The ash from combustion of bio-fuel in power plants contains heavy metals and is currently regarded as a waste product only to be re-circulated to plantations and cultivated fields in small amounts (3 ton/ha three times over 75 years, Miljøstyrelsen 2008). Obviously, the economy of biofuel would improve if more ash was re-circulated. Implementation of the Danish energy strategy requires increased bio-fuel import, which makes increased ash re-circulation even more pertinent. Knowledge on safety and side-effects of ash-recirculation is urgently needed.
We will investigate the toxicity to soil organisms of combustion ash, with special emphasis on their contents of heavy metals, especially Cadmium (Cd): How does ash-toxicity depend on fuel source, receiver-ecosystem, and soil type? We will further examine bioaccumulation of heavy metals, which eventually leads to export to other environments via plant-feeding, fungal-feeding and carnivorous animals. Metal export via percolating water will also be examined. Ash addition will change nutrient content and pH of the soil. Depending on soil type, this may lead to accumulation or reduction of organic carbon in the soil. Besides toxicity of the ash, it is therefore essential to reveal if recirculation of ash will increase or reduce the amount of carbon retained in the soil, and thus improve or impoverish the CO2 budget of the whole energy production sector. Three major fuels, Norway spruce, willow, and cereal straw differ markedly in several respects. Hence ash toxicity, export of toxic compounds, and carbon accumulation in soil may also differ. We will amend ash at field scale, and in reduced laboratory systems. This combined effort is necessary to properly evaluate the effects of ash. Based on the results, we will modify toxic ash by addition or organic waste or by immobilization of heavy metals to decrease toxicity.