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Department of Natural and Environmental Sciences, Mid Sweden University, 851 70 Sundsvall, Sweden
* Corresponding author (andreas.aronsson{at}mh.se).
Received for publication October 10, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONCHARACTERISTICS OF WOOD ASHBIOLOGICAL EFFECTS OF WOOD...BIOTOXIC EFFECTS OF WOOD...CONCLUSIONSREFERENCES |
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Abbreviations: DOC, dissolved organic carbon • WAA, wood ash application
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONCHARACTERISTICS OF WOOD ASHBIOLOGICAL EFFECTS OF WOOD...BIOTOXIC EFFECTS OF WOOD...CONCLUSIONSREFERENCES |
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CHARACTERISTICS OF WOOD ASH |
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TOPABSTRACTINTRODUCTIONCHARACTERISTICS OF WOOD ASHBIOLOGICAL EFFECTS OF WOOD...BIOTOXIC EFFECTS OF WOOD...CONCLUSIONSREFERENCES |
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Olanders and Steenari (1995) have concluded that wood ash is dominated by the macroelements Ca, Si, Al, K, and Mg. This corresponds well with the findings of Demeyer et al. (2001) and others (Steenari and Lindqvist, 1997; Hansen et al., 2001; Holmberg, 2003). Among the microelements (of which several are toxic at high concentrations), Demeyer et al. (2001) show that Fe is the most abundant, followed by Mn, Zn, and Cu. This pattern is also recognized in other studies (e.g., Miller et al., 2002).
The physical properties of the wood ash are obviously of great importance in predicting the magnitude of the environmental impact of the ash application. Coarser fractions of ash are less reactive to the environment (Larsson and Westling, 1998). Obernberger et al. (1997) investigated the influence of different combustion technologies on the ash properties. It was concluded that the concentrations of environmentally relevant heavy metals (especially Cd and Zn) increased with decreasing precipitation temperature and particle size. This was also valid for K, Na, Cl, and S. Different methods and techniques are being developed to make the ash as harmless to the environment as possible, without losing its positive characteristics. Three main forms of wood ash can be identified: fly ash or loose ash (unstabilized), self-hardened crush-ash (stabilized), and pelleted and agglomerated ash (stabilized). Steenari et al. (1999) conclude that the liming effect of the ash is extended and the possible shock effects of high pH are avoided when using stabilized wood ash. In the humus layer, pH can increase up to 2.5 units when using loose ash; when using granulated ash the effect has been greatly lessened or even not detectable (Egnell et al., 1998). A decrease in Ca and S leaching when using stabilized wood ash was also reported in the study by Steenari et al. (1999). Considering the indications of the results above, pelleted wood ash would be the preferred stabilization form utilized in WAA. However, the extra time and energy consumption needed to agglomerate the ash will increase the cost of the product, explaining why the self-hardened crush-ash is the most likely product to be used in large-scale activities.
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BIOLOGICAL EFFECTS OF WOOD ASH APPLICATION |
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TOPABSTRACTINTRODUCTIONCHARACTERISTICS OF WOOD ASHBIOLOGICAL EFFECTS OF WOOD...BIOTOXIC EFFECTS OF WOOD...CONCLUSIONSREFERENCES |
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Ground Vegetation
Jacobson and Gustafsson (2001) conducted a major study on the effects of WAA (3–9 Mg ha–1) on ground vegetation. The overall result was a minor change in vegetation cover. However, the three most common bryophytes [dicranum moss, Dicranum polysetum Sw.; splendid feather moss, Hylocomium splendens (Hedw.) Schimp. in B.S.G.; and Schreber's big red stem moss, Pleurozium schreberi (Brid.) Mitt.] reacted rapidly, initially displaying severe visible damage. After 2 yr, recovery was initiated and after 5 yr no visible damage could be seen. This damage was not observed when pelleted wood ash was used, and no visible damage to lichens was found for any treatment. However, 5 yr after treatment the cover of the lichens had decreased, most significantly on the ash-treated plots. The authors suggest legal restrictions on the chemical and physical properties of wood ash (e.g., stabilization form and heavy metal contents) to avoid a negative impact on ground flora. Arvidsson et al. (2001) conclude that WAA (3 Mg ha–1; crush-ash) is not likely to affect biodiversity or lead to a decrease of species abundance. Overall, the effect of WAA on ground vegetation was limited in this study, and in a study on mineral soils by Nilsson (2001) the contents of different nutrients (e.g., Ca, Mg, K) or heavy metals (e.g., Cd, Zn) in bilberry (Vaccinium myrtillus L.) were not affected by WAA. However, a long-term study by Moilanen et al. (2002) of a drained mire in Finland showed radical and long-lasting consequences on biological activity after wood ash treatment. In the wood ash–treated plots the ground flora consisted of grasses and herbs typical for upland forests, while ground vegetation in the untreated plots consisted of mosses and dwarf shrubs. A study on ground flora establishment by Olsson and Kellner (2002) showed an overall positive correlation of WAA with the pH of the humus layer and with the number of established species. They concluded that wood ash has similar effects as lime on the establishment of new species. They also suggested that germination could be affected by changes in pH (increase as well as decrease), and recommended further studies on the topic. A study by Levula et al. (2000) showed that cowberry (Vaccinium vitis-idaea L.) cover was slightly reduced by the application of wood ash (highest dose, 5 Mg ha–1).
Forest Growth
Nilsson (2001) studied elemental turnover in a cutover peatland and uptake in vegetation. For the different tree species studied [alder, Alnus spp.; birch, Betula pubescens Ehrh.; willow, Salix spp.; Norway spruce, Picea abies (L.) H. Karst.; and Scots pine, Pinus sylvestris L.], the increase in soil pH and nutrient pools after WAA seemed sufficient to maintain a sustainable biomass production. Moilanen et al. (2002) found that the stem volume growth of Scots pine on peatland was promoted by ash treatment and the total wood production was 13 to 17 times greater than that of the control plot. It was also concluded that nutrient concentrations in the peat had increased as well as the development of CO2 emissions. Since the content of N is very low in wood ash, the most positive effects of wood ash application have occurred on N-rich sites or with ash amended with N fertilizers. An exception to this seems to be WAA on an acidic and highly N-loaded site (4.2 Mg ha–1), where accelerated formation (due to N mineralization) and leaching of nitrate has been reported (Högbom et al., 2001a, 2001c). In Finland, extensive and long-term studies on wood ash–ameliorated peatland have been conducted (Silfverberg and Moilanen, 2001). The overall results from these studies show that a manifold in tree growth rate and stem volume is achieved after ash application, as well as a shift in ground vegetation species and germination (Silfverberg and Huikari, 1985; Silfverberg and Hotanen, 1989; Silfverberg, 1991, 1995, 1996; Hytönen and Kaunisto, 1999; Moilanen et al., 2002). As the limiting growth factors in these sites are deficiencies of P and K (Silfverberg and Moilanen, 2001), and not necessarily N, increased vegetation growth is commonly seen. However, a majority of peatland forests are of low productivity by nature and tree harvesting is usually rare. At these sites, wood ash should be considered mainly as a fertilizer and not as recycled nutrients.
In contrast, the growth-inhibiting factor in the majority of mineral soil and upland forest stands is N deficiency (Jacobson et al., 2000); thus the effects of wood ash application are less pronounced here. Jacobson (2003) studied stem growth on coniferous stands on mineral soils affected by N fertilization or WAA, and a combination of the two. The results show a significant increase in stem growth when using N or N and wood ash, but small or no response to wood ash alone. An interesting result was the indication of increased stem growth on fertile sites, but a decrease on less fertile sites when using wood ash. The effects were, however, nonsignificant. Increased growth of Norway spruce has been reported after WAA (4 Mg yr–1 ha–1) on an acidic brown forest soil in Switzerland (Hallenbarter et al., 2002).
Needle Chemistry
Arvidsson and Lundkvist (2002) conducted a study on needle nutrient concentration 5 yr after application of hardened wood ash (3 Mg ha–1). They concluded that P, K, and Ca concentrations were increased and that the results were consistent over all stands, irrespective of climate zone or fertility. This strongly indicates that needle nutrient status is likely to be explained by the characteristics of the ash and the dose applied. The authors (Arvidsson and Lundkvist, 2002) concluded that wood ash could be used as a replacement of removed nutrients at whole-tree harvesting. In a study by Jacobson (2003) the nutrient concentrations in the needles of coniferous stands, except for N, tended to increase when wood ash was applied. The only significant increase, however, was seen in K and B, 3 to 5 yr after WAA. On the contrary, Hallenbarter et al. (2002) did not detect any changes in nutrient contents or ratios in needles during a 3-yr period and an application rate of 4 Mg yr–1 ha–1.
The discrepancies in the results of tree and vegetation growth can be discussed from several points of view. As mentioned in the Characteristics of Wood Ash section, above, the chemical and physical properties of the wood ash are crucial for the explanation of the biological effects. In the study by Jacobson and Gustafsson (2001), no visible effects on bryophytes or lichens were seen when using pelleted wood ash. These organisms are very sensitive to alkaline substances and severe damage is common when applying unstabilized ash or lime (Silfverberg, 1995; Ingerslev et al., 2001).
Together with the aforementioned explanations of differences in the results between several studies, perhaps one of the most obvious explanations is the wide range of application doses. Egnell et al. (1998) have compiled a vast variety of applied doses of wood ash (0.3–30 Mg ha–1). The wide range of doses, together with the different degrees of stabilization discussed above, makes it very difficult to interpret and compare the results from different studies. In addition to this, the different time scales (ranging from 2–3 yr to 40 yr) of different studies bring a further dimension to the difficulty of the interpretation. There is also a climatic zone aspect, while precipitation influences the weathering processes in different areas. Southern parts with high precipitation are likely to have faster treatment effects, as mentioned by Arvidsson and Lundkvist (2002). Closely associated with climatic aspects is another factor, actually not very often discussed, namely the time of the year the ash is applied. Applying ash onto the snow in the winter should, at least in the short term, yield different results than from application in summer conditions. This argument is somewhat supported by the results of Piirainen (2001), indicating a greater leaching of base cations (Ca, K, Mg) and S when applying ash in winter.
For future research on tree growth, it would be interesting to determine at what age of the tree stands WAA would have the greatest influence. Nutrients might be of greatest importance during the younger life stages, in which case WAA should be recommended to forest stands of a certain age. According to present knowledge, application of an intermediate dose (2–4 Mg ha–1) of pelleted wood ash on N-rich sites (preferable peat) in late summer would provide a positive result on forest growth, and minimize the negative effects on the environment.
Fungi and Soil Fauna
Fungi
Ectomycorrhizal fungi are involved in the nutrient uptake of forest trees and the determination of any effects of wood ash treatment on this coexistence is of great importance. Hagerberg and Wallander (2002) investigated the influence of intensive harvesting and wood ash fertilization on the external ectomycorrhizal mycelium in forest soil. Wood ash amendment resulted in 2.4 times more ectomycorrhizal fungal biomass. However, the fungi mycelium had no effect on the dissolution rate of the wood ash. Recent studies have shown that ectomycorrhiza can play a vital role in the mobilization of nutrients in the wood ash by various weathering processes, as discussed by Mahmood et al. (2001)(2002, 2003). The potential ability of these fungi to mobilize nutrients may increase their importance in wood ash–treated forests. The same authors have shown that wood ash granules normally are colonized by fungal mycelia; in this study the incubation time was about 7 yr (Mahmood et al., 2001). The potential ability of mycorrhiza to release elements from wood ash has also been investigated by Wallander et al. (2003), who found that WAA increased the amount of Ti, Mn, and Pb in the rhizomorphs. Frostegård et al. (1993) showed that in spite of increased pH the amount of fungi was unaffected by ash treatment. Small changes in the mycorrhizal community after addition of wood ash (4.28 Mg ha–1) were also recorded by Taylor and Finlay (2003).
Soil Microbial Studies
The findings within the research field of microbial studies are somewhat ambiguous. Zimmermann and Frey (2002) found an increase in microbial activity and biomass in soil treated with wood ash, as well as an increase in the growth rate of soil microorganisms. The authors concluded that increased pH and quantity of nutrients following WAA were closely related to the results in the investigation. The higher activity was also linked to increased mineralization of organic matter. These results correlate well with similar results from other investigations (Bååth et al., 1992; Bååth and Arnebrant, 1994; Fritze et al., 2000; Mahmood et al., 2003; Perkiömäki and Fritze, 2002a, 2002b). The opposite effect, namely a decrease in microbial activity and biomass after WAA (clear-cut area; highest dose of 5 Mg ha–1) has been shown by Bååth et al. (1995). Attempts to correlate these results to soil pH, bacteria pH response patterns, or substrate quality were only partly successful. Bååth et al. (1995) hypothesize that altered substrate quantity, in regards to the availability of substrates after treatment, could explain the decrease. Changes in humus microbial activity are shown to be detectable 18 yr after wood ash treatment (Perkiömäki and Fritze, 2002a). There are also studies showing no significant effects. Fritze et al. (1994) found that wood ash treatment did not change the level of microbial biomass. Bååth et al. (1992) found no differences in total bacterial numbers between an alkaline polluted area and a reference plot, although a 1.6-fold increase in bacterial activity in the alkaline polluted site was seen. Differences in bacterial community structure influenced by wood ash treatment have been reported. Mahmood et al. (2003) showed a clear difference between samples collected from wood ash–treated pots and controls without ash. Bacterial activity was significantly greater in ash treatments and also the community structure, measured as changes in the phospholipid fatty acids (PLFA) composition, was different. These differences in bacterial community structure were also shown by Frostegård et al. (1993) and Liiri et al. (2002b).
Soil Decomposer Animal Studies
Responses of soil decomposer animals to WAA have been studied by Lundkvist (1998). The abundance of enchytraeids was not significantly different from the untreated controls, although a high dose of wood ash (8 Mg ha–1; crush-ash) caused a downward movement of enchytraeids from the upper layers of the soil within 1 to 3 yr after treatment. Liiri et al. (2001) found WAA, as well as drought, to be negative for enchytraeid populations. Haimi et al. (2000) also found that the enchytraeid Cognettia sphagnetorum responded negatively (decreased numbers) to a wood ash treatment of 5 Mg ha–1. Studies by Liiri et al. (2002a) of microarthropods affected by wood ash in a pine forest stand (3 Mg ha–1) and in laboratory (5 Mg ha–1) showed no changes in the community structure after wood ash treatment and it was concluded that soil microarthropods are rather resistant to changes in soil pH. However, Haimi et al. (2000) found that even though the total numbers of soil animals increased after ash treatment (1 and 5 Mg ha–1; 3 yr after treatment), a decrease in microarthropods was observed when exposed to the highest dose of ash (5 Mg ha–1). A general conclusion is that soil fauna, in terms of enchytraeids and microarthropods, seem to be quite tolerant of the soil chemical change caused by wood ash treatment (Haimi et al., 2000; Liiri et al., 2001, 2002a).
In summary, doses of wood ash in the above-mentioned studies have ranged between 1 and 9 Mg ha–1 (loose or hardened ash); studies have usually been conducted for 1 to 3 yr after WAA and humus layer organisms were studied. Contrary to the results of the vegetation studies, the environmental conditions seem to have less influence on the results for fungi, bacteria, and soil decomposer animals. Perkiömäki and Fritze (2002a) have showed that irrespective of forest site fertility, microbial activity and community structure were increased after wood ash fertilization. These changes were related to the dose and form of ash applied (3 and 9 Mg ha–1; loose and hardened ash, respectively). Changes in microbial species composition could be explained by the increase of DOC and changes in humus quality following WAA, as hypothesized by Bååth et al. (1995).
Future studies on fungi after WAA are of great interest, since alterations of community structure composition of ectomycorrhiza have been observed after lime treatment (Taylor and Finlay, 2003). Such alterations might as well occur after WAA. This could have an environmental impact and perhaps influence tree growth; the role of ectomycorrhiza cannot be emphasized enough. Further research on soil fauna affected by WAA is also of great importance, as most present studies indicate a higher activity and hence an increase in soil respiration (CO2 emissions). The magnitude of these processes could be of great importance, since the forest is commonly regarded as a carbon sink. It would be highly undesirable to decrease the ability of the forest to act as a sink to greenhouse gases, such as CO2, or possibly contribute to a shift from sink to source.
Aquatic Life
Few studies on the effects of wood ash, as well as other types of terrestrial fertilization, on aquatic environments have been conducted. In the review by Nohrstedt (2001) no Swedish reports were found for lime, wood ash, or N-free fertilizers. Tulonen et al. (2002) conducted a 3-yr limnological study on the effects of WAA (6.4 Mg ha–1; crush-ash) in the drainage basins of two small, humic lakes. Their results showed slightly increased amounts of K+, SO2–4, and Cl– in the water, as well as increased phytoplankton biomass. Additional tank experiments were undertaken to investigate immediate effects (wood ash solution added ranged between 5 and 50 mg L–1). These studies showed an increase of pH, alkalinity, conductivity, and Ca and P concentrations, along with decreased phytoplankton growth. Rapid changes in pH seem to affect the metabolism of phytoplankton and bacteria very rapidly, and aquatic microorganisms seem to be very adapted to the prevailing acidity (Salonen et al., 1994). Such changes could be due to liming or ash treatment, but could also be of natural origin, such as forest fires (Patoine et al., 2000). The above-mentioned changes have also been found in investigations of fossil microorganisms and by analyzing chemistry of old lake deposits in the United States (Rhodes and Davis, 1995). To avoid rapid changes of pH and to decrease the liming effect of the wood ash, different stabilization techniques of the ash are of great interest.
The lack of knowledge in the research field of aquatic ecosystems and their response to WAA makes this an important issue for future research. The few studies conducted at present mainly consider changes in water chemistry. It is likely that WAA could induce changes in the periphyton community of streams. These changes could influence the benthic animal community and, finally, the different species of fish in the stream. Legal restrictions in terms of buffer strip zones between wood ash–treated areas and watercourses and lakes are needed to avoid unwanted effects of WAA. Based on research and findings linked to the liming activity in the Scandinavian countries, similar effects can be expected from WAA. Further research is needed at the system, community, and species level to achieve sustainable development in forestry and avoid interference with the natural environment.
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BIOTOXIC EFFECTS OF WOOD ASH APPLICATION |
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TOPABSTRACTINTRODUCTIONCHARACTERISTICS OF WOOD ASHBIOLOGICAL EFFECTS OF WOOD...BIOTOXIC EFFECTS OF WOOD...CONCLUSIONSREFERENCES |
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Several studies have shown the toxicity of Cd to different types of biophysiological functions in various types of organisms. Swiergosz et al. (1998) found Cd to be extremely dangerous as it is easily absorbed and remains in tissues for a long time. In an investigation of accumulation of Cd in bank vole (Clethrionomys glareolus), examination of the tissues revealed pathological changes in the structures of kidneys, liver, and testes (Swiergosz et al., 1998). Ogoshi et al. (1992) confirmed that Cd has a lesion effect on the mechanical strength of bone, for both young and old rats. Effects of raised concentrations of Cd in small mammals are also discussed by Ma et al. (1991), Leffler and Nyholm (1996), Nickelson and West (1996), and Lodenius et al. (2002b). It appears that the food source of the animals could be of great importance, as discussed by Lodenius et al. (2002b). The difference in Cd concentrations following WAA between the bank voles and the common shrews (Sorex araneus) in that study could be explained by different food habits. The seasonal variations of Cd concentrations in plants (Lodenius, 2002) or other food sources, might also explain differences in Cd amounts in animals and should be considered when planning experiments with WAA.
An inhibitory effect of Cd on the growth of wood-rotting fungus was observed by Baldrian et al. (1996). Cadmium at concentrations higher than 0.5 mmol L–1 significantly inhibited the activity of all of the tested enzymes from the fungi. Another study showed that the most sensitive fungus did not grow at concentrations exceeding 0.1 mmol L–1 (Baldrian and Gabriel, 1997). It has also been shown that colonization of ectomycorrhiza could be decreased by Cd. In contrast, the same study concluded that mycorrhizae alleviate the Cd-induced reduction in growth of Norway spruce seedlings (Jentschke et al., 1999). Lodenius et al. (2002a) report higher mean Cd concentrations for nine fungi species out of ten in ash-treated areas. Results from investigations by Fritze et al. (2000) suggest that while Cd alone induced decreased microbial activity and changed phospholipid fatty acids pattern, wood ash protected the humus microflora from the harmful effects of Cd. Another study by Perkiömäki and Fritze (2002b) showed that Cd in ash did not leach into the humus layer due to increased deposition of acidified rain. The Cd concentration of wood ash used in a study by Levula et al. (2000) was 1.4 mg kg –1 and it had no effect on the Cd concentrations in the berries of cowberry seven growing seasons after application. Nilsson and Eriksson (1998) conducted a thorough investigation on the uptake of nutrients and heavy metals in bilberry following WAA. Samples were collected 3 and 13 mo after treatment (loose and hardened wood ash; 2–8 Mg ha–1). No significant differences in Cd contents in the berries were observed between the control plots and the plots treated with wood ash, irrespective of ash characteristics or application dose. However, since this study was made only during the first year after treatment, the long-term effects could prove to be different. It is likely that the mobility of Cd (and other heavy metals) could increase when the alkalinity of the wood ash is depleted, followed by a decrease in pH. A similar study by Rüling (1996) on the uptake of heavy metals in cowberry and bilberry after WAA (1.5–10 Mg ha–1; loose and pelleted wood ash; 2 mo to about 10 yr) showed no increase in Cd contents in berries, irrespective of treatment. Block and Part (1992) showed that the Cd uptake through the gills of fish can be decreased due to higher concentrations of Ca. Considering the high amounts of Ca in wood ash, Cd uptake in fish and aquatic insects might be reduced, and possibly negated following WAA. However, these speculations must be followed by adequate research to avoid future biotoxic effects. A recent study on N fertilization of coniferous forest implied that the stream water was not affected by this treatment, although increased levels of transition metals (Cd and Zn) in soil water were detected (Högbom et al., 2001b). Whether this reaction is similar for wood ash treatment must still be investigated.
Among aquatic invertebrates, disturbances in behavior due to Cd have been reported, as well as lower recruitment of reproducing organisms (e.g., Wolf et al., 1998, Söderberg-Savelli, 2002). As for bank vole and shrew, mentioned above, it was concluded that the type of food is the prevailing variable to Cd uptake by the aquatic insect Chaoborus punctipennis. Cadmium content in larvae exposed to water with high levels of Cd did not increase; in larvae fed with prey of high Cd content it did (Munger and Hare, 2000). The possibility of using aquatic insects as bioindicators for Cd and other trace elements has been discussed by several authors (Cain et al., 1992; Hare, 1992; Beauvais et al., 1995; Nummelin et al., 1997; Goodyear and McNeill, 1999) and should be considered when applying wood ash.
Cesium
In the future, large-scale practice of wood ash recycling could lead to an increase in the levels of radioactive 137Cs in soil and biota in the forest. The distribution of radioactive elements can be of natural and anthropogenic origin. In the northeastern United States, 137Cs in wood ash originates primarily from aboveground nuclear weapons testing in the 1950s and 1960s (Ohno and Hess, 1994). Recently, concern has arisen regarding Cs content of wood ash–amended soil. In Sweden, and other Scandinavian countries as well, the problem is of another nature. Due to the fire in Reactor 4 at the Chernobyl nuclear power plant in 1986, large areas of central Sweden were contaminated. Application of recycled wood ash originating from contaminated areas could lead to an increase in the concentration of radioactive 137Cs. Recommendations by the Swedish Radiation Protection Institute say that no wood ash containing radioactive 137Cs concentration exceeding 5 kBq kg–1 should be recycled to the forests (Swedish Radiation Protection Institute, 1999). In a study by Högbom and Nohrstedt (2001), six of the seven sites treated with wood ash showed no statistically significant effects on the 137Cs activity. Conversely, there was a decrease of radioactive 137Cs at one plot treated with wood ash. It was concluded that application of wood ash containing 137Cs does not necessarily increase radioactivity in plants and soil. Ohno and Hess (1994) also concluded, considering the permitted ash application rates to soil, there was no statistically significant effect on the levels of 137Cs in wood ash–amended soils. In fact, Levula et al. (2000) found a decrease of 137Cs in cowberry caused by wood ash fertilization. As reported by Ravila and Holm (1996), wood from ash treated plots showed small differences in the levels of 137Cs from untreated plots. In a report by Munthe et al. (2001), WAA has been suggested to increase the immobilization of Cs and decrease the uptake in vegetation due to increased uptake of K.
Mercury
Organic material plays a central role in solubility of metals such as Hg. Generally, the solubility of cations decreases with increasing pH. Mercury binds strongly to organic substances and the effects of WAA are expected to be similar to that of liming. Parkman and Munthe (1998) investigated effects of Hg in runoff water. Both MeHg (methyl mercury) and total Hg were lower in the runoff water in the limed area, compared with an area treated with wood ash and the control. Total Hg in the ashed area did not significantly differ from the reference area. Evaluations of the liming activities in Sweden show a slight decrease in Hg of pike (Esox lucius) after liming lakes or catchment areas. However, some results indicate a great increase of Hg as well and the discrepancies among Hg contents in E. lucius have not been correlated to any specific chemical parameter. There is also no experimental evidence of leaching of either Hg or MeHg after acidic deposition, liming, or WAA and no consistent results can be presented (Munthe et al., 2001). Along with Cd, Hg has been found to be very toxic and strongly inhibits growth on wood-rotting basidiomycetes. Changes in mycelial morphology were observed when cultivated in the presence of Hg (Baldrian and Gabriel, 1997).
Aluminum
It is well documented that increasing acidity in forest soils increases Al solubility. Dise et al. (2001) concluded that forests that release dissolved Al are primarily located in areas receiving the highest amounts of acid rain. The connection between acid atmospheric deposition and increasing levels of mobile Al has also been discussed by Mulder et al. (2001). Forest monitoring studies have, nevertheless, failed to show correlations between soil acidification and forest health (De Wit et al., 2001). In a field experiment trees were not affected by potentially toxic concentrations of Al even after 3 yr. The only observed effect was a decrease in Mg concentrations in needles and it is suggested that Al blocks Mg uptake at the root surface (De Wit et al., 2001).
However, Al toxicity and mortality has been documented for several organisms; including soil organisms (Mulder et al., 2001), salmon (Salmo salar) (Kroglund et al., 2001; Magee et al., 2001), and benthic invertebrates (Hermann, 2001). According to the Swedish National Board of Forestry (2001b), one of the most important reasons for WAA is the neutralization of acidified soils and surface waters. The main reasons for applying alkaline substances are to increase pH and to decrease the amount of toxic Al. In several studies, these objectives have been achieved. Saarsalmi et al. (2001) concluded that neutralization as well as fertilization effects of WAA were of long duration (in this study, 16 yr). An ash-induced pH increase in soil solution of 0.6 to 1.0 pH units was detected, along with a decrease in concentrations of exchangeable Al. A decrease in toxic Al has also been observed in tropical acid soils after WAA, as well as neutralization of soil acidity (Nkana et al., 1998). In a study by Meiwes (1995), although the alkalinity of soil solution was increased after liming, decreases in Al concentrations were less pronounced. A decrease of exchangeable soil solution Al after WAA has also been reported by Bundt et al. (2001b) and by Kahl et al. (1996). In the O horizon (uppermost, organic layer) wood ash treatments (6, 13, and 20 Mg ha–1) resulted in increased pH and higher levels of Ca, K, Mg, and cation exchange capacity (CEC), while Al decreased (Kahl et al., 1996). Combined application of wood ash and lime was studied by Clapham and Zibilske (1992), who found a greater positive effect on soil pH than with lime only. Calcium, K, and Na were leached in greater amounts in wood ash–treated soils and soil Al decreased linearly with wood ash amendment in the wood ash + lime admixtures. Clapham and Zibilske (1992) also found an increase in the uptake of Al in plants treated with this admixture. The authors suggest wood ash as a liming complement rather than as a replacement.
Recent studies imply that application of wood ash can increase the concentrations of inorganic Al and decrease pH, at least at an initial period (1–3 yr). Ring et al. (1999) showed that granulated wood ash increased concentrations of Al in the humus layer of a Scots pine stand. Högbom et al. (2001c) investigated soil solution Al in a highly N loaded Norway pine site after application of wood ash (4.2 Mg ha–1; pelleted ash). The results showed a decrease of pH in deeper soil solution (50 cm) along with increased Al concentrations. Leaching of Al following WAA has also been reported by Lundell et al. (2001) and discussed by Rumpf et al. (2001). The complexity of soil Al and its interaction with organic substances has been discussed by Munthe et al. (2001), who concluded that both sudden acidification and alkalinization can induce solubilization of inorganic Al. The reasons for this are highly related to different ion exchanging reactions and processes. High concentrations of base cations can induce an ion exchange on the surface of the soil particles, leading to mobilization of Al3+ and H+ in the soil solution.
Aluminum is probably the most investigated element associated with liming and use of wood ash, due to Scandinavian problems of acidification and strategies to lower its harmful effects to biota. Most scientists seem to agree on the positive effects of WAA regarding increased pH and decreased concentrations of inorganic Al. However, some controversial results indicate leaching of Al in certain layers of the soil, and future research should further investigate these findings.
Other Metals and Toxic Compounds
In addition to the elements mentioned above, there are several other metals and compounds that derive from wood ash. Some of the more common metals in ash that can harm organisms are Fe, Ni, Zn, Co, As, and Cr (Levula et al., 2000; Perkiömäki and Fritze, 2002a). Together with Hg and Cd, Co has been found to be the most toxic heavy metal, inducing strong inhibition of tested basidiomycetes in a study by Baldrian and Gabriel (1997). The leachability of Cu and Ni has been studied by Chirenje et al. (2002b). The results indicated that Cu (and to some extent Ni) in wood ash–amended soil formed soluble complexes with DOC at high pH (around 10). The same authors also investigated leachability of As and Cr in wood ash–amended soil columns. The amounts of As and Cr in the leachate were higher in the water-leached columns than in the acid-leached ones. The authors suggest the lower Cr leaching in humic acid (HA)–leached columns was due to the high reaction between Cr and HA. Arsenic was found to flocculate with DOC in the columns, hence lowering the amounts of As when mixing ash with soil (Chirenje et al., 2002a). In a study by Timmermans et al. (1992), the uptake of Cd and Zn in two species of aquatic insects was investigated. It was concluded that Cd uptake from food and Zn uptake from water dominated in both species. Thus, the fairly high amounts of Zn in wood ash (e.g., 700–800 mg kg–1; Demeyer et al., 2001) may be of threatening consequence to aquatic organisms.
An essential trace element for plants is B. On mineral soil in eastern Finland, growth disturbances due to supposed B deficiency have been encountered (Hynönen, 2000). Recovery of tree growth to normal levels after B fertilization has been achieved, as reported by Braekke (1983). Since wood ash contains B, which likely is accessible for plants, ash could be a source of B for the plants. This is supported by the findings of Jacobson (2003), where concentrations of B significantly increased 3 to 5 yr after WAA. If this is generally valid, increased uptake of B could be induced by WAA.
Wood ash application can also be a source of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). Studies by Bundt et al. (2001a) show that the concentrations of PAHs in the organic horizon of the soil can increase up to sixfold after WAA. The PCB concentrations, on the other hand, decreased. This was probably caused by the mobilization of stored PCB due to higher mobility of dissolved organic matter, caused by increased pH (Bundt et al., 2001a) following WAA. However, the overall effects of increased pH caused by WAA seem to be more positive than negative regarding metals and toxic compounds, in the present day situation.
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The effects of WAA on aquatic organisms and ecosystems are highly uncertain. Wood ash application will increase pH in the water, increase nutrient content, and possibly mobilize toxic compounds. This is likely to have negative effects for aquatic organisms. Consequently, we recommend that buffer strip zones between wood ash–treated areas and watercourses and lakes are made to avoid unwanted effects of WAA. Future limnological research on the effects of WAA must be undertaken before large-scale application is implemented, to assess the impact on aquatic environments.
The amounts of toxic compounds associated with wood ash show large variations and due to different mobility of elements like Cd, Al, and Cs, caution must be exercised if application is to occur in natural environments. It is essential to analyze all wood ash before application, and more research must be completed before the initiation of WAA programs to avoid future biotoxic effects when using wood ash.
Assuming the purpose of WAA is to avoid depletion of essential soil nutrients, we recommend site- and wood ash–specific application practices, rather than broad and general guidelines for wood ash application to forests. Assuming that the purpose of WAA is to reduce the harmful effects of acidification of surface waters, we would rather advise the use of lime to avoid drastic changes of the mobility of elements in soil and water. The final conclusion of this review is that the potential beneficial effects of WAA may not outweigh the diverse potential negative impacts to the environment.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONCHARACTERISTICS OF WOOD ASHBIOLOGICAL EFFECTS OF WOOD...BIOTOXIC EFFECTS OF WOOD...CONCLUSIONSREFERENCES |
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