CHARACTERIZATION OF 2 HUMIC-ACID FRACTIONS FROM A CALCAREOUS VERMICULITIC SOIL - IMPLICATIONS FOR THE HUMIFICATION PROCESS

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 extrac ted from this soil with and without decalcification. The HA extracted without decalcification - the mobile HA (MHA) pool - was dated as mode rn by C-14-dating, while the radiocarbon age of the HA bound to Ca (Ca HA) was estimated at 290 yr. Compared to the MHA, the CaHA had increas ed aromaticity and C condensation, increased substituent carboxyl grou ps, and decreased substituent amino acids. Whereas MHA was evenly dist ributed through a 0-60 cm soil profile, the CaHA was concentrated in t he 0-20 cm surface layer. The CaHA pool is probably protected from che mical and biological degradation through development of bonds to excha ngeable Ca and possibly structural Fe. The MHA may be bound to the soi l through exchangeable monovalent cations, through attachment of perip heral amino acids to exchange sites or fixation sites, or through othe r SOM molecules. No evidence was found to suggest different source mat erials for the two HA fractions. Although the MHA and CaHA are chemica lly distinct, these carbon pools may represent early and late stages o f the humification process in aerated soils in temperate climates.