Composts (e. g., from green and kitchen waste) and soil differ in their compo­sition and properties in almost every parameter. The amount of organic carbon, the amount of organic nitrogen, pH, electrical conductivity (EC), cation exchange capacity (CEC), salinity, chloride concentration, and sulfate concentration are higher in compost. Therefore, depending on the quality and quantity of compost, climate, and soil characteristics, their application will result in a modification of the soil organic matter composition (Fuchs et al. 2004). Compost application increases soil organic carbon and total nitrogen content at the upper horizon in both sandy and clay soils, and that increase can be observed even more than 10 years after application (Melero et al. 2007; Delschen 1999). Generally, an increase in soil pH after compost use is observed because most composts are basic and have a considerable buffering capacity. The mineralization of carbon and the subsequent production of OH~ ions by ligand exchange as well as the introduction of basic cations such as K+, Ca2+, and Mg2+ leads to an increase of soil pH (Mkhabela and Warman 2005). Compost application can ameliorate soil acidity by increasing soil pH and allows large quantities of lime required for these soils to be saved (Haynes and Mokolobate 2001). Application of municipal, mixed green and animal waste compost has been reported to proportionally increase the EC and salt effects of soils (Stamatiadis et al. 1999; Walter et al. 2006). Agricultural soil EC levels ranged from 0 to 4 dS m-1, whereas soil EC levels of municipal solid waste composts ranged from 3.69 to 7.49 dS m-1 (Brady and Weil 1996); therefore, the increase of EC could be of concern. In some cases, soil EC levels were excessive and inhibited plant growth; nevertheless, Zhang et al. (2006) reported that the increased soil EC values decline over time because of nutrient removal by crops and leaching, but the long-term soil biological activity could be negatively affected (Iglesias-Jimenez and Alvarez 1993). Incorporation of compost into soil, especially at high doses, increases the CEC (Bengtson and Cornette 1973); the rise is generally linked to an increase in the level of organic material, the pH, and in base saturation (Ca, K, Na) (Fuchs et al. 2004). Nitrate leaching is one of the concerns regarding compost utilization. NO3~ is highly mobile in soils and is susceptible to leaching through the soil profile and into the groundwater by the infiltrating water. When compared with organic and mineral fertilizers, the nitrate leaching potential of composts is very low (Insam and Merschak 1997). Composts for agricultural use should come from source-separated organic waste and green waste only; then heavy metals are not of concern, as they would be if municipal solid waste compost were used (Epstein et al. 1992; Sharma et al. 1997).

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.

During incineration of wood and other types of plant biomass, a solid residue is formed, representing about 2% (e. g. willow wood) to 20% (e. g. rice husks) of the input material (Jenkins et al. 1998). Depending on the plant species, the origin of plant, the plant parts used for combustion, the process parameters during incinera­tion and the storage conditions of combustion residues, ashes differ considerably regarding their physical and chemical properties (Demeyer et al. 2001). These characteristics determine the quality of different ash types and their suitability for further applications (Karltun et al. 2008). Moreover, different treatments after combustion (self-hardening, thermal treatment or hardening with the addition of a binding material such as a potassium silicate) affect leaching properties of the ash. Ash pellets with a denser structure and a smaller specific surface area display lower leaching rates (Mahmoudkhani et al. 2007). The application form of biomass ash is of great concern, as untreated ash is difficult to apply evenly to soil and may lead to burning of the plant surfaces. Pretreatment of ash may thus be necessary to prevent such damage by lowering the reactivity of the ash. Pretreated ash products are assumed to be more suitable for application purposes, result in less dust formation during spreading, facilitate even spreading and prolong the fertiliser effect owing to slower decomposition rates (Sarenbo et al. 2009).

The chemical composition of biomass ash varies depending on the combustion technique used, the type of material used in the combustion (tree species and

original material, bark, stem, braches, etc.), the storage conditions, etc. (Vance 1996; Blander 1997; Obernberger et al. 1997; Larsson and Westling 1998). The material used in this study was bottom ash generated in moving grate furnaces in three biomass plants in Spain (FINSA, Financiera Maderera). Chemically, this type of ash is less reactive than ash generated in fluidized bed combustors (equivalent neutralizing value 18% CaCO3) since it contains a high percentage of unburned materials. The composition and the quantity of elements provided by such ash are shown in Table 6.3. The concentration of heavy metals is very low (bottom ash), especially in comparison with other waste such as sewage sludge and coal ash (below the limits established by the EU-European Community (1986) and the US Environmental Protection Agency).

However, there are some drawbacks associated with bottom ash which may hinder the spreading process, such as the high humidity (55%) and the high

proportion of coarse elements (more than 20% larger than 4 mm) such as slug, unburned wood swath, and metals.

The environmental impact analysis is focused on emissions, especially those of CO2, NOx, and SO2. The emissions related to ash transport and fertilizer production and transport need to be calculated to obtain the total environmental impact.

8.4.1 Combustion of the Cacao Shells in the Bioenergy Plant

Per hectare cacao plantation in Cote d’Ivoire, 74 kg cacao shells are produced. These shells are combusted in a bioenergy plant. In Table 8.8, an overview is given of the main characteristics of the circulating fluidized bed combustion plant for the combustion of 74 kg cacao shells. The combustion of 74 cacao shells produces

5.3 kg filter ashes. Only the filter ash is suitable as a fertilizer. The bed ash consists of approximately 69% bed material (sand) and additive, whereas the filter ash contains only 13% bed material and additive. It is assumed that all potassium is present as K2CO3.

The use of high-quality biomass ashes for fertilisation processes or for geotechnical and industrial purposes is advisable; however, there are several open questions for fostering different areas of application:

• If ash is to be used as fertiliser or fertiliser supplement on a large scale, it has to be turned into a marketable product in a cost-effective manner. Therefore, a standard product has to be developed, making it applicable on different sites with specific nutrient supply and pH regulation requirements (Table 1.1). Separation of high-quality and low-quality ashes at the incineration plant is highly desir­able. Combustion processes that do not blend the entire ash but provide fractions of bottom and fly ash are preferable.

Table 1.1 Important steps for development and assessment of a user-friendly and cost-effective fertiliser with ash admixture Product development and assessment Formulation of the product

Detailed assessment of the processing conditions (granulation, pelletising)

Production of prototypes Chemical assessment of prototypes

Analysis of effects of soil application regarding physicochemical and microbial parameters as well as plant productivity Optimisation of product and procedures Economic assessment and cost optimisation of product Development of marketing strategies Registration of product

Development of monitoring programmes (fertilisation effect, ecotoxicology)

• Ash applications to soils and the use of ash for geotechnical and industrial purposes have to be accompanied by quality controls to guarantee short-term and long-term harmlessness and the usefulness of these amendments/admixtures. International programmes have to be implemented to build up and merge knowledge on the recycling of biomass ashes and to work out general user guidelines, accompanied by standardised tests.

• Whereas wood ash application in forest ecosystems is commonly accepted in northern European countries, this is not so in other European countries. It is thus necessary to increase public awareness of the importance of sustainable forest management, including the recycling of wood ashes.

• On a national and European level, end-of-waste solutions for ashes should be envisaged, based on strict quality limits.

1.2 Conclusions

Biomass ash is the inorganic residue produced during incineration of biomass for heat and electricity production, containing valuable macronutrients and micronu­trients from the combusted biomass. Physical and chemical characteristics of biomass ashes depend strongly on the plant type (plant species, origin of plants, plant parts combusted) as well as the process parameters during incineration and the storage conditions of ashes; thus, the quality of different biomass ashes differs considerably, even within the same incineration facility.

Investigations on the suitability of biomass ash application to forests as well as agroecosystems are difficult to compare, as different types and amounts of ash have been tested in various environments, hindering the establishment of general guide­lines for ash amendments. Biomass ash applications have been shown to be beneficial to soil ecosystems, as long as ashes of good quality are used and care is taken not to accumulate heavy metals or organic pollutants as a consequence of high application doses or repeated applications. These positive effects have been confirmed in long-term experiments in forest ecosystems in northern European countries, where wood ash applications have a long tradition. Long-term investiga­tions aside from these forest ecosystems are, however, scarce as are clear regula­tions and guidelines for biomass ash recycling to soil. The same is true for the use of combustion residues for geotechnical and industrial purposes.

Research on the recycling of biomass ashes is a relevant and timely issue. More specific information and recent research outcomes on the recycling of biomass ashes as fertiliser or fertiliser supplement in forests and agroecosystems as well as for geotechnical and industrial applications are provided in other chapters of this book.

Acknowledgements We wish to thank the University of Innsbruck, Austrian Federal Ministry of Science and Research, Federal Ministry of Agriculture, Forestry, Environment and Water Man­agement, Land Tirol and Tiroler Zukunftsstiftung, Osterreichische Bodenkundliche Gesellschaft, Tiroler Wasserkraft AG, Vorarlberger Kraftwerke AG, Salzburg AG and Dettendorfer Wertstoff GmbH & Co. KG as well as all project partners of K-Regio BioTreaT (http://www. biotreat. eu) for supporting the conference “Recycling of Biomass Ashes”.

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.

Jouni K. Nieminen

Abstract Wood ash effects on soil animals in a boreal forest ecosystem are reviewed focusing on recent results on interactive effects of wood ash and organic amendments, and laboratory microcosms as a tool to understand soil food webs are discussed. Loose wood ash can reduce the populations of enchytraeids, collembo — lans and mites, but increase nematode populations particularly in experimental laboratory ecosystems with little or no primary production. Recent studies indicate that the repressive effect on enchytraeids depends on carbon availability. Carbohy­drate supply seemed to alleviate the negative wood ash effect on enchytraeid body size and abundance. The fact that carbon alleviated wood ash effects on enchy — traeids without any change in pH supports the view that wood ash effects on soil animals are partly indirect consequences of altered food resources. Experimental evidence suggests that the negative wood ash effect on enchytraeids is partly linked to increased bacteria-to-fungi ratio after wood ash application, and that this may be counteracted by carbohydrate addition.

4.1 Introduction

Only a fraction of the wood ash generated in power plants is recycled back to the forest ecosystem. For example, in Finland some 50% of the wood ash generated in energy production is utilized, and most of this is used for purposes other than forest fertilization (Finnish Forest Industries Federation 2008). Although the forest eco­system would, in principle, benefit from recycling the nutrients back to the har­vested sites, there are also problems such as short-term effects of elevated pH and levels of heavy metals on soil biota (Pitman 2006; see Chap. 1, Knapp and Insam 2011). In their review, Aronsson and Ekelund (2004) concluded that wood ash effects on soil fauna need to be investigated further.

J. K. Nieminen

Department of Biological and Environmental Science, University of Jyvaskyla, 40014 Jyvaskyla, Finland

e-mail: jouni. k.nieminen@jyu. fi

H. Insam and B. A. Knapp (eds.), Recycling of Biomass Ashes,

DOI 10.1007/978-3-642-19354-5_4, © Springer-Verlag Berlin Heidelberg 2011

Wood ash effects on soil animals have been investigated both in small-scale laboratory microcosms and in large-scale field experiments. Most laboratory experiments did not include any plants or included only one small tree seedling. Microcosms containing only heterotrophs are subsequently called heterotrophic microcosms, and microcosms including plants are called autotrophic microcosms. Further, loose wood ash was used in most studies, but today granulated ash is increasingly used as a forest fertilizer. Ash granules dissolve slowly in forest soil and increase the pH and the concentration of exchangeable cations slower than loose ash (Eriksson 1998). The properties of pure wood ash used in some studies are well known. The ash content of young and deciduous trees is higher than that of old and coniferous trees and the ash content of bark is manyfold that of stem wood (Hakkila and Kalaja 1983). Wood ash contains 1-6% phosphorus (P), 6-22% potassium (K), 19-33% calcium (Ca) and 2-5% magnesium (Mg), depending on the tree species and the component (Hakkila and Kalaja 1983). The Ca content of bark and branch ash is higher in relation to P, K and Mg than that of stem wood ash, and compared with other tree species, birch and alder ashes are P-rich (Hakkila and Kalaja 1983). The quality of ash obtained from power plants is more variable, depending on the fuel used (Korpijarvi et al. 2009). For example, in Finland the ash used in forest fertilization is typically a mixture of wood (logging residue, bark, sawdust) and peat ash, which contains less P, K, Ca and Mg but more aluminium and iron than wood ash (Hakkila and Kalaja 1983), and some nutrients are lost during the process. Consequently, the nutrient concentrations of ash from power plants are somewhat lower than those of pure wood ash. It is difficult to see whether differing wood ash effects were due to experimental conditions or ash properties. Specifically, tests of wood ash effects on soil animals at different levels of organic carbon availability were lacking until recently.

In this minireview I first briefly summarize the literature on wood ash effects on soil animals in general. Then I focus on recent laboratory microcosm experi­ments (plants excluded) and greenhouse experiments (grasses and conifer seedlings included) testing the effects of loose wood ash at different levels of carbon availability on enchytraeids and nematodes. Finally, the results are discussed focusing particularly on the use of laboratory microcosms as a research method.

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).