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 m ineralization in soil is that the result depends on the type of substrate a dded (Giller et al., 1998). Carbon mineralization in metal-contaminated soi ls 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 hush, following two drying and rewetti ng 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 d uring the 23 days of moist incubation following spiking. Metals slightly re duced 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 concent rations significantly (P < 0.05) increased by factors between 2.5 and 5.3. The Rush 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 degradatio n constant varied between 34 x 10(-3) and 90 x 10(-3) day(-1) for the uncon taminated soils. The degradation constants at the highest Zn rate were sign ificantly 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) w as only significant (P < 0.05) in two soils. Since drying-rewetting events are natural processes that promote C-mineralization in the topsoil, we beli eve that the decomposition of the DOC Rush may be a relevant indicator of t he effects of contaminants on C-mineralization in the long term. (C) 2001 E lsevier Science Ltd. All rights reserved.