Jesper Liengaard Johansen:
Biofuel such as wood biomass is gaining ground as a CO2 neutral alternative to fossil fuels. Recycling of wood ash, which is a by-product from the combustion of wood biomass, to production forests secures a sustainable circular flow of nutrients, and have numerous beneficial effects on soil fertility. However, wood ash also contains considerable amounts of the heavy metal cadmium (Cd), which may cause problems to the soil ecosystem and ultimately, due to bio-accumulation, affect human health. The methods traditionally used for risk-assessments and threshold values may be out-dated and based on precautionary principles. These methods generally test the xenobiotic, here Cd, in a standardized growth conditions. Therefore, studies that focus on the specific target area; the soil, and the product; the ash, is needed.
I studied different aspects of Cd toxicity and bio-accumulation in relation to wood ash recycling. The specific research areas were 1. Toxicity of Cd in soil; 2. Wood ash induced changes in the soil bio-availability of Cd; 3. Synergistic effect of Cd and Zn; 4. The effect of mycorrhiza on Cd accumulation; and 5. Bioaccumulation of Cd.
I found that very large amounts of Cd is needed to affect soil-living organisms and processes, because sorption processes in the soil bind Cd in an insoluble form which leave only a small fraction available to the organisms. In addition, wood ash increase the sorption capacity of the soil and thus decrease Cd toxicity further to soil organisms, and may decrease plant accumulation of Cd. I found that Zn, which is present in wood ash in 100:1 ratio with Cd, generally decrease plant accumulation and toxicity of Cd. Indeed, I saw a reduction in Cd accumulation by plants, and a decrease in Cd toxicity to soil protists grown in aqueous media when Zn was present. However, I did not see this phenomenon in protists living in soil. I also found that mycorrhiza have the ability to decrease Cd accumulation in their host plant, and that the mycorrhizal colonization increases under moderate Cd stress. This may be important under natural conditions where the mycorrhizal symbiosis is extremely common. Lastly, I measured the Cd accumulation in different organisms exposed to wood ash. In most cases, the organisms did not accumulate more Cd, but in three cases, we found an increase in Cd concentration, however, these concentrations were still low and not to be considered toxic.