Use of biofuel ashes in forestry

Negative environmental impacts of fossil fuel combustion have increased the use of biomass for energy production. As a consequence of the increased use of biofuels, the production of ashes will increase greatly in the near future. Because of relatively high contents of plant nutrients, biofuel ashes can be recycled as mineral fertilisers to compensate for the loss of nutrients resulting from tree harvesting. The present review aims to summarise the available information on factors affecting the quality of the wood and peat ash and the implications arising from ash application as fertiliser in the forest ecosystems. The understanding of the wood ash as fertiliser originates from the traditional slash-and-burn agriculture. During combustion most elements in wood are retained in the ash. The quality and chemical composition of ashes depends on many factors. The major elements in the ashes are calcium, potassium, magnesium, manganese, sodium, iron, phosphorus and sulphur. Trace elements found in different ash include barium, boron, cadmium, copper, mercury and zinc. The most abundant elements in peat ash are silicon, iron and aluminium. Ashes are characterised by high alkalinity with pH in the range from 11 to 13. Ashes raise the pH and reduce the total acidity in the humus layer and in the top of the mineral soil. The addition of wood ash does not result in a significant growth increase on mineral soils but increases the tree growth on peat soils. The Cd in wood ash did not become bioavailable and harmful to forest biota. The application of wood ash did not change or even decreased the ¹³⁷Cs activity within forest soil. Wood ash application increased the coverage of the ground vegetation in upland forests and on peatland. Increase in microbial activity and growth rate after ash treatment was observed.

The positive effect of wood ash on plant growth was understood long ago as better growth of grasses on areas burnt by natural fires was noticed. Scientific study of the effects of ashes began in the first half of the 20ᵗʰ century when the fertilisation experiments with wood ash were started in Sweden (1918) and in Finland (1937). Experiments of fertilisation with wood ash of drained peatlands in Finland are classical and the plots are used for studies up to now.The quality and chemical composition of ashes depends on many factors, including type of the fuel, tree species, type of the plant tissue, degree of the processing of the fuel before combustion, type of the burner and incineration conditions, and proportion of bottom and fly ash in the end product. The major elements in the ashes are calcium, potassium, magnesium, manganese, sodium, iron, phosphorus and sulphur. Ash is generally low on nitrogen because it is vaporises during combustion. Trace elements found in different types of ash include barium, boron, cadmium, copper, mercury and zinc. Compared to wood ash, peat ash contains essentially less mineral elements. The most abundant elements in peat ash are silicon, iron and aluminium. Ashes are characterised by high alkalinity with the pH ranging from 11 to 13.When applied to a soil, ashes will raise its pH and reduce the total acidity in the humus layer and in the top of the mineral soil. A rise in the concentration of base cations has been reported. Wood ash application has been found to increase the levels of extractable phosphorus in humus layer when higher ash doses are used and has little impact on total nitrogen concentrations in soil due to its low levels in the ash.The addition of wood ash does not result in a significant growth increase of trees on mineral soils. The limiting factor for tree growth on mineral soils is in most cases the availability of nitrogen. Wood ash promotes the growth of trees and improves the growth conditions on peat soils, and these positive effects are long lasting. Application of low peat ash doses did not increase the biomass production of trees but has a positive effect when larger amounts are applied.The effects of cadmium on ecosystems and a hazard to human health are of particular concern. There is currently not enough scientific knowledge available to recommend restrictions on the wood ash use as forest fertilizer due to high cadmium concentrations. Generally, the cadmium in wood ash did not become bioavailable and harmful to forest biota. Radionuclide content of forest biomass harvested for energy is related with wood ash application. Results based on experiments show that the application of wood ash does not change or even decreases the ¹³⁷Cs activity within forest soil and vegetation. On peatlands an increase in the coverage of herbs and grasses and colonisation by nitrophiles has been observed. In addition, sphagnum mosses are substituted by forest mosses. However, wood ash may cause burn damages to bryophytes and lichens.According to some published data, the effect of wood ash on fauna consists in changes in the concentrations of heavy metals and radionuclides in body tissues. In most wood ash studies increased microbial activity and growth rate after ash treatment are reported.