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 controlled laboratory experiments ard used to highlight pertinent differences between non-humified (root exudates, simple ligands) soil organic compounds and humic substances. Particular attention is paid to the role that mycorrhizae-forming fungi play in the production of chemical weathering agents. Using aluminum oxide as a model mineral, it is shown that exudates from non-mycorrhizal roots and humic substances do not significantly affect weathering rates, while exudates from ectomycorrhizal roots effectively enhance dissolution rates. It is proposed that weathering rates in the rhizosphere may be fast in comparison with rates in the bulk soil, due to exudation of simple (non-humified) organic ligands by mycorrhizal roots. On the other hand, the influence of humified soil organic matter on mineral dissolution may be smaller than generally expected.