Journal of Environmental Quality 31:946-953 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Surface Water Quality
Limnological Effects of Wood Ash Application to the Subcatchments of Boreal, Humic Lakes
Tiina Tulonen*,a,
Lauri Arvolaa and
Susanna Ollilab
a Univ. of Helsinki, Lammi Biological Station, FIN-16900 Lammi, Finland
b Pirkanmaa Regional Environment Centre, P.O. Box 297, FIN-33101 Tampere, Finland
* Corresponding author (tiina.tulonen{at}helsinki.fi)
Received for publication April 13, 2001.
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ABSTRACT
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To assess environmental risks of wood ash, limnological effects of ash application to the drainage basins of two small, humic lakes and one reference lake in southern Finland were examined in this three-year study. Treated areas corresponded to 12 and 19% of the total catchment and the amount of wood ash added was 6400 kg ha-1. Immediate effects of wood ash on lake water were investigated in three tank experiments each lasting 1.5 wk. In tank experiments, addition of wood ash increased pH, alkalinity, conductivity, and Ca and P concentrations of humic lake water, while growth of phytoplankton decreased. After wood ash application to the subcatchments, pH, alkalinity, conductivity, and concentrations of K+, SO2-4, and Cl- slightly increased, both in inflowing waters and in the lakes, but no increased leaching of Ca, N, or P from the treated subcatchments occurred. Phytoplankton biomass increased in both experimental lakes in comparison with the reference lake. In the lake with 19% application rate to the catchment, zooplankton biomass also increased. The results indicate that, over the short term, a small-scale ash treatment to a forested drainage basin will not necessarily cause significant changes in the water quality of boreal humic lakes, but at higher application rates, changes in water chemistry and biology are more evident.
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INTRODUCTION
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IN FINLAND, the forest industry and power plants annually produce approximately 200000 metric tons of ash. Previously, wood ash has been deposited in landfills or in waste disposal sites, but in recent years methods for ash utilization have been considered (Campbell, 1990). Applying wood ash to forested areas could replenish nutrients and trace elements removed during harvesting, which often occurs on short rotations and causes nutrient depletion in forests (Williams et al., 1996; Larsson and Westling, 1998). In addition, there has been increasing interest in the utilization of wood ash to reduce the acidification of forest soils (Meiwes, 1995; Ring et al., 1999).
Wood ash contains many essential nutrients for the growth of trees and other plants, mainly calcium, potassium, magnesium, and phosphorus. The nitrogen content is low due to losses during combustion. Because of this high nutritional value, the utilization of wood ash as a forest fertilizer has received increased attention in many countries (Silverberg and Issakainen, 1996; Silverberg, 1998). Wood ash is particularly recommended for peatlands that suffer from a shortage of trace elements, but are rich in nitrogen (Saarela, 1991; Ferm et al., 1992). According to Finnish recommendations, the wood ash that is applied should contain similar amounts of phosphorus to commercial fertilizers (30–40 kg P ha-1) in order to produce favorable effects on forest growth. The amount of wood ash applied usually varies between 3000 and 8000 kg ha-1 depending on its phosphorus concentration. Wood ash producers have estimated that approximately 25000 ha of drained peatland in Finland could be fertilized with wood ash annually. However, phosphorus may potentially leach from catchments into nearby lakes and cause eutrophication. Furthermore, the strongly basic and reactive wood ash can increase pH and cation concentrations in water. Liming of surface waters has been used, particularly in Sweden, to mitigate the effects of acidification (Henrikson and Brodin, 1995) and ash application on water ecosystems might have similar effects to those caused by liming. However, liming is selectively applied to mineral soils and, moreover, the phosphorus concentration in lime is much lower than in wood ash.
Leaching of ash components into soil solutions has been studied both under laboratory conditions and in catchments (Fransman and Nihlgård, 1995; Kahl et al., 1996; Nilsson and Lundin, 1996; Steenari et al., 1996; Larsson and Westling, 1998; Silverberg, 1998), but to date, no extensive investigation into the effects of wood ash application on lake ecosystems has been conducted. Ecosystem effects may depend on characteristics of the recipient lakes, as well as on local soil type, date of the application, and weather conditions after wood ash is applied.
This study evaluates the effects of wood ash application on small, humic headwater lakes in southern Finland. The immediate effects of wood ash on water chemistry and phytoplankton were first investigated with tank experiments, where different amounts of wood ash were added to humic lake water. Leaching of nutrients from treated subcatchments, consisting of both mineral soil and peatland, and the changes in water chemistry and plankton communities in the lakes downstream, were investigated over a two-year period.
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EXPERIMENTAL METHODS
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Wood Ash
The wood ash used in this experiment was produced as a by-product of a paper mill (Metsä-Botnia, Äänekoski, Finland). This so-called self-hardened ash was first dampened and then, once dry, crushed into small particles. The moisture content of the ash was approximately 25% and neutralization capacity approximately 10% Ca as determined by titration. The elemental composition of the ash is shown in Table 1. Elements were analyzed after nitric acid extraction with the coupled plasma source emission spectrometry (ICP–ES) technique.
Tank Experiments
Tank experiments, each lasting 8 to 9 d, were performed in June, July, and August 1997. Sample water, collected from the humic Lake Horkkajärvi, was immediately poured through a 50-µm mesh net to remove metazooplankton. After mixing, the water was divided into light gray plastic tanks (80 L). The tanks, each containing 70 L of sample water, were placed in a large ventilated greenhouse. The experiments were performed under a natural light regime and temperature conditions.
Three treatments were used in the tank experiments; one control and two concentrations of wood ash, each with three replicates. In Experiments I and III, 10 and 50 mg L-1 wood ash was added and in Experiment II, rates were 5 and 25 mg L-1.
For chemical and biological analyses, subsamples were collected from the experimental tanks immediately after mixing of the water, one hour after ash addition, and every second day from then on. Water temperature and pH were measured daily. Analyses for PO4–P, total P, NO2 + NO3–N, and total N were performed with standardized methods using an Akea autoanalyzer (Lachat [Milwaukee, WI] QC 8000) (Murphy and Riley, 1963; Wood et al., 1967; Koroleff, 1979). Electrical conductivity, alkalinity (Gran titration), K, Ca, Mg, and Fe concentrations (atomic absorption spectrometry [AAS] analyzer, Varian [Palo Alto, CA] SpectrAA 220 Fast Sequential), and concentrations of SO2-4 and Cl- (Dionex [Sunnyvale, CA] 2000i ion chromatograph) were determined at the beginning and end of each experiment. Chlorophyll a was analyzed with a Hitachi (Tokyo, Japan) F-4000 fluorescence spectrophotometer with excitation and emission wavelengths of 435 and 671 nm. All analyses were performed in the laboratory at Lammi Biological Station.
Primary production of phytoplankton was measured using a radiocarbon technique (Schindler et al., 1972). Phytoplankton biomass and species composition were determined from preserved samples (Lugol solution) with a Nikon (Tokyo, Japan) Diaphot inverted microscope using Utermöhl's (1958) technique.
Field Study
The field studies focused on three small (0.4–1.1 ha) humic headwater lakes situated in the Evo forest area (61°14' N, 25°12' E) in southern Finland. The subcatchments of two lakes, Lake Tavilampi and Lake Nimetön, were treated with approximately 6400 kg ha-1 of self-hardened wood ash (Table 2). Ash was spread over the snow-covered forest floor by hand in February 1998. The ash application added 37 to 41 kg ha-1 of phosphorus to the forest soil. The resultant doses of Cd, Cr, and Zn to the soil were relatively high in comparison with annual atmospheric deposition (Table 2). Lake Horkkajärvi served as the control lake. The treated subcatchments of Lake Nimetön consisted of both ditched peatland and mineral soil with two brooks flowing through them (Brook 1 and 2). One brook flowed through the untreated peatland (control brook). The subcatchment of Lake Tavilampi consisted of peatland with Norway spruce [Picea abies (L.) H. Karst.] as the predominant tree species. This lake had no aboveground inflow.
The brooks and lakes of the study area were monitored for one year before wood ash application. Treatments were carried out in February 1998 and monitoring continued until autumn 1999. Brooks and lakes were sampled biweekly or monthly from May to September. During winter, lakewater samples were collected three to four times. Samples for chemical, bacterial, and phytoplankton analyses were collected with a Limnos (Turku, Finland) tube sampler (volume 2.1 L) from the middle of the lake, at depths of 0, 1, 2, and 6 m (5 m in Lake Tavilampi). Samples for zooplankton measurements were collected with the tube sampler from the surface to a depth of 3 m, and the pooled sample was concentrated using a 50-µm mesh net. Metazooplankton samples were preserved in formaldehyde and counted under an inverted microscope. Bacterial density was determined from stained samples with a Nikon Optiphot epifluorescence microscope (Bergström et al., 1986). Concentrations of Cd and Cr in the brook water were analyzed in the spring 1998 with an atomic absorption spectrometry analyzer (Varian SpectrAA-400 with a GTA-96 graphite tube atomizer). Other chemical and biological analyses were performed as described for the tank experiments.
The Mann–Whitney U test was used to test whether changes in the brooks and in the lakes were statistically significant. The time periods before (1997) and after (1998 or 1999) wood ash application were the grouping variable.
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RESULTS
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Tank Experiments
Addition of wood ash to lake water explained approximately 85% of the increase in water pH (R2 = 0.84) and alkalinity (R2 = 0.86) and approximately 75% of increase in the conductivity (R2 = 0.74) and phosphate (R2 = 0.76) (Fig. 1
, Table 3). At the end of the experiments, concentrations of SO4, Ca, and K were higher in ash treatments than in the control. No marked increase was found in Cl, Mg, or Fe concentrations. Addition of wood ash decreased phytoplankton production and chlorophyll a concentrations in Experiments I and III performed in June and August (Fig. 2)
, while in Experiment II, in July, ash addition slightly stimulated phytoplankton growth. This increase was most obvious on the fourth day of the experiment. Mallomonas akrokomos (Chrysophyceae) dominated the phytoplankton community in June (approximately 97% of biomass) and Closterium acutum var. variable (Conjugatophyceae) in August (approximately 80% of biomass). In July, the lowest concentration of wood ash (5 mg L-1) stimulated growth of Chrysococcus sp. (Chrysophyceae) and C. acutum var. variable.
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Fig. 1. Water pH, alkalinity, conductivity, and PO4–P concentration of lake water in tank experiments 8 d after addition of wood ash.
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Field Studies
Leaching of wood ash components was monitored in the catchment of Lake Nimetön. During summer 1997–1999, quantity of runoff varied widely in the brooks investigated. The year 1998 was exceptionally rainy and the average discharge in brooks during May–September was 136 L min-1, which was almost twice as high as the year before ash application. In spring 1999, the discharge was still high, but subsequent lack of rain dried the brooks up completely by the beginning of July.
After ash application, the alkalinity and conductivity of the water increased in the brook flowing from the treated peatland (Fig. 3)
. A tendency toward pH increase was found in the brook flowing through the treated mineral soil one year after ash application. The same tendency was found after two years in the brook flowing through the treated peatland (Table 4). In the rainy year of 1998, N and P concentrations increased in comparison with the previous year in all tributaries, including the control brook flowing from the untreated area. Leaching of Cl, K, and SO4 from the peatland was greater from the treated subcatchments than from the control, although the changes were only statistically significant for K (Table 4, Fig. 3). Concentrations of Fe decreased in the brooks flowing from the treated subcatchments after ash application. In spring 1998, Cr and Cd concentrations in the brooks were near the detection limits and no difference between the brooks was observed.
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Fig. 3. Alkalinity, conductivity, and concentration of K in the brooks of Lake Nimetön in 1997–1999. Brook 1 flows from ash-treated peatland and the control brook from untreated peatland.
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Table 4. Average annual water quality of the brooks flowing into Lake Nimetön before (1997) and after (1998–1999) ash application. Statistical significances are indicated for changes between the control year (1997) and the years after ash treatment (1998 or 1999).
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In Lake Tavilampi and Lake Nimetön, mean pH, alkalinity, and conductivity of the surface water (0–2 m) were slightly higher in 1998 over 1997, while in the control lake (Lake Horkkajärvi), they were lower in 1998 than in 1997 (Fig. 4
, Table 5). After two years the pH in both lakes with treated catchments had risen more than in the control lake. In Lake Tavilampi and Lake Horkkajärvi, P and N concentrations were generally higher in 1998 than in the previous year. In Lake Nimetön no increase in nutrient concentrations was observed and in 1999 concentrations decreased. In comparison with the control lake, there was a small tendency for an increase in K, Cl, and SO4 concentrations in Lake Tavilampi and Lake Nimetön. The increase in Cl concentration for 1997 to 1998 was statistically significant (p < 0.001) in both lakes.
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Table 5. Average annual water quality (at depth 0–2 m) in the lakes in 1997–1999. Statistical significances are indicated for changes between the control year (1997) and the years after ash treatment (1998 or 1999).
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In Lake Tavilampi, the mean chlorophyll a concentrations were not significantly different either before or after ash application (Table 6). However, in summer 1999, the mean concentration increased slightly compared with the previous two years. Phytoplankton biomass increased in 1998, mainly due to increases in the algal groups Cryptophyceae and Chlorophyceae. Moreover, in 1999 phytoplankton biomass doubled from the previous year, mostly because of the more vigorous growth of green algae (Chlorophyceae). In 1997, the year before ash application, green algae composed 16% of total phytoplankton but two years after ash application the proportion of green algae had risen to 64%. Bacterial densities were almost the same in all lakes during the study period.
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Table 6. Average bacterial density, chlorophyll a, phytoplankton biomass, and number of Rotatoria and Crustaceae in the lake water in May–September before (1997) and after (1998, 1999) ash application.
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In Lake Nimetön, no clear differences in the mean concentrations of chlorophyll a were found. However, mean phytoplankton biomass in 1998 was twice as high as in the previous year, mostly because of the peak in Peridinium umbonatum (Dinophyceae) in May. In the control lake, no pronounced changes in biomass or species composition of phytoplankton during the first two years were found, and in 1999 the phytoplankton biomass was lower than in previous years (Table 6).
After ash application, the number of rotifers increased in Lake Tavilampi and decreased in Lake Nimetön (Fig. 5)
. In Lake Horkkajärvi, the rotifer numbers were low and no variation between years was found. The vigorous growth of rotifers in Lake Tavilampi was mostly due to the increase of Kellicottia bostonensis. In Lake Nimetön, rotifer numbers decreased because of decline of Polyarthra sp. The number of Crustaceae was low in Lake Nimetön compared with the two other lakes, where no changes between study years were found.
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DISCUSSION
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The addition of wood ash caused changes in water quality both in the tank experiments and in the field, but these changes varied. The increases in pH, P, and Ca concentrations in tanks were not found in the brooks or lakes of the treated areas. In previous laboratory studies, P and Ca have been found to leach rapidly from different types of ash (Larsson and Westling, 1998). Moreover, Nilsson and Lundin (1996) and Fransman and Nihlgård (1995) have observed an increase in Ca concentrations of ground and stream water after ash application. In this study, increased Cl, K, and SO4 levels in brook water were apparent after ash application, which is consistent with the results of other field studies, where Cl, K, and SO4 have been the first ash components to leach from the catchment (Steenari et al., 1996; Williams et al., 1996).
In the forest soil, Ca and P may form more stable compounds such as orthophosphate or apatite (Steenari et al., 1996), resulting in reduced leaching. Furthermore, the high concentrations of Fe or humic substances in peatlands can fix phosphate into rather insoluble compounds (Heikkinen, 1990). In this study, P leaching from the catchment appeared to be correlated with quantity of runoff. Previous studies in Lake Nimetön have also indicated that the brooks have higher nutrient concentrations during wet summers than during dry ones (Arvola et al., 1990). In addition to chemical reactions and variations in discharge, biological processes may also play an important role in the P solubilization and transport in forest soils. Furthermore, the rather low solubility of the self-hardened wood ash added suggests that leaching of excess Ca and P from the catchment may be gradual and take up to several years. In some studies leaching of N compounds into soil solution has been observed after ash addition, but it is unclear how readily these compounds are exported to ground water or to recipient lakewaters (Kahl et al., 1996; Högbom et al., 2000). In this study we did not find any evidence of leaching of N compounds after ash application.
Although the nutrient concentrations of the lakes did not increase after treatment of their catchments with wood ash, phytoplankton biomass in Lake Nimetön and Lake Tavilampi was higher in the rainy year of 1998 than in the previous year. In 1999, chlorophyll a concentrations and particularly phytoplankton biomass in Lake Tavilampi showed a stimulation of phytoplankton growth. Moreover, higher rotifer densities also suggest enhanced lake productivity. Increased rotifer numbers have been observed after forest fires in the Canadian Boreal Shield lakes (Patoine et al., 2000).
In Sweden, liming of surface waters applied to mitigate the harmful effects of acidification resulted in increased productivity of lakes and decreased heavy metal concentrations in lake waters (Henrikson and Brodin, 1995). Similarly, wood ash application can decrease leaching rates of heavy metals already present in the catchment area, by increasing the soil pH. In this study, no Cd or Cr leaching was detected, which is consistent with other studies (Williams et al., 1996). However, the amounts of heavy metals added the forest soil in ash are substantially higher than the amount from annual atmospheric deposition (Ruoho-Airola, 1995); hence, long-term monitoring is needed to reveal the true potential for accumulation of heavy metals in aquatic environments.
The tank experiments demonstrated that pH increase may cause an immediate decrease in phytoplankton growth, which has also been observed in laboratory conditions and in limed lakes (Salonen et al., 1994; Henrikson and Brodin, 1995; Järvinen et al., 1995). Liming often increases water pH by as much as two units, but, at the application rates used in our field study (approximately 6000 kg ha-1, 12 and 19% of the catchment), ash did not significantly change the water pH of recipient lakes. Even so, application of wood ash to the catchment may reduce variation in the pH of lake water during different drainage conditions. The pH of the control lake decreased in 1998 because high summer precipitation increased the amount of runoff such that more acidic water flowed into the lake (Fig. 4). However, in the two ash-treated lakes, the pH did not decrease in spite of the wet summer season.
When considering lakewaters, it is difficult to set any strict thresholds on recommended ash application rates because the size of the application area in relation to the catchment area is a more significant factor. In this study some effects of ash application on biological processes were seen when 19% of catchment, mostly peatland, was treated, but treating 12% of the catchment (both mineral soil and peatland) did not cause any pronounced changes in the lake ecosystem. Many forestry practices, such as ditching or site preparation, often lead to a higher load of nutrients to recipient waters than wood ash fertilization (Holopainen and Huttunen, 1998; Manninen, 1998). This has also been documented in Lake Nimetön, where clearcutting and burning of the catchment in the early 1980s caused immediate leaching of P and N, and the enhancement of primary production (Arvola et al., 1990).
In conclusion, we showed that laboratory tests alone do not reveal the true effects of ash application and that field studies are also needed. Over the short term, a small-scale application of wood ash to forest soil seems to have only a minor effect on the water quality of humic headwaters. However, 19% treatment of the catchment might already stimulate phyto- and zooplankton growth in the lake.
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ACKNOWLEDGMENTS
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This work has been funded by Metsäteho Oy. We thank Jaakko Vainionpää and Riitta Ilola for carrying out the chemical analyses, Mikael Pihlström for the cadmium and chromium analyses, Anja Lehtovaara for counting zooplankton, and Mirkka Jones for proofreading the manuscript.
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TOP
ABSTRACT
INTRODUCTION
