Background and hypotheses

Renewable energy sources (wind, biofuel, sun etc.) constitute 1/5 of the total Danish energy consumption. Wood and cereal straw are by far most important, and accounts for about 75% of the renewable energy (Energistyrelsen, 2010). The use of wood in energy production has increased 400% during the last 20 years; imported wood makes up more than half of the increase. This trend will likely continue; thus a doubling of the present level is expected before 2020. Annual costs of wood ash disposal add up to DKr. 25-40 mill. Plant nutrients contained in wood ash are lost when ash is deposited. Thus recycling of the ash as fertilizer, will be a gain for the bio-fuel production and should be encouraged, provided unacceptable side effects, such as toxicity, bioaccumulation of heavy metals, or impoverished ecosystem functioning, can be avoided.

Presently, due to the Cd-content of the ash, ash must not be applied more than three times during a 75 year period, and each delivery must not exceed 3 T ha-1 (Miljøstyrelsen, 2008). The basis for setting this limit, however, is a single project (Eksamensarbete from 2004 at Borås Högskola, Sweden). The main immediate effect on ecosystem functioning of ash addition is probably mediated via soil pH changes (Nohrstedt 2001). Previous studies have indicated that the threshold between slight and dramatic ecosystem effects for single-dose applications is around 12 T ha-1 (Augusto et al. 2008). Estimates of threshold levels for ash addition that keep Cd levels below critical levels are rather inconclusive (Pitman 2006, Augusto et al. 2006). However, if all ash from biofuel in Denmark was to be returned to Danish plantations, the annual increase in soil Cd would be 0.02% p.a., i.e. 2% over a century, which does not imply serious side-effects. We thus expect that ash can be administered, at higher loads than presently allowed, without any risk for the environment. To do so safely require, however, documentation that the soil ecosystem is not affected in an unacceptable way.

A healthy community of soil organisms is pivotal for the performance of essential ecosystem services; i.e. provision of nutrients to plants and accumulation of organic material, which sequesters carbon and prevents leaching of Cd. Further, the composition of the soil microbial communities governs the production of green house gasses such as N2O and CH4. Last but not least, the Cd from the ash may accumulate in the soil food webs, and thus potentially harm e.g. birds feeding on earthworms or humans feeding on fungi. Norway spruce, willow, and cereal crops are important bio-fuel producing plants. Spruce and willow plantations, and arable cultivated fields, are ecologically very contrasting environments, with different pH, mineral nutrient conditions and biological communities. Hence, their potential for retaining Cd, soil carbon and other ecosystem services will differ.

In ASHBACK, we will perform controlled experiments on toxicity of ash to soil organisms, assess bioaccumulation of heavy metals in organisms in the food web, analyse whether ash will induce an elevated or reduced production of trace gases to the atmosphere, and whether ash will increase or diminish breakdown of soil organic matter in plantations. If ash increases organic matter breakdown and/or production of trace gases it will be a drawback on ash return, as the reduction in emission of CO2 from fossil fuel with use of bio-fuel is then replaced by increased emission of greenhouse gases from soils.

General hypotheses

A. More ash than currently allowed can be applied without the risk of (i) toxic effects, (ii) dangerous bioaccumulation, or (iii) significant changes in ecosystem functioning.
B. At moderate concentrations, ash effects are caused by changes in system pH and nutrients in the ash, and are not due to toxic effects.
C. Ash hardening does not affect toxicity.
D. Mixing of ash with organic waste products reduces toxicity considerably, and, results in a product that is a valuable fertiliser for both plantation and farmed field soils.

Working (testable) hypotheses

1) Toxic effects of ash are more pronounced at low soil pH due to the higher mobility of metals.
2) Critical bio-accumulation will correlate positively with size and lifetime of organisms and with food chain length.
3) Bioaccumulation in surface dwelling fauna is most critical in systems where earthworms are abundant. This can lead to unacceptable export via birds.
4) Bioaccumulation in fungal fruit bodies prevail in more acidic plantations. This may lead to unacceptable uptake of Cd in mammals including humans.
5) Bioavailability of heavy metals in ash is reduced if the ash is mixed with organic waste products such as animal slurry and/or biogas sludge.
6) Addition of easily available nutrients in ash stimulates microbial activity and decomposition in fertile soils giving humus degradation and increased flow of CO2 to the atmosphere.
7) Easily available nutrients in ash alter the microbial activity patterns and thus decomposition rates. In low fertility soils, humus degradation is reduced and carbon sequestration increased.
8) Leaching of Cd to the subsoil aquifers is not a problem; however, metal mobility in the upper layer is larger in plantations of lower pH.
9) Greenhouse trace gas (N2O) production can increase due to ash if decomposition rates, and hence oxygen consumption, increase with a resulting higher frequency of anaerobic sites.