Dc. Olk et al., CHARACTERIZATION OF 2 HUMIC-ACID FRACTIONS FROM A CALCAREOUS VERMICULITIC SOIL - IMPLICATIONS FOR THE HUMIFICATION PROCESS, Geoderma, 65(3-4), 1995, pp. 195-208
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.