INFLUENCE OF HUMIFIED AND NON-HUMIFIED NATURAL ORGANIC-COMPOUNDS ON MINERAL DISSOLUTION

The most important solution chemical controls on weathering rates are the pH and concentration of chelating ligands. Here, data from control led laboratory experiments ard used to highlight pertinent differences between non-humified (root exudates, simple ligands) soil organic com pounds and humic substances. Particular attention is paid to the role that mycorrhizae-forming fungi play in the production of chemical weat hering agents. Using aluminum oxide as a model mineral, it is shown th at exudates from non-mycorrhizal roots and humic substances do not sig nificantly affect weathering rates, while exudates from ectomycorrhiza l roots effectively enhance dissolution rates. It is proposed that wea thering rates in the rhizosphere may be fast in comparison with rates in the bulk soil, due to exudation of simple (non-humified) organic li gands by mycorrhizal roots. On the other hand, the influence of humifi ed soil organic matter on mineral dissolution may be smaller than gene rally expected.