through exchangeable monovalent cations, through attachment of peripheral amino acids to exchange sites or fixation sites, or through other SOM molecules. No evidence was found to suggest different source materials for the two HA fractions. Although the MHA and CaHA are chemically distinct, these carbon pools may represent early and late stages of the humification process in aerated soils in temperate climates.

publication date

  • 1995
  • 1995
  • 1995