The parent material is the geological horizon from which soil horizons form; it is a key factor that in many cases determines the kinds and contents of secondary minerals of soils (Arbestain et al. 1999). Soils that develop from granite are likely to be more acidic than soils developed from calcareous limestone. In tropical and subtropical areas, under rainfall and high temperatures throughout the year, the process of acidification occurs over a long or a short time with the weathering of the soil parent material that liberates significant amounts of silica, iron, and aluminum and subsequent association of minerals of low crystallinity and aluminum-humus complexes (Garcia-Rodeja et al. 1987). In the humid tropics, most silicate minerals in the parent material are weathered away by desilication, leaving little other than the oxides of iron and aluminum (Sumner and Noble 2003).

A sample of WBFA was dry-mixed with Portland cement (CEM I 42.5R) with an ash-to-cement ratio of 30:70 by mass, and the resulting blended cement (binder) was used for environmental compatibility studies. Table 10.2 gives the chemical

and mineralogical compositions of the Portland cement, the composition of the latter being calculated by the Bogue method.

Cement pastes were prepared manually by using the blended cement and deio­nized water as mixing water, at a water-to-binder weight ratio (w/b) of 0.50.

Cubic specimens, with sides of 40 mm, were cast from the cement paste and, after 24 h of curing within the molds, the specimens were demolded and cured for 28 days in a controlled temperature and humidity environment (20° C and relative humidity above 95%). Afterwards, the cubic specimens were subjected to the monolith leaching test.

Effect on Soil Microorganisms

Heterotrophic organisms in the soil are ultimately responsible for ensuring the availability of nutrients for primary production (Wardle 2002). Microorganisms play a very important role in many biogeochemical cycles in agroecosystems including organic matter decomposition, nutrient mineralization, and trace gas emission and consumption (Carney et al. 2004). The principal “players” in the decomposition process are microorganisms, i. e., bacteria, archaea, and fungi.

Bacteria are able to perform an extremely wide range of chemical transforma­tions, but are, however, only active over a very narrow range of environmental conditions (Lavelle and Spain 2001). As with all microorganisms, bacteria have a system of external digestion mediated through the production of extracell­ular enzymes, and some of the metabolites released by extracellular digestion may be used by other organisms, thus creating a trophic stimulus for opportunistic or cooperating microorganisms (Hattori 1973; Lavelle and Spain 2001). Until recently, archaea were considered to occur in extreme environments only, but their presence was also reported in numerous other habitats, including forest and agricultural soils, where their potential for ammonia oxidation was demonstrated (Bintrim et al. 1997; Pace 1997; Prosser and Nicol 2008). Bacteria and archaea, on the one hand, and fungi, on the other, differ biochemically and morphologically. Fungi are larger than bacteria and have hyphae that can grow into and explore distant microhabitats, and translocate carbon and nitrogen and other nutrients within the hyphal network. Thus, fungi are regarded as being more capable than bacteria and actinobacteria in degrading polysaccharides (Atlas and Bartha 1998; Lavelle and Spain 2001). The broad functions of fungal mycelium in soil and litter are decomposition and nutrient cycling. In contrast to bacteria, fungi can remain active in soils at very low water potential (—7,200 kPa) and are better suited than bacteria to exist in interpore spaces (Shipton 1986). These microorganisms influ­ence or control ecosystem processes and form mycorrhizal interactions with plants (Coleman 2001; Wardle 2002).

Soil microbial community diversity has been suggested as a way of assessing the “health” or “quality” of soils (Chapman et al. 2007). High biodiversity may be vitally important in structurally diverse ecosystems such as soil because it may promote productivity and stability of this environment (Grime 1997; van Bruggen and Semenov 2000). The biodiversity of fungal or bacterial populations in the rhizosphere is closely related to growth of crops; hence, crop yield may be used as an indicator of soil health associated with greater stability in productivity (Lynch et al. 2004).

The effect of different composts on the microbial biomass and diversity depends in part on the amount used and very strongly on the compost quality (Ros et al. 2006). Populations of rhizosphere microorganisms were reported to increase in relation to increasing inputs of composted organic matter to soil, and compost application has been found to enhance biomass nitrogen, carbon, and sulfur content and microbial activity over several years (Perucci 1990; Ros et al. 2006). Single and repetitive applications of different amounts of organic wastes signifi­cantly increase the amount of soil microbial biomass and enhance nitrogen mineralization potential, but excessive rates of application (100 t ha—1) reduce the functional diversity of the microbial community (Banerjee et al. 1997). Several studies have reported modification of both bacterial and fungal community structure following application of compost (Crecchio et al. 2004; Ros et al. 2006; Innerebner et al. 2006). Increases in dehydrogenase, p-glucosidase, urease, nitrate reductase, and phosphatase activities were observed 3 months after application of municipal solid waste compost (Crecchio et al. 2004). Some composts rich in heavy metals (Zn, Cu, and Pb) have been reported to decrease enzyme (phosphatase and urease) activities, whereas other enzymes (dehydrogenase, catalase, protease) were not affected (Garcia-Gil et al. 2000).

The two types of fluidised bed furnaces commonly distinguished are the circulating fluidised bed furnace and the bubbling fluidised bed furnace. The bed material, usually sand, is finer for the circulating fluidised bed furnace than for the bubbling fluidised bed furnace, 0.3 and 2 mm respectively. Part of the bed is bled out to keep an acceptable chemistry in the bed: alkalis tend to dissolve in the sand and reduce

There are other types with special designs, e. g. cyclone furnaces, but they are not common.

the temperature at which the particles sinter together. The combustion temperature is low, approximately 850°C.

The largest part of the residue is fly ash. The proportion of bottom ash depends on how much material is bled from the bottom of the bed. Both streams of ash contain not only ash from the fuel but also bed material, oversized material in the bottom ash and fine particles from attrition in the fly ash.

The combustion temperature is too low to allow the Portland cement reactions in the fly ash (these require at least 1,400°C). However, they still have binding properties through other reactions which are not fully known. Because of the low combustion temperatures, the particles have irregular shapes. The compressive strength is good even in the green stage if they have been well packed. Among other properties, this makes them very good materials for road building. Mixed in equal proportions with digested sewage sludge, they also provide dense sealing layers to landfills.

The bottom ash contains mostly bed material. It is often too fine and has too narrow a size distribution to be useful in construction, other than as low-quality fill material. However, under certain circumstances, e. g. absence of binding properties, it could be used as backfill material in trenches for piping.

Analysis of variance was used to test for effects of multiple wood ash application on soil chemical properties and foliar concentration of macroelements and microele­ments, with the PROC GLM procedure of SAS (SAS Institute 2004).

Normal diameter and total height were measured annually in all trees within the plots at the end of the growing season for a period of 3 years. The breast diameter was measured in two directions with a caliper, to an accuracy of less than 1 mm. Tree height was measurement with a Vertex III hypsometer. A volume equation based on the allometric model of Schumacher and Hall was used to calculate the tree volumes. The parameters of this model were estimated by Castedo (2004) by use of the following expression:

v = 0.000048 x d20062 x ht0’86691,

where v is the bark volume (m3) of individual trees; d is the breast diameter in centimeters, and ht is the total height in meters. The total height and breast diameters were compared by repeated measurement analysis with the PROC GLM procedure of SAS (SAS Institute 2004) after prior confirmation of the assumptions of equal (Levene test), normal (Kolmogorov-Smimov test), and inde­pendent variance. The mathematical model used was as follows:

yij = m + pxit + Ti + Dj + Ti x Dj + eij,

where yij is the random variable representing the value in the jth observation of the ith treatment, m is a constant representing the mean response of the variable; pxit models the linear relationship between the response and the covariate (initial height or diameter measured before the application of ashes), Ti and Dj are the effects of treatment i (control, WA, and WAP) and the time j (0,1,2,.. .4 measurements), respectively, Ti x Dj is the interaction effect of treatment i by time j, and eij is the experimental error.

In Table 8.9, an overview of the amounts of fertilizer/ash to be used on 1-ha plantations is given for both scenarios. The amount of fertilizer in scenario 2 needed to compensate for the amount of filter ash used in scenario 1 is 1.1 kg/ha for triple superphosphate and 4.1 kg/ha for potassium chloride.

Table 8.8 Overview of the main characteristics of the combustion of 74 kg cacao shells in the

Decay of organic matter, inappropriate use of nitrogenous fertilizers, and the removal of alkaline plant material from the field in cropping systems further accelerate soil acidification (Vieira et al. 2008). Some studies have shown that legumes increase soil acidification in pastures and arable cropping systems (Williams 1980; Burle et al. 1997). Acid production during carbon and nitrogen cycles is considered to be the most relevant in agricultural and pasture ecosystems (Helyar and Porter 1989). In non — polluted areas, soil acidification is mainly caused by the release of protons during the cycling of carbon, nitrogen, and sulfur in the soil-plant-animal system (Ulrich and Summer 1991). In a balanced system, carbon, nitrogen, and sulfur cycling processes are coupled and there is no generation of acidity. Perturbations such as uncoupling of nutrient cycles, accumulation of soil organic matter, leaching of nutrients (mainly NO3~) and the mobile exchangeable basic cations (Ca2+, Mg2+, K+, and Na+), and application of nitrogen fertilizers (which cause oxidation of NH4+ to NO3~) generate soil acidity. NO3~ is not strongly adsorbed by the soil and will leach, and if it is not totally taken up by the crop, increased soil acidification will occur as a consequence (Bolan and Hedley 2003).

The monolith leaching test used in this study was a modified version of the conventional NEN 7345 leaching test (Dutch Standardization Institute 1995).

The conventional NEN 7345 test consists of immersing the test specimen in a closed polypropylene reactor containing deionized water as a leachant (initially acidified to pH 4.0 by nitric acid addition). A liquid-to-solid volume ratio (L/S) of

5.0 was used. During the test, there was no agitation of the leaching solution and the pH was free to change according to the acid or basic characteristics of the com­pounds released from the test specimen. The leachate was periodically replaced with an equal volume of leachant, after cumulative leach times of 0.25,1,2.25,4, 9, 16, 36, and 64 days. The eight leachates were filtered through a 0.45-p. m membrane filter and, after pH and electric conductivity measurements had been conducted, they were acidified to pH 2.0 with 5 M nitric acid. Finally, each leachate was analyzed for the concentrations of selected heavy metals (in this study, Cd, Cu, Cr, Ni, Pb, and Zn) by using an atomic absorption spectrophotometer equipped with a graphite furnace.

The modified leaching test used in this study differed from the conventional NEN 7345 test only for the continuous and moderate agitation of the leaching solution within the reactor (realized by a magnetic stirrer) and, especially, for the pH evolution throughout the test. In particular, the pH was kept constant to a predetermined value by means of an automatic titrator. The pH was monitored constantly, and the titrator compensated for alterations of pH by small additions of 0.5 M HNO3 from a burette. A pH of 6.0, which is characteristic for most natural waters, was selected for the modified NEN 7345 test. The addition of nitric acid was motivated by the necessity of compensating for the pH rise due to the release of alkaline compounds (alkalies and calcium hydroxide) from the cemen­titious test specimen, without altering the leachability of heavy metals through complexation reactions. Indeed, the nitrate ion is known as a poor complexing agent toward most heavy metals. Figure 10.1 illustrates the apparatus used for the monolith leaching test.

Three replicate leaching tests were performed and the test results were averaged. In each test, one cubic specimen (64 cm3 in volume) was used and was moistened with 320 cm3 of deionized water (L/S = 5.0; eight leachant renewals).

The relative contributions of the soil fauna to microbial turnover and nutrient mineralization are directly related to the demographics of the soil biota (Coleman et al. 1983). Many invertebrates (e. g., earthworms, termites, and ants) play an important role for soil fertility; they produce macropores (e. g., galleries, chambers) and organomineral structures that influence hydraulic properties, macroaggre­gation, and organic matter dynamics in soil (Lavelle 1997, 2002). The importance of earthworms in enhancing nutrient availability and raising the rate of nitrogen turnover through the breakdown and incorporation of organic matter into the soil was demonstrated (Basker et al. 1992). Leroy et al. (2007) and Moreira et al. (2008) reported a significant increase of earthworm density and biomass after compost application. Earthworms can affect other soil-inhabiting invertebrates by altering their resource base, affecting soil structure, by direct ingestion, and by dispersing them (Blair et al. 1995); a direct effect of earthworm activities following compost application is an increase in the number of trophic groups of soil-inhabiting arthropods (phytophages, predators, omnivores, and saprovores) (Gunadi et al. 2002). Nematodes play a role in decomposition and nutrient cycling; free-living nematodes that feed on bacteria and fungi (as opposed to plants) contribute as much as 27% of the readily available nitrogen in the soil and promote rhizosphere colonization of beneficial rhizobacteria (Ekschmitt et al. 1999; Knox et al. 2003). Compost supply stimulates nonparasitic nematodes and encourages predator nema­todes and parasitic fungi which specifically destroy the eggs of certain parasitic nematodes (Fuchs et al. 2004). By boosting the key functional species in soil defined as “ecosystem engineers” (Jones et al. 1994), compost allows optimal conditions for plant health and growth.

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.