Rates of dissolved organic matter release and sorption in forest soils

Rapid percolation enhances dissolved organic matter (DOM) transport in soils. One reason for this is nonequilibrium conditions, which make the time-dependence of the sorption/desorption an important parameter. We investigated the time-dependent DOM release and sorption of four soils (one topsoil, three subsoils) with different properties (organic carbon (OC): 2-56 g kg(-1); dithionite-citrate-bicarbonate-extractable Fe: 2.4-21.9 g kg(-1); clay: 30-450 g kg(-1)) in batch experiments during a 24-h period. Release of dissolved organic carbon (DOC) when a DOC-free solution was added occurred mainly within the first 15 min and then decreased drastically. After 2 to 4 h, the release almost stopped, but steady-state conditions were not reached within the experimental period. DOM, fractionation with XAD-8 adsorber resin showed that the major portion of the released DOC exhibited hydrophilic properties. At an addition of 14 mmol DOC kg(-1), the topsoil still released DOC. One subsoil, which was rich in Al and Fe oxides and low in OC, sorbed both hydrophilic and hydrophobic DOC strongly, with a preference for the hydrophobic fraction. The other two subsoil horizons showed a sorption of hydrophobic DOC that was accompanied by a steadily increasing release of hydrophilic DOC. This indicates competition between the two DOC fractions, with hydrophobic DOC having the stronger affinity to sorbent and displacing soil-bound hydrophilic substances. The reactions approached steady-state conditions after 2 to 4 h. The time-dependency of the release and sorption can be described adequately by the Elovich equation and the fractional-power model. In a few cases, the DOC release from soil was represented better by the parabolic-diffusion equation. The slope of the Elovich equation, which may be considered the apparent rate constant, was higher for the hydrophilic than for the hydrophobic DOC fraction. It is related to the ratio between OC and Al and Fe oxide content.