EFFECTS OF ROOT EXUDATES AND HUMIC SUBSTANCES ON WEATHERING KINETICS

The effect of complex natural organic ligands on the weathering kineti cs of aluminum oxide was investigated in laboratory experiments. A pea t-derived humic substance and root exudates obtained from ectomycorrhi zal (Picea abies - Hebeloma crustuliniforme) and non-mycorrhizal Norwa y Spruce trees; and gamma-Al2O3 were used as a model system. The exper imental weathering rates are in accordance with a surface-controlled d issolution mechanism. The effect of the humic material on dissolution rates appears to depend on the degree of protonation of the humic (mac ro)molecules: we observed dissolution-enhancement or -inhibition at pH 3 and 4, respectively. Ectomycorrhizal exudates proved to be effectiv e weathering agents at pH 4, as opposed to humic material and non-myco rrhizal exudates. Our results suggest that (i) the role of humic mater ials in mineral weathering and podzolization is different from what is commonly thought, and (ii) mineral weathering rates in the rhizospher e may be higher than in the bulk soil.