Decomposition of dissolved organic carbon after soil drying and rewetting as an indicator of metal toxicity in soils

One of the drawbacks of respiration tests to identify metal toxicity on C mineralization in soil is that the result depends on the type of substrate added (Giller et al., 1998). Carbon mineralization in metal-contaminated soils was measured using the native soil organic matter as the substrate. The method is based on monitoring the decrease in the dissolved organic carbon (DOC) in soil solution after the DOC flush, following two drying and rewetting cycles. Four agricultural topsoils were spiked with ZnCl2 at 50, 150 and 500 mg Zn kg−1. The DOC concentration in soil solutions did not change during the 23 days of moist incubation following spiking. Metals slightly reduced the DOC in all soils but this effect was significant (P<0.05) only in one soil. After the two air-drying and rewetting cycles, the DOC concentrations significantly (P<0.05) increased by factors between 2.5 and 5.3. The flush in carbon after rewetting consistently decreased in the following 24 days of moist incubation in all uncontaminated soils, and this decrease was less pronounced in metal-contaminated soils. The first-order degradation constant varied between 34×10−3 and 90×10−3 day−1 for the uncontaminated soils. The degradation constants at the highest Zn rate were significantly lower by between 2.4 and 12-fold compared to the control for all soils (P<0.05). Inhibition of the DOC decomposition at 150 mg Zn kg−1 was only significant (P<0.05) in two soils. Since drying–rewetting events are natural processes that promote C-mineralization in the topsoil, we believe that the decomposition of the DOC flush may be a relevant indicator of the effects of contaminants on C-mineralization in the long term.