Sustainability in the use of solid woody biofuels requires that forestry is sustain­able. Conventional forestry has an acidifying effect on soils, and intensive harvesting has an even greater effect. “Intensive” is taken to mean whole-tree­harvesting, i. e. timber and pulp wood as well as logging residues. “Sustainabil­ity” implies that the mineral nutrients removed in an intensive harvest are returned to the forest soil, e. g. as ash. A good yield of biomass in the future is an important motive, but maybe even more important is to restore the buffering capacity of the soil.

Compensation with ash after harvesting logging residues is an environmental measure encouraged and regulated in Sweden. The details can be found in the recommendations from the Swedish Forest Agency (2002, 2008), which are the fruit of extensive research activities stretching back to the 1980s and financed largely by the government. The environmental impact assessment of removing logging residues published by the National Forest Agency presents the collected scientific basis for the recommendations (Egnell et al. 1998).

To return the desired quantities of mineral nutrients, one needs to keep track of the quantities of logging residues actually removed, which vary depending on soil fertility, essence, etc., and the ash content. Regarding the composition of ash, the basic principle behind the recommendations is that what has been taken away may be returned, nutrients as well as less desirable constituents. A guideline on limit concentrations for elements in ash is then obtained; see Table 11.3.

A consequence of the governing principle of balance is that the tolerable concentration of certain elements in ash is higher than if ash had been considered as a fertiliser. In the latter case, everything in the ash is a net addition to the load on the soil, and the net increase of the concentration of these elements in the soil should be limited. When compensating, one need only to see that the net addition is negligible. Table 11.4 (H. Eriksson and Swedish Forest Agency, personal commu­nication) provides a background for Table 11.3: it summarises the results of analyses of ashed softwood with respect to these contentious trace elements. For each element, the median of the available set of data and the 85th percentile are presented; the number of data ranges from a dozen to slightly more than 100.

The main result of studies of the environmental impact of ash is that these high concentrations of trace elements are not an important issue with the low doses of ash that are being spread for compensation purposes (Egnell et al. 1998). The basicity is a greater problem: ash may not be used fresh from the furnace. Ash must be stabilised by mixing it with water and allowed to mature during storage so that its pH is lowered. To these recommendations, one should add the regulation from the Radiation Protection Authorities that limit the concentration of caesium-137 in ash that is to be used. This is of course owing to the impact from the Chernobyl accident.

In principle, it should always be possible to spread ash from clean wood to forest soils. However, it is unavoidable that some residues will have concentrations in excess of these recommended limits. Some trees (birch, pine and willow) may naturally contain excessively high quantities of cadmium. Fractionation in the furnace will remove cadmium and caesium from the bottom ash of a grate furnace and concentrate them in the fly ash. Fractionation does though also reduce the concentration of potassium and zinc in the bottom ash. Other elements that may cause concern are chromium and sometimes nickel, a result of corrosion in the furnaces or in the fuel handling equipment. An illustration of the span of actual concentrations is given in Table 11.5 for some nutrients and in Table 11.6 for trace elements.

The only combustion residues that consistently meet the requirements are fly ashes from FB furnaces. One should remember that their content of objectionable substances is diluted by the bed material that is entrained with the fly ash.

Phosphorus is especially important as conventional harvest already creates a deficiency in many stands, and the deficiency is accentuated by intensive harvesting where logging residues are removed. The experience acquired hitherto is that the phosphorus content of the combustion residues generally does not reach the desired levels. Zinc is also problematic, as the zinc content of bottom ashes seldom exceeds the lowest level in the recommendations.

A guideline for the classification of ash from solid biofuels and peat has been published by Nordtest (Haglund and Expert Group 2008) targeting its use in recycling and fertilising. A table comparing guideline values for elements in ash in the Nordic countries is provided there. If an ash does not meet all of the requirements for spreading on soils as a compensation measure, it should be used as material in civil works.

However well accepted by authorities, compensation has not yet really taken off: only part of the logging sites that should receive a compensatory dose of ash has actually received one. The Swedish Forest Agency has intensified the distribution of information to stakeholders in order to push adoption of compensation with ash, for example through the EU-Life project RecAsh (Emilsson 2006). The Web site of RecAsh has been closed down, but all information is now available in Swedish, Finnish and English on the Web site of the Swedish Forest Agency (http://www. skogsstyrelsen. se).

The Ash Programme has chosen to complement the RecAsh project as well as the extensive R&D programmes on wood ash by focusing on biomass growth. It was perceived that one essential non-technical barrier is the absence of an economic incentive: spreading ash to soils in doses of a couple of tonnes per hectare is costly but the forest owner is not to expect an immediate return as increased growth. If increased growth could nevertheless be demonstrated for at least some circum­stances, one could expect an increased general interest. However, this is not an easy proposition as growth in Swedish forests in most cases is limited by the availability of nitrogen.

In these discussions one has to consider mineral soils separately from organic soils (peat lands), as they are two different cases. Much of the R&D leading to the official recommendations dealt with mineral soils. Although some increased growth was observed on fertile sites, some reduced growth could be observed on less fertile sites. None of the figures in isolated studies are statistically significant, but the general trend is convincing. However, public opinion has mostly retained the negative part of the conclusions, i. e. decreased growth. On the other hand, there is plenty of historical evidence for increased growth of drained peat lands as a result of applying ash. The adverse effects of ash on mineral soils are now well known, but it is feared that applying ash to peat lands will, for example, increase the emission of greenhouse gases.

There is controversy regarding the causes for increased growth on mineral soil. In one school of thought, the basicity of ash increases the availability of nitrogen to vegetation. In the other school of thought, increased growth is a result of an improved nutrient supply, primarily phosphorus. Older experimental fields have been revisited within the Ash Programme and new ones have been established (Sikstrom et al. 2006, 2009a; Thelin 2006, 2009). Evidence is coming in, but slowly. So far, none of the interpretations have been invalidated.

Peat land is usually taken to be only the present peat bogs or peat cutovers. However, in the 1950s and 1960s almost one million hectares was drained and afforested in Sweden. Growth is very variable as peat lands are almost always deficient in several mineral nutrients. In a prestudy by Hanell and Magnusson (2005), it was found that spreading ash as a fertiliser on approximately 190,000 ha, i. e. compensating for the future harvest before it is actually carried out, could yield a short-term profit. Larger areas could benefit from an application of ash, but it will take more time to obtain harvestable trees.

The priority of the Ash Programme is to address possible adverse effects. In a series of studies, the effect of ash on water chemistry, on the production of greenhouse gases (methane, nitrous oxide, carbon dioxide), on biodiversity and on microbial mass was investigated (Sikstrom et al. 2006, 2009b), as was also done by other authors (Kuba et al. 2008; Bougnom et al. 2010). The results do not indicate any important negative effect: the production of methane is mostly unaf­fected, that of carbon dioxide decreases, biodiversity increases…

The Ash Programme is now shifting its emphasis towards growth on fertile mineral soil. Peat lands remain interesting targets in northern Sweden, but nature conservation requirements as well as difficulties to use modern and heavy machines on such soils make them less interesting in southern Sweden.