Soil oven-drying led to a significant increase in the DOM
concentration. The increase in Hy DOM accounted for 64 to 77%
of the increase in total DOC. This finding is consistent with our
hypothesis of microbial cell lysis being responsible, at least in
part, for the elevated DOM levels found after soil oven-drying.
Concentrations of FA and HA increased to a lesser extent after
soil oven-drying. The increases in these DOM fractions can be
attributed to disruption of organomineral complexes. Alternatively,
decreased binding of HA and FA by metal oxides converted
into more crystalline minerals during soil oven-drying may have
contributed to their increased concentrations. Soil oven-drying
caused significant increases in the total dissolved Cu and Ni
concentrations, due to complexation of these metals with the
elevated levels of HA and FA. Measured free concentrations of
Cd2+, Cr3+, Cu2+, Ni2+, and Zn2+ remained unchanged following
soil oven-drying, because the major part of the total amount of
sorbed metals in soil is generally present in the solid phase, in
particular SOM. Measured free Cd2+, Cu2+, Ni2+, and Zn2+
concentrations were predicted very well for both field-moist and
oven-dried soils using ORCHESTRA, which includes the NICADonnan
model. These results confirm the importance of humic
DOM in controlling the solubility of Cu and Ni. However, poor
predictions were obtained for Cr3+, for which better NICA
parameters need to be derived. For the measurement of total
dissolved Cu and Ni concentrations in soil solution extracts, it is
better to use field-moist soils. Both field-moist and oven-dried
soils can be used for the measurement of free metal
concentrations.