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 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”.

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.

Although carbon (C) is mostly oxidised and nitrogen (N) is emitted in the form of gaseous compounds during combustion, most other elements present in the plant material are retained in the ash (Steenari et al. 1999). Wood ash mainly consists of calcium, potassium, magnesium, silicon, aluminium, phosphorus, sodium, manga­nese and sulphur, whereas it is N-deficient. Trace elements found in wood ash are iron, zinc, arsenic, nickel, chromium, lead, mercury, copper, boron, molybdenum, vanadium, barium, cadmium and silver, again found in varying concentrations in different types of wood ash (Demeyer et al. 2001; Karltun et al. 2008). The behaviour patterns of these elements differ considerably, as some elements are partially or completely volatilised during combustion, whereas others are retained to a high degree (Miller et al. 2002). Owing to the incomplete combustion of biomass, remaining C can be found in the ash to some extent, usually as charcoal (Karltun et al. 2008). Whereas the amounts of K, S, B, Na and Cu were reported to decrease with furnace temperature, the amounts of Mg, P, Mn, Al, Fe, Si and Ca were not affected by temperature (Misra et al. 1993). However, these effects depend on the tree species incinerated (Pitman 2006). Moreover, a lack of standardisation concerning the methods used for the assessment of major and minor ash-forming elements causes further divergences (Baernthaler et al. 2006).

Katja Schiemenz, Jurgen Kern, Hans-Marten Paulsen, Silvia Bachmann, and Bettina Eichler-Lobermann

Abstract The reutilization of biomass ashes in agriculture is important to create nutrient cycles. In field and pot experiments we investigated the fertilizing effects of different biomass ashes (rape meal ash, straw ash, and cereal ash) for eight different crops on a loamy sand and a sandy loam. Special emphasis was given to phosphorus (P). The ashes showed large differences in their elemental composition. The highest P contents (10.5%) were measured in the cereal ash, and lowest in straw ash (1% P). The solubility of P in water was low; however, about 80% of P was soluble in citric acid. Generally, the P fertilizing effect of ashes was comparable to that of highly soluble P fertilizers such as triple superphosphate. The ash supply resulted in an increase of P uptake of cultivated crops as well as in increased soil P pools (total P, water-soluble P, double-lactate-soluble P, oxalate-soluble P) and P saturation. The ash effects depended also on the cultivated crop. Good results were found in combination with phacelia, buckwheat, and maize. Provided that biomass ashes are low in heavy metals and other toxic substances, the ashes can be applied in agriculture as a valuable fertilizer.

2.1 Introduction

Renewable energy sources are important for reducing the EU’s dependence on fossil fuels and cutting greenhouse gas emissions and other pollutants. “Biomass is one of the most important resources for reaching our renewable energy targets.

K. Schiemenz, S. Bachmann, and B. Eichler-Lobermann (H)

Faculty of Agricultural and Environmental Sciences, University of Rostock, J.-von-Liebig-Weg 6,

18059 Rostock, Germany

e-mail: bettina. eichler@uni-rostock. de

J. Kern

Leibniz-Institut fiir Agrartechnik Potsdam-Bornim e. V., Max-Eyth-Allee 100, 14469 Potsdam, Germany

H.-M. Paulsen

Institute of Organic Farming vTI, Trenthorst 32, 23847 Trenthorst, Germany

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

DOI 10.1007/978-3-642-19354-5_2, © Springer-Verlag Berlin Heidelberg 2011

It already contributes more than half of renewable energy consumption in the European Union” said Gunther Oettinger, Europe’s energy commissioner in Feb­ruary 2010, when the European Commission adopted a report on sustainability requirements for the use of solid biomass and biogas in electricity, heating, and cooling (European Commission 2010).

In the context of increasing bioenergy production, the recycling of the residues in agriculture can contribute to realize nutrient cycles and reduce the necessity of commercial fertilizer application (see Chap. 3, Haraldsen et al. 2011; Chap. 7, Bougnom et al. 2011). This has special importance for phosphorus (P), since the P resources are strongly limited worldwide. Ashes from combustion of biomass are the oldest mineral fertilizer. Biomass ashes are nearly free of nitrogen but contain P and other nutrients needed for plant nutrition (Sander and Andren 1997; Vance and Mitchell 2000; Patterson et al. 2004; Uckert 2004). Furthermore, biomass ashes can also be used as liming agents (Muse and Mitchell 1995; Mandre 2006), and can stimulate microbial activities in the soil (Demeyer et al. 2001; see Chap. 1, Knapp and Insam 2011).

The nutrient compositions of ashes are affected by different factors. Particularly, the kind of biomass combusted influences the quality and the nutrient values of ashes. Besides the raw material used, the combustion process itself affects the chemical composition of the ashes. The P concentration in biomass ashes may range from less than 1% up to 10% (Table 2.1).

Positive effects of biomass ashes on crop yields were found in different studies. Among others, Krejsl and Scanlon (1996) found wood ashes to increase oat and bean yields. Phongpan and Mosier (2003) found positive effects of rice hull ash on rice yield, Ikpe and Powell (2002) reported positive impacts of millet ash on millet yields, and Haraldsen et al. (2011, see Chap. 3) found particularly positive combi­nation effects with meat and bone meal residues on barley.

Nutrient uptake efficiency and mobilization mechanisms of crops are important for high utilization of applied P (Schilling et al. 1998; Neumann 2007), and the utilization of P in ashes also depends on the cultivated crop. However, research findings concerning interactions between biomass ashes and crop species are rarely available.

In our studies we investigated three different crop biomass ashes and eight crop species within field and pot experiments. The objectives of our work were to evaluate the P fertilizing effect of biomass ashes on different soils, and to investi­gate possible interactions between the effects of ashes and cultivated crops.

Table 2.1 P contents in biomass ashes

Type of ash P content (%)

Bagasse ash (Jamil et al. 2004) 0.01

Alfalfa stem ash (Mozaffari et al. 2002) 0.90

Horticulture ashes (Zhang et al. 2002) 0.04-1.00

Wood ashes (Erich and Ohno 1992; Saarsalmi et al. 2001; Hytonen 2003) 0.90-1.70

Wheat straw ash (Hytonen 2003) 1.30

Rape straw ash (Hytonen 2003) 2.10

Poultry litter ash (Yusiharni 2001; Codling et al. 2002) 5.00

Cereal ash (Eichler et al. 2008b) 10.4

A summary of the effects of different ashes on forest soil animal groups detected in both field and laboratory experiments is given in Table 4.1. Loose wood ash was used in most experiments (Table 4.1). Because the dissolution rate of granulated wood ash is much lower than that of hardened or loose ash (Eriksson 1998; Nieminen et al. 2005), the ash effects on soil organisms also differ between ash types. The solubility of CaCO3 formed during stabilization is 2 orders of magnitude lower than that of CaO and Ca(OH)2, and, therefore, stabilized ash causes a much smaller pH shock in soil than loose ash (Steenari and Lindqvist 1997).

Macroarthropods react slowly to wood ash application (Huhta et al. 1986). Although very high wood ash doses (7 Mg ha—1) can cause negative effects on microarthropods (Koskenniemi and Huhta 1986), their populations and community structure tolerate moderate wood ash application (Liiri et al. 2002a, b). The numbers of some collembolans even increased after wood ash application in a field study (Vilkamaa and Huhta 1986). In contrast, 1-5 Mg loose wood ash per hectare, particularly if mixed with humus, has been shown to reduce the populations of the enchytraeid Cognettia sphagnetorum (Vejd.) both in many laboratory studies and in a field study (Huhta 1984; Haimi et al. 2000; Liiri et al. 2001, 2002c, 2007; Nieminen 2008a). A higher dose, however, was needed for a negative response in the field than in laboratory experiments (Haimi et al. 2000). In contrast to loose ash, hardened or granulated ash did not affect enchytraeid abundance, although a transient increase in the cadmium content of enchytraeid biomass was detected in one field study (Lundkvist 1998).

In contrast to its effects on enchytraeids and microarthropods, loose wood ash has been found to increase the total numbers of nematodes (Hyvonen and Huhta 1989; Liiri et al. 2002b; Nieminen 2009).

C. sphagnetorum is an omnivorous litter — and microbe-feeding oligochaetan worm dominating the mesofaunal community in boreal coniferous forests, and it is considered a keystone species in those ecosystems (Huhta et al. 1998). Because this species is adapted to acid soil (Baath et al. 1980; Standen 1982; Abrahamsen 1983; Huhta 1984), the negative wood ash effects have been explained by the increase of soil pH after wood ash application (Aronsson and Ekelund 2004). Changing pH can affect soil animals indirectly by changing the community com­position of microbes (Perkiomaki and Fritze 2002).

In addition to nutritional effects, wood ash can also modify enchytraeid popula­tions. When Nieminen and Haimi (2010) transferred enchytraeids exposed to wood ash in the forest to laboratory microcosms containing unamended Norway spruce forest humus, they found that enchytraeid populations originating from wood-ash — treated soil propagated slower and mineralized less nitrogen (N) than populations from untreated control forest.

In summary, the most conspicuous wood ash effect on soil fauna is the reduction of enchytraeid populations after loose wood ash treatment, especially in small-scale laboratory experiments including small plants or no plants and, hence, very limited carbon flow to the soil. In contrast, up to 8 Mg granulated ash per hectare had no effect on enchytraeid abundance in the forest (Lundkvist 1998).

The heavy metals that may accumulate in wood ash are of special concern when it is used for fertilisation purposes. Compared with coal ashes, ashes derived from wood are lower in heavy metals, but are more alkaline (Campbell 1990). High concentra­tions of As, Cd, Cr, Pb, Zn and Cu may, however, occur owing to the incineration of surface-treated waste wood and wood treated with industrial preservatives (Krook et al. 2006). Cu concentrations in biomass ashes were frequently shown to exceed critical values according to national regulations in Austria (Neurauter et al. 2004) and Germany (Ministerium fur Umwelt und Verkehr Baden-Wurttemberg 2003). Average Cu contents found in three studies dealing with ash composition are illustrated in Fig. 1.1 (Neurauter et al. 2004; Niederberger 2002; Toth(Sva 2005). Whereas the incineration of pure wood led to moderate Cu concentrations in the resulting ash, high Cu contents were found when other biomass, especially roadside greenery and material derived from wood processing, was combusted together with natural wood (samples 9, 10).

Wood ash is better applicable for fertilisation purposes if it is separated into fly and bottom ash during combustion, as heavy metals — except for Zn — accumulate in the fly ash (Pitman 2006; Stockinger et al. 2006). Fly ash is the lightest fraction formed during combustion, being deposited within the boiler or in the filters (Pitman 2006). Ashes may also include organic pollutants such as polychlorinated dibenzodioxin, biphenyls, dibenzofuran and polycyclic aromatic hydrocarbons (PAHs), which are of interest because of their toxic, mutagenic and carcinogenic effects (Lavric et al. 1994; Enell et al. 2008). High amounts of PAHs are ascribed to a poor combustion performance (Sarenbo 2009). Wood ashes may pose a risk not only because of the direct input of organic pollutants, but also because a rise in soil pH following wood ash amendments enhances remobilisation of PAHs and polychlorinated biphenyls (Bundt et al. 2001). An elevated pH also affects metal solubility in soil; however, changes in solubility do not necessarily correlate with incorporation of heavy metals in plants grown on the respective soils (Dimitriou et al. 2006). Another essential issue in regard of ash amendments to soils is leaching of toxic substances to the ground­water (Williams 1997), especially in combination with an elevated pH and high Na content (Morris et al. 2000). Leaching is frequently evaluated in laboratory tests, but

sample

these tend to overestimate or underestimate on-site leaching processes and thus it is difficult to assess the real situation in the field (Reijnders 2005).

2.2.1 Treatments and Experimental Design

Two field experiments were conducted on different soil types at the agricultural experimental stations of the University of Rostock (loamy sand) and at the Institute of Organic Farming in Trenthorst (sandy loam). The same soil types were also used for the pot experiments in Rostock; in 2007 the experiments were established on loamy sand and in 2008 they were established on sandy loam (Table 2.2).

The rape meal ash (RMA) was produced at the University of Rostock in a fluidized bed combustion at 860°C. The rye straw ash (SA) was produced via grate firing at 750°C and was supplied by the Leibniz Institute for Agricultural Engineering in Potsdam-Bornim (Germany). The rye cereal ash (CA) was manu­factured at the Agricultural Technical School of Tulln (Austria) also via grate firing at 650-850°C. The fertilization treatments in the field and pot experiments were established in respect of the nutrient contents of the ashes (Table 2.3). Heavy metal contents are given in Table 2.4.

In the 2-year field experiments with different crop plants (see Table 2.5) the three ashes were applied once at the beginning of the experiments and incorporated into the top soil. Nitrogen was given in all treatments according to good fertilization practice.

In the pot experiments six different fertilization treatments were established. Besides the ash treatments, other treatments included triple superphosphate (TSP) as a highly soluble P source, potassium chloride (KCl) as a highly soluble potas­sium source, and a control (CON) without P and potassium. The ashes/fertilizers were applied on the soil surface and mixed into the upper 5 cm of soil. For nitrogen supply, 1.4 g NH4NO3 per pot was given. Mitscherlich pots with 6 kg soil each were used for crop cultivation.

Eight different crops were investigated in the pot experiments. Depending on the favourable growing time of cultivated main crops and catch crops, two experiments per year were established (Table 2.5). The main crops were seeded in April and the catch crops were seeded in August. The crop growing period in the pot experiments was about 7-10 weeks until the time of maximum biomass. In field and pot experiments all treatments were replicated four times.

The functioning of ecological systems can be studied at small spatiotemporal scales in controlled laboratory microcosms. In particular, the microcosm method has been used as a tool to understand the functioning of decomposer food webs (Scheu 2002; Huhta 2007). Although the experimental approach has limitations such as limited community composition and restricted mobility of animals (Kampichler et al. 2001), idealized model systems help to reduce the natural variability and exclude variables which are considered beyond the scope of the research. The dynamics of decomposer food webs and nutrients in such systems are fairly well known. Decomposer population dynamics in laboratory microcosms containing no auto­trophs, and hence no carbon input, consists of one growth phase and a subsequent decline phase (Nieminen 2008b). The length of the population growth phase depends on the initial population size (Nieminen 2002). For example, the biomass of fungal-feeding nematodes increased exponentially for 3.5 weeks in organic forest soil, then crashed and remained low for 16 weeks (Nieminen 2008b). In a laboratory experiment using larger pieces of forest floor, nematode populations crashed after 3 weeks, then collembolan and enchytraeid populations crashed (Huhta et al. 1983). In such a system lacking carbon input and nutrient uptake by plants, inorganic N accumulates in soil (Nieminen 2008b).

In such severely carbon limited systems, addition of organic carbon available to microbes increases microbial biomass, and reduces the N concentration in soil (Sparling and Williams 1986; Schmidt et al. 1997; Dunn et al. 2006). In an early experiment, glucose increased both the biomass of fluorescein diacetate active mycelium and yeasts as well as fungal-feeding nematodes in pine microcosms (Baath et al. 1978). However, the soil properties were quite unnatural, for example the pH was high (above 6.8), and algae thrived in microcosms but enchytraeids were lacking (Baath et al. 1978). In coniferous forest soil, the pH and moisture usually limit yeast and algal growth. Extra carbon input is also reflected in the biomasses of higher trophic levels of the soil decomposer food web. Addition of carboxymethyl cellulose to microcosms containing mineral and organic soil, needle litter and a Scots pine seedling increased the biomass of both saprotrophic fungi and hyphal-feeding nematodes (Nieminen and Setala 2001). Although the biomass of filamentous fungi increases after cellulose addition, addition of labile carbon such as sucrose can enhance the growth of early successional (r-strategist) microbes such as bacteria (Moore-Kucera and Dick 2008; Nottingham et al. 2009) and Zygomy — cota fungi (Hanson et al. 2008). Nieminen (2010) found that a sucrose addition equalling 100 kg C ha-1 maintained a stable nematode population for one growing season, and an increasing enchytraeid population. Sucrose addition did not alter net N mineralization rates, indicating that N mineralization by increased animal popula­tions exactly balanced the N immobilization in microbial biomass. When the carbon addition rate is increased above 100 kg C ha-1 in Norway spruce forest soil, soil animals, which have orders of magnitude lower growth rates than microbes, cannot consume all the extra microbial biomass in one growing season, and as a conse­quence, N is immobilized in microbial biomass.

As versatile as wood ash is, its potential areas of application are:

• Ash application in forest ecosystems

Wood ash is commonly applied to forest ecosystems to return nutrients extracted through whole-tree harvesting and to counteract soil acidification (Sect. 1.3.1).

• Wood ash as fertiliser or fertiliser supplement in agroecosystems

Wood ash rich in nutrients but displaying a low concentration of heavy metals or organic pollutants is also suitable as fertiliser or fertiliser supplement for agricul­tural and horticultural purposes (Sect. 1.3.2).

• Wood ash for geotechnical constructions and industrial processes

Typical applications in this field are the construction of roads and parking areas, the use of ash as a surface layer in landfills and admixture of ash for concrete, brick or cement production (Sect. 1.3.3).

1.3.1 Ash Application in Forest Ecosystems

The effect of wood ash application on forest ecosystems has been intensively studied in northern European countries where ash is used as fertiliser in boreal forests (Aronsson and Ekelund 2004). Owing to extensive forest harvesting (espe­cially whole-tree harvesting), reuse of ashes was established to avoid base element depletion of forest soils, leading to increasing acidity as well as decreasing amounts of nutrients and organic matter in the soil, thus threatening forest productivity (Stupak et al. 2008).