Wood ash decreases cadmium toxicity to the soil nematode Caenorhabditis elegans

Wood ash is a beneficial fertilizer and liming agent in nutrient depleted soils, but it also contains considerable amounts of cadmium (Cd), which can be toxic to organisms in the environment. Therefore, risk assessments regarding utilization of wood ash is required. Here, we studied how wood ash (applied in doses equivalent to 0, 3 and 6 t ha^-1) and Cd (applied in doses of 0, 10, 150, 300, 600, 1200 and 2000 mg kg^-1) affected growth of the soil nematode Caenorhabditis elegans. The treatments were combined in a full factorial design. Wood ash alone greatly stimulated both soil respiration and growth of C. elegans, whereas Cd alone had a toxic effect. However, unrealistically high Cd levels were needed to severely affect growth of C. elegans and soil respiration, especially soil respiration was very resilient to Cd amendment. Ash addition decreased Cd toxicity to C. elegans, with an EC₅₀ value of 390 mg Cd kg^-1 in the 3 t ash ha^-1 treatment, and an increase of EC₅₀ to 1894 mg Cd kg^-1 in the 6 t ash ha^-1 treatment. This is probably because ash increases the Cd sorption capacity of the soil, and thereby decreases the bio-availability of Cd. The results suggest that there is no acute toxic effect of Cd to nematodes associated with wood ash recycling; in fact, our results suggest that ash actually decrease Cd toxicity.