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.

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).

In each experimental plot, samples were collected from each soil layer, at a depth of 0-40 cm, from the center of each plot, following a zigzag route. Soil samples were mixed to ensure homogeneity, dried at 40°C, and sieved through a 2-mm screen; microoelements and macroelements were extracted by the Mehlich 3 procedure (Mehlich and Mehlich 1984). Soil pH was measured in H2O and KCl (0.1 M). For elemental analysis (CNS), the soil samples were ground in a mortar to obtain a fine powder.

Sampling was carried out in spring and autumn. Needle samples were also collected, approximately 25% from the floor of the plot (Ballard and Carter 1986), approximately 25% from the upper third, approximately 25% from the growth of the year, and approximately 25% from the sunniest branches (Will 1985). The needle samples were dried at 60°C to constant weight and were ground for CNS analysis by inductively coupled plasma optical emission spectrometry.

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

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

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).

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 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.

11.4.1 Landfills and Mining Waste Deposits

Combustion residues are used to provide a barrier against the penetration of water or oxygen, or both, into the body of the landfill or of the heap or impoundment of mining waste. The properties that make fly ash from solid biofuels suitable are their high pH and their self-binding properties. The potential for use in several contexts is quite high and the demand could easily exceed the availability of suitable residues.

Fly ash mixed into sewage sludge in equal dry substance proportions raises the pH of the sludge, thus preventing its biological degradation. The percolation rates achieved in field experiments with fly ash mixed into sewage sludge are on the order of 12 l/m2/year, both initially and after a few years, which is sufficiently low for sealing layers on landfills for non-hazardous waste (Carling et al. 2006; Macsik et al. 2005). Its shear strength is acceptable and it withstands settling in the body of the landfill.

On the Tveta landfill, the functional requirements on a sealing layer are fulfilled using monolithic layers of ash through diffusional processes, i. e. particle size distribution, moisture content and reactivity of the combustion residues, all con­tributing to the minimisation of the pore volume (Tham and Andreas 2008).

A sealing layer of fly ash from solid biofuels, covered by a protective layer of vegetated sewage sludge, prevents oxygen from reaching sulphidic tailings from mining. The experimental object is the 80-ha large tailings impoundment of Gillervattnet, where an experimental field covering 3 ha was established in 2003-2005. The hardened layer of ash and the toxicity of the fresh ash prevent roots from breaking through the sealing layer and providing channels for air entry.

However, reed canary grass has some capacity to weaken even hardened fly ash sealing layers with a resistance of approximately 5 MPa. The study suggests that the secretion of saccharides by some plant roots may contribute to this effect (Greger et al. 2006, 2009). It is thus best to avoid these particular plants.