EFFECT OF NATURAL ORGANIC SOIL SOLUTES ON WEATHERING RATES OF SOIL MINERALS

The rate at which minerals in the soil weather is affected by pH and c oncentration of organic solutes (DOC). The rates of release of Al, Ca, Fe, K, Mg, Na, P and Si from a mineral soil sample to solutions of na tural organic solutes and HCl (control) were determined at pH 3 and 4 for up to 17 weeks. Soil solutions were collected by centrifuging mate rials of O horizons from various soil types under four tree species (s pruce, birch, oak, beech) and passed through a cation-exchange resin t o yield H+-saturated organic solutes. The acceleration of the elements ' release by the organic solutes was shown directly by the relative li gand effect (RLE), that is, the release rate in the organic solute sol ution divided by the release rate in the HCl solution (control) at the same pH. The RLE was greater at pH 4 than at pH 3, and it decreased f or the elements in the order Fe > Ca > Mg > Al approximate to Si > K a pproximate to Na. This indicates that natural organic solutes are more important weathering agents at higher than lower pH and for weatherin g of mafic minerals rich in Ca, Fe and Mg than of felsic minerals such as K- and Na-feldspars. For all elements and at both pHs, RLE was str ongly correlated with the concentration of DOC, which was also closely correlated with titratable acidity of the organic solutes. The import ant effect of soil type and tree species in producing weathering-promo ting organic solutes therefore seems to be expressed through the conce ntration and not the composition of the organic soil solutes.