effects of wood ash dose and formulation on soil chemistry at two coniferous forest sites

EFFECTS OF WOOD ASH DOSE AND FORMULATION ON SOILCHEMISTRY AT TWO CONIFEROUS FOREST SITES

STAFFAN JACOBSON1,∗, LARS HOGBOM1, EVA RING1

and HANS-ORJAN NOHRSTEDT2

1Skogforsk, The Forestry Research Institute of Sweden, Uppsala Science Park, SE-751 83 Uppsala,Sweden; 2Formas, P.O. Box 1206, SE-11182 Stockholm, Sweden

(∗author for correspondence, e-mail: [email protected], Tel: +46-(0)18-18 85 47,Fax: +46-(0)18-18 86 00)

(Received 29 November 2002; accepted 6 April 2004)

Abstract. Harvesting stem biomass from the forest inevitably involves exporting nutrients from theecosystem. The amount exported is increased when the logging residues are also removed for useas fuel. Recycling of the resulting wood ash has been advocated as a measure to compensate for thenutrient losses and to sustain future forest production. The physical formulation of the wood ash mayhave an important influence on its effects on soil properties. In this paper, we report effects of twodifferent types of wood ash (one self-hardened and crushed, the other pelleted), with differences insolubility, on soil chemistry in the humus layer and upper 15 cm of the mineral soil, at two coniferoussites in south-central Sweden, 5 yr after their application. The crushed ash was applied at three doses(3, 6 and 9 ton ha–1), while the pelleted ash was applied at only one dose (3 ton ha–1). At both sites thesoil was podzolized. The two sites differed with respect to soil conditions, despite being situated onlya few kilometers apart. The application of wood ash increased both soil pH and base-cation content inthe humus layer at both sites. In the mineral soil, the effects were less pronounced. Treatment effectson soil chemistry did not differ between the two ash formulations. The retention (i.e. the extractableamount of nutrients found in the soil that could be attributed to the ash application) of nutrients variedstrongly between the two sites, and K retention (ca. 10%) was generally lower than that of Ca and Mg.

Keywords: ammonium, calcium, magnesium, nitrate, pH, phosphorus, potassium

1. Introduction

Interest in using logging residues for bioenergy production has increased in Swedenduring the last decade. Forest fuels, together with by-products from the forestindustry, contribute 21% of the total energy used in Sweden (Anonymous, 2000).However, harvesting logging residues involves increased extraction of biomassfrom the forest, including removal of nutrient-rich components, such as twigs andneedles. Thus, this slash extraction increases nutrient export from the site and soilacidification (Staaf and Olsson, 1991; Olsson et al., 1993; Sverdrup and Rosen,1998). Recycling the nutrients in the wood ash has been suggested as a means toprevent, or at least reduce, the undesirable effects of intensive biomass harvesting(Vance, 1996; Olsson et al., 1996; Eriksson, 1998). Wood ash has high pH andANC values, and retains a major portion of most macronutrients (except nitrogen(N)) and micronutrients from the biomass in inorganic forms.

Water, Air, and Soil Pollution 158: 113–125, 2004.C© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

114 S. JACOBSON ET AL.

Application of friable (nonhardened), highly soluble wood ash can cause rapidand drastic increases in the concentration of salts and the pH of the upper forestsoil horizon (Khanna et al., 1994), resulting in increased microbial activity and soilrespiration (Fritze et al., 1994; Zimmermann and Frey, 2002). Poorly stabilizedwood ashes, i.e. ash with large proportions of fine particles (<0.25 mm), may alsonegatively affect the ground vegetation (Kellner and Weibull, 1998; Jacobson andGustafsson, 2001). Furthermore, it has been shown that a pronounced increase inpH can increase nitrate leaching in N-rich soils (Huttl and Zottl, 1993; Kreutzer,1995; Hogbom et al., 2001). Therefore, in order to reduce the risks of unwantedenvironmental effects, it is advisable to transform the very reactive oxides of the ashinto compounds with low solubility (e.g. carbonates), and to form larger aggregateswith a dense, stable matrix, thus limiting the potential contact area for water aroundthe ash particles (Steenari et al., 1998).

Here we report changes in soil chemistry observed following the applicationof different doses of self-hardened and crushed wood ash. In addition, at one ashdose, the effects were compared with those of another type of wood ash product,originating from a different source and stabilized in a different way. The studywas performed at two sites, with different soil types, both typically N limited andrepresentative of vast areas of forest land in Sweden.

2. Material and Methods

The two experimental sites used in this study are situated a few kilometers apartin south-central Sweden. Site 249-Riddarhyttan is covered by a mixed Picea abies(L.) Karst/Pinus sylvestris L. stand growing on a podzolized soil formed on glacialtill (referred to as the mixed site in the following text), while site 250-Riddarhyttanis dominated by P. sylvestris growing on a podzolized soil that has developed on asandy sediment (referred to hereafter as the pine site). The highest marine shorelinefollowing the latest glaciation was in altitude between the two sites studied. Bothsites are characterized as permanent recharge areas for groundwater. Stand and siteproperties of the two sites are given in Table I. Wood ash (3, 6 and 9 ton ha–1

(d.w.)) was applied in September 1995 to experimental plots (30 × 30 m at thepine site and 22 × 22 m at the mixed site) in a randomized block design with threereplicates. Plots were arranged in blocks based on stand basal area, number of stemsand forest vegetation type at the pine site and by soil-solution chemistry parametersat the mixed site.

2.1. THE WOOD ASHES

The two types of ash used in these field experiments originated from two differentpulp mills. The crushed ash came from the pulp mill in Pitea. This ash was pretreatedwith water and left in a heap for a month for self-hardening, before crushing. Aftercrushing, the ash was sieved (mesh size = 10 mm), and the finer fraction was

EFFECTS OF WOOD ASH DOSE AND FORMULATION ON SOIL CHEMISTRY 115

TABLE I

Site and stand characteristics of the two studied sites at the time of establishment

249-Riddarhyttan, mixed site 250-Riddarhyttan, pine site

Location 59◦48′ N:15◦30′ E 59◦48′ N:15◦32′ E

Altitude (m asl) 180 135

Annual mean temperature (◦C)a 3.9 3.9

Precipitation (mm yr–1)a 730 730

Soil type Podzol Podzol

Soil texture and genesis Sandy-silty till Sandy sediment

Dominating tree species Pinus sylvestris/Picea abies Pinus sylvestris

Site quality (m3 ha–1 yr–1) 7.7 5.9

Stand age (yr) 60 50

Stem mean height (m) 20 16

Standing stem volume (m3 ha–1) 195 150

aAlexandersson et al. (1991).

spread. Data on particle size distribution can be obtained from Jacobson (2003).Analyses of the crushed ash revealed that it was highly water-soluble. The pelletedash originated from the Ortviken pulp mill. The pelleting was done after adding8–10% “pine-oil” as a binding agent. A notable feature of this ash was its high levelof unburnt material, reflected in its high carbon content (Table II). Despite this, thepellets had become fairly hard and water resistant.

TABLE II

Chemical composition (% of d.m.) of the two ashes

Self-hardenedcrushed ash Pelleted ash

Ca 13.7 15.2

Mg 1.4 1.4

K 6.4 3.5

P 0.8 0.8

Al 1.9 3.1

Mn 0.8 1.0

Fe 1.1 2.1

Na 10.8 1.1

Ti 0.1 0.1

S 10.9 1.6

C 2 21.6

Si 5.6 9.4


The chemical composition of the two ashes is detailed in Table II. Ca, Mg,K, P, Al, Mn and Si were determined by ICP-AES after being fused with LiBO2

and dissolved in dilute HNO3. Trace elements were determined by ICP-MS afterdigestion by microwave heating in closed teflon vessels with HF:HNO3:HCl (1:3:1by volume).

2.2. SOIL SAMPLING AND ANALYSES

In June 2000, i.e. 5 yr after treatment, 16 samples of the humus layer were takenfrom each plot, using a soil auger with a diameter of 6 cm. The humus samples weresieved through a 6-mm mesh. From the mineral soil, 32 samples per plot were takendown to a depth of 15 cm using a soil auger with a diameter of 3 cm. The mineralsoil samples were divided into two layers: 0–7.5 and 7.5–15 cm. The mineral soilmaterial was sieved through a 4-mm mesh. Soil samples from the same plot werepooled within layers to form a general sample representing each plot. Before sieving,the soil samples were stored in a refrigerator (2 ◦C). After sieving and mixing,the material was divided into three subsamples for various analyses. Subsamples(humus only) for total C and N determination were dried at room temperature in adust-free environment, while samples for pH and extractable nutrient (Ca, Mg, Kand P) determinations were kept in the refrigerator until analysis. Subsamples fordetermination of inorganic N were kept frozen until analysis.pH was determined using a glass electrode in a soil:water slurry (1:2 by volume).Potassium (K) and phosphorus (P) were measured with an Auto Analyzer II (Tech-nicon) after extracting 5 g fresh material in 100 ml 0.1 M ammonium lactate (pH3.75) for 1.5 h and filtering the resulting suspension through pleated filters (00A,125 mm). P was analyzed using a colorimetric method (ammonium molybdate-ascorbic acid, abs. 660 nm) and K using a flame photometer. The same extractwas used when analyzing calcium (Ca) and magnesium (Mg), but these cationswere measured with an inductively coupled plasma (ICP) optical emission spec-trometer (Optima 300 DV, Perkin-Elmer). Concentrations of NH+

4 and NO–3 were

determined with flow injection analysis (TRAACS 800) after extraction with 1 MKCl. Total C and N concentrations were analyzed on air-dried soil samples (0.5 ghumus) by combustion at 1250 ◦C with a LECO 2000 analyzer (LECO EquipmentCorp.). All chemical analyses were performed at the Department of Soil Science,Swedish University of Agricultural Sciences in Uppsala.

Recovery was calculated as the proportion of nutrient added via the ash thatcould be retrieved by extraction with 0.1 M ammonium lactate on ash plots, minusthe amount found in the control plots.

2.3. STATISTICAL ANALYSIS

Treatment effects were analyzed within soil layer using a two-way ANOVA.For all tested variables, except pH, log-transformed values were used in order


to improve the normality of the distribution of the residuals. Differences be-tween treatment means were evaluated according to Tukey’s test for multiplecomparisons. In addition, the strength and significance of dose–response relation-ships was tested by linear regression analysis. The General Linear Model proce-dure of the SAS-package (SAS Institute, Inc., 1999) was used for the statisticalanalyses.

3. Results

Effects of applying self-hardened and crushed wood ash on the soil chemistry weredetected, even 5 yr after application. The results of the 3 ton ha–1 (3T) treatmentwith crushed ash did not differ significantly (P > 0.05) from those of the pelletedash treatment with respect to pH, Ca, Mg, K, P, N and C in any of the studiedlayers.

3.1. pH

At the mixed site, the pH of both the humus and upper mineral soil horizon sig-nificantly differed between plots given the 6T treatment and the control plots(Figure 1A). However, the pH values of the other treatments did not significantlydiffer from the control. This was in marked contrast with the pine site where a cleardose response (p < 0.01) for the crushed ash was found in the humus layer (Figure1B). A similar pattern was also found in the 0–7.5 cm layer of the mineral soil(Figure 1A), but in this case the lowest dose (3T) did not differ significantly fromthe control.

3.2. Ca AND Mg

The contents of Ca (Figures 2A and 2B) and Mg (Figures 3A and 3B) followedthe same pattern as pH. In the humus layer at the mixed site the contents in plotsgiven the largest doses differed significantly from those found in the control and 3Tplots. In the upper mineral soil horizon a significant treatment effect was found forMg (Figure 3A), but not for Ca (Figure 2A), at the mixed site. At the pine site bothCa (Figure 2B) and Mg (Figure 3B) contents were significantly increased after theapplication of crushed ash.

3.3. K

Significant treatment effects for K were found only in the two mineral soil lay-ers (Figures 4A and 4B). Furthermore, there was a clear dose–response effect(P < 0.01) for this element at both sites and in both mineral soil layers.


Figure 1. pH at different soil depths, comparing the effects of different wood ash doses at (A) themixed site and (B) the pine site, and effects of crushed and pelleted ash at (C) the mixed site and(D) the pine site. ◦ = control plots, � = 3 ton ha–1 crushed ash, • = 6 ton ha–1 crushed ash,� = 9 ton ha–1 crushed ash , � = 3 ton ha–1 pelleted wood ash. Mean values ± 1 s.e. of the mean.Symbols not followed by the same lower case letter indicate significant differences at the P < 0.05level.

3.4. P

At the mixed site the two highest doses (6T and 9T) increased the P content signif-icantly in both the humus and upper mineral soil horizons (Figure 5A). At the pinesite (Figure 5B) statistically significant differences were found only in the humuslayer.

3.5. NUTRIENT RECOVERY

There were large differences between the recoveries of the applied nutrients be-tween the two sites (Table III). Ca recovery values following the crushed ashtreatments were much higher at the pine site (45–54%) compared to the mixedsite (16–24%). For the pelleted ash the differences were much smaller betweenthe sites (10 and 19% for the mixed and pine site, respectively). The recoveryof Mg followed the same pattern as the Ca recovery. Similarly, the recovery ofP was much lower at the mixed site (1–11%) than at the pine site (40–78%).


Figure 2. Extractable Ca content (kg ha–1) at different soil depths, comparing the effects of differentwood ash doses at (A) the mixed site and (B) the pine site, and effects of crushed and pelletedash at (C) the mixed site and (D) the pine site. Mean values ± 1 s.e. of the mean. Symbols as inFigure 1.

The recovery of K was low at both sites (ca. 10%), regardless of ash dose orformulation.

3.6. N AND C

The NH+4 content of the humus layer was significantly affected by the ash treatments

at the pine site (Figure 6B), and the response increased with increasing ash dose.In the two mineral soil horizons no effect of ash was found. No effect of ash wasfound at the mixed site in any layer (Figure 6A). The extractable amounts of NH+

4found in the control plots did not differ between the sites. There were no significantdifferences in NH+

4 contents between the two different formulations at either site(Figures 6C and D). As for NO–

3 , trace amounts were only sporadically found inthe soil, preventing statistical analysis.

The amounts of total C in the humus layer at the control plots were 13.7 and15.3 ton ha–1 for the mixed site and pine site, respectively. As for total N, an equalamount (420 kg N ha–1) was found at the control plots at both sites. No significanttreatment effects were found on either total C or total N contents in the humus layer.Consequently, the C to N ratio in the humus layer was also unaffected.


Figure 3. Extractable Mg content (kg ha–1) at different soil depths, comparing the effects of differentwood ash doses at (A) the mixed site and (B) the pine site, and effects of crushed and pelleted ash at(C) the mixed site and (D) the pine site. Mean values ± 1 s.e. of the mean. Symbols as in Figure 1.

4. Discussion

The ash-induced increases observed in pH and extractable amounts of Ca and Mg inthe humus layer following wood ash application are in accordance with the resultsof several other studies (Bramryd and Fransman, 1995; Kahl et al., 1996; Eriksson,1998; Saarsalmi et al., 2001; Ludwig et al., 2002; Geibe et al., 2003).

The mobility in the soil differs substantially between the measured cations. Thiswas clearly reflected in our results, especially at the pine site, where treatment-related Ca and Mg contents were mainly increased in the humus layer, while in-creases in K were detected further down the profile. Analogously, large increases inK concentrations were found in soil–water samples (taken by means of suction cupsat 50-cm depth) from all crushed ash-treated plots at the mixed site. The Ca concen-tration in the soil water increased as well, but the rise was statistically significantonly in 9T, and its duration was shorter (E. Ring, personal communication). The lowamounts of recovered K found at these sites are consistent with findings of earlierwood ash studies (Kahl et al., 1996; Eriksson, 1998). As for P, the low recovery atthe mixed site may be related to the formation of insoluble Ca phosphates due tothe Ca added with the ash (Condron et al., 1993). Increased amounts of extractable


Figure 4. Extractable K content (kg ha–1) at different soil depths, comparing the effects of differentwood ash doses at (A) the mixed site and (B) the pine site, and effects of crushed and pelleted ash at(C) the mixed site and (D) the pine site. Mean values ± 1 s.e. of the mean. Symbols as in Figure 1.

Ca have been found to have a strong negative influence on the mineralization oforganically bound P in soils (Harrison, 1982), and decreases in P availability afteradditions of lime or wood ash have frequently been reported (Condron et al., 1993;Clarholm, 1994; Long et al., 1997; Geibe et al., 2003).

Unexpected results from this study were the large differences in the recovery ofapplied nutrients between the two sites. We can only speculate about the reasonsfor this. The mixed site can be characterized as a mesic site located on a longslope, in contrast to the much drier, lichen- and dwarf-shrub dominated pine site.Since the two sites are close to each other, there are unlikely to be any majordifferences in precipitation between them. Possibly, the divergence in recoverylevels between the sites could be due to differences in the formation of secondarycompounds related to differences in humus quality. Differences in nutrient uptake byvegetation is an additional possible explanation for the divergent levels of recoverybetween the two sites, with higher growth and probable nutrient uptake at the mixedsite. Nevertheless, the observed differences in nutrient recovery have to be furtherinvestigated in studies including measurements of the total amount of base cationsand phosphorus, preferably further down in the mineral soil, and nutrient contentsof the vegetation (above and below ground).


TABLE III

Recovery ±1 s.e of extractable amounts (in percent) of added nutrientsat the two sites

Self-hardened crushed ash

3 ton 6 ton 9 tonPelleted ash3 ton

Mixed site

Ca 16 ± 12 24 ± 5 19 ± 2 10 ± 5

K 10 ± 2 8 ± 1 12 ± 2 10 ± 6

Mg 17 ± 12 16 ± 5 23 ± 2 16 ± 5

P 3 ± 4 15 ± 2 11 ± 1 1 ± 4

Pine site

Ca 45 ± 1 45 ± 2 54 ± 5 19 ± 1

K 10 ± 3 13 ± 1 9 ± 1 13 ± 1

Mg 43 ± 1 45 ± 2 52 ± 5 16 ± 5

P 40 ± 23 56 ± 28 52 ± 11 78 ± 22

Figure 5. Extractable P content (kg ha–1) at different soil depths, comparing the effects of differentwood ash doses at (A) the mixed site and (B) the pine site, and effects of crushed and pelletedash at (C) the mixed site and (D) the pine site. Mean values ± 1 s.e. of the mean. Symbols as inFigure 1.


Figure 6. NH+4 content (kg N ha–1) at different soil depths, comparing the effects of different wood

ash doses at (A) the mixed site and (B) the pine site, and effects of crushed and pelleted ash at (C) themixed site and (D) the pine site. Mean values ±1 s.e. of the mean. Symbols as in Figure 1.

In the present study no effects of wood ash were found on total N and C contentsin any of the soil horizons studied. As for NH+

4 , there was a significant ash doseeffect in the humus layer at the pine site (Figure 6B), which was not found at themixed site (Figure 6A). The addition of an alkaline compound often results in ahigher decomposition rate, which in turn may increase N mineralization (Perssonet al., 1990). Ash treatment (6 and 9 ton ha–1) resulted in increased amounts ofNH+

4 in the humus layer at the pine site. However, these increases are inconsistentwith measurements of tree stem growth at this site, which showed a slight negativecorrelation between ash application and growth, and there were no correspondingincreases in the N concentration of the needles (Jacobson, 2003).

When compared at the same dose, the effects on soil chemistry of the two dif-ferent ash formulations did not significantly differ. However, the ashes used did notoriginate from the same mill, and differed, at least to some extent, in chemical com-position. This difference confounds the interpretation of our results, and precludesany general conclusions regarding the effects of different ash formulations.

Recycling wood ash may possibly help counter the acidification of soil and soil–water. However, the practice has been mainly advocated as a method to replacenutrients removed by whole-tree harvesting (Olsson et al., 1996; Eriksson, 1998),


and thus sustain future forest productivity (Clarholm, 1994; Sverdrup and Rosen,1998). According to investigations of the forest yield and nutrient status of the trees,the two sites were clearly N-limited (Jacobson, 2003, for the pine site; unpublisheddata for the mixed site) and hence did not benefit from the nutrients containedin the wood ash. In this context, and from the forest stand growth perspective,recycling wood ashes must be regarded as a preventive measure against possiblenutrient imbalances in the future, presupposing that the ecosystem has the long-termcapacity to retain the nutrients added in available forms. However, considering themodest, short-term and varying levels of nutrient recovery found in the soil in thisstudy just 5 yr after the treatment, the value of such preventive action is debatable.

Acknowledgements

This work was financially supported by the Swedish Energy Agency, Assi DomanAB Forestry, Holmen Skog AB, Korsnas AB, SCA Skog AB and Stora Enso.We would also like to thank Sophia Herbertsson for help with soil sampling andpreparations.

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effects of wood ash dose and formulation on soil chemistry at two coniferous forest sites

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