Wood ash in boreal, low-productive pine stands on upland and peatland sites: Long-term effects on stand growth and soil properties

Forest Ecology and Management 327 (2014) 86–95

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Wood ash in boreal, low-productive pine stands on upland and peatland sites: Long-term effects on stand growth and soil properties Anna Saarsalmi a,⇑, Aino Smolander a, Mikko Moilanen b, Mikko Kukkola a a b

Finnish Forest Research Institute, P.O. Box 18, FI-01301 Vantaa, Finland Finnish Forest Research Institute, Oulu Research Unit, Paavo Havaksen tietie 3, FI-90014 Oulu, Finland

a r t i c l e

i n f o

Article history: Received 13 February 2014 Received in revised form 14 April 2014 Accepted 23 April 2014

Keywords: Soil acidity Pinus sylvestris L. Microbial processes N cycling C cycling Cellulose decomposition

a b s t r a c t The effect of wood ash on growth of Scots pine was studied in 64- to 75-year-old stands on three upland sites (Exps. 402, 407 and 408) for 20 years and in a 30-year-old Scots pine stand on an oligotrophic peatland site (Exp. 251) for 25 years. In Experiments 407 and 251 the responses of soil chemical properties and soil microbiological processes related to C and N cycling were also studied. The upland experiments included a control and a treatment with 3 Mg ha 1 of wood ash. In Exp. 407, 120 kg N ha 1 was applied together with ash; this experiment also included a treatment with N alone. The peatland experiment included a control and a treatment with 4.8 Mg ha 1 of wood ash. All experiments had 3 replications. Wood ash significantly decreased soil acidity on all sites. On the upland site, after 20 years, the concentration of K2SO4-extractable DOC and the rates of C mineralization (CO2–C production) and net N mineralization were all higher in the Ash + N treatment than in the control or N treatments. However, the treatments did not significantly affect the amounts of C or N in the microbial biomass or the concentration of NH4–N. On the peatland site, after 27 years, ash stimulated C mineralization and cellulose decomposition, but microbial biomass C or N, net N mineralization or the concentration of N were not affected significantly. On both the upland and peatland site, net nitrification was very low in all treatments. In Exp. 408, the volume growth in the control and Ash treatment was during the 20-year study period 60 and 64 m3 ha 1, respectively, and in Exp. 402 108 and 120 m3 ha 1, respectively, the latter difference being significant. In Exp. 407, the volume growth in the Ash + N treatment was during the 20-year study period significantly higher (92 m3 ha 1) than in the N and control treatments (76 and 73 m3 ha 1, respectively). On the peatland site during the 25-year study period the growth was 145 and 169 m3 ha 1, in the control and Ash treatments, respectively. In conclusion, the long-term positive response of stem growth to wood ash on peatlands and N fertilized upland sites can be partly explained by changes in soil nutrient status and by microbial processes related to C and N cycling. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction In the Nordic countries, forest biomass is increasingly being used as a source of energy in order to reach targets for reductions of CO2 emissions set by the European Union. In Finland in 2012, the annual domestic use of forest chips originating from harvesting residues and stumps was 8.3 million m3; and the proportion of all woodbased fuels in the total consumption of energy was 24% (Ylitalo, 2013). The National Climate and Energy Strategy indicates that annual production of forest chips in Finland is to be increased to 13.5 million m3 by the year 2020 (Ministry of Employment and the Economy, 2010). Consumption of primary biomass for energy

⇑ Corresponding author. Tel.: +358 10 211 5478. E-mail address: Anna.saarsalmi@metla.fi (A. Saarsalmi). http://dx.doi.org/10.1016/j.foreco.2014.04.031 0378-1127/Ó 2014 Elsevier B.V. All rights reserved.

production is generating increasing quantities of wood ash. In Finland, the total amount of wood ash produced annually by the forest industry and heating plants is estimated to be 200,000–300,000 Mg. Recycling wood ash back into the forest is one possible way to close the nutrient cycle and to counteract increased soil acidity (Karltun et al., 2008). Wood ash contains all the major mineral nutrients in plants except for N and when returned to the soil has a liming effect. A decrease in soil acidity and an increase in base saturation following the application of wood ash on both upland and peatland soils have been widely reported (Khanna et al., 1994; Kahl et al., 1996; Saarsalmi et al., 2001, 2012; Ludwig et al., 2002; Moilanen et al., 2002, 2013; Brunner et al., 2004; Huotari, 2011). The effect of wood ash on acidity of the organic layer can be of long duration (Saarsalmi et al., 2001, 2012; Moilanen et al., 2002).

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On upland sites the availability of N is generally a limiting factor in biomass production (Viro, 1967; Kukkola and Saramäki, 1983; Tamm, 1991). Although wood ash does not contain N, it is thought, that due to its soil-ameliorating effect, on upland soils wood ash may also have a long-term positive impact on volume growth. So far, studies carried out in middle-aged and older coniferous stands within the boreal zone have not shown evidence of a growth response caused by wood ash during a short study period (613 years) (Sikström, 1992; Jacobson, 2003; Saarsalmi et al., 2004, 2005; Moilanen et al., 2013). However, a positive growth response to wood ash has sometimes been reported in young stands on infertile upland sites (Tamminen, 1998; Perkiömäki et al., 2004; Saarsalmi and Levula, 2007; Mandre et al., 2006). When wood ash was given together with N, the positive growth response to Ash + N lasted longer than addition of N alone (Saarsalmi et al., 2012). However, this has not always been the case (Saarsalmi et al., 2010). Most studies of ash on peaty soils have been carried out on mesotrophic and N-rich site types where the mineral nutrient deficiencies and imbalances in nutrient status of trees are most severe. On such peaty soils, long-lasting – even four or five decades – positive effects of wood ash on growth and nutrient status of conifers have been reported in several studies (e.g., Silverberg and Huikari, 1985; Moilanen et al., 2002, 2005). In oligotrophic peatlands – where nitrogen availability to trees is low – the effects of wood ash on soil or on tree stand have remained moderate (Silverberg and Huikari, 1985; Moilanen et al., 2013). Wood ash has been shown to stimulate litter and cellulose decomposition and carbon mineralization in forest soils over the long term (Moilanen et al., 2002, 2012; Perkiömäki and Fritze, 2002, 2005; Perkiömäki et al., 2004; Rosenberg et al., 2010). The positive effect of wood ash has also been observed to counteract the negative effects of N fertilization on the amount of C and N in the microbial biomass and on C mineralization (Saarsalmi et al., 2010, 2012). Increase in microbial activities related to C cycling after application of wood ash has been explained by both direct and indirect effects of the increase in soil pH (Jokinen et al., 2006). Much less is known about the long-term effects of ash on N cycling. Some results have indicated increased net N mineralization (Högbom et al., 2001), but lowered mineral N concentrations have also been reported (Eriksson, 1996). Wood ash and N fertilization given together increased net N mineralization in two Scots pine stands over the long term (Saarsalmi et al., 2010, 2012). Saarsalmi et al. (2004) investigated the response of Scots pine growth and the chemical properties of soil to wood ash application on three N-poor upland sites 10 years after application; on one of the sites, wood ash was applied together with N fertilizer. They found no essential difference in growth response between the control and the wood-ash-alone treatments. If wood ash was added together with N, volume growth still tended to be higher than in the control during the second 5-year period when the response to the N-alone treatment had ceased. In the present study, we report results of the same three stands after 20 years. Studies comparing the stand responses between N-poor upland and peatland soils are almost lacking, except the study by Moilanen et al. (2013). Therefore, for comparison, the response of Scots pine growth to wood ash application on an oligotrophic peatland site after 25/ 27 years is also reported. Our aim was also to determine whether changes in soil chemical properties and microbiological processes related to C and N cycling could explain the growth response. Our hypothesis was that addition of wood ash on peatlands and addition of wood ash together with N on upland soil sites increase stem growth over the long term. We also hypothesized that changes in soil nutrient status and microbial processes related to C and N cycling explain the growth responses.

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2. Materials and methods 2.1. Sites and treatments 2.1.1. Upland sites Field experiments were established in three naturally regenerated, thinned 64- to 75-year-old Scots pine (Pinus sylvestris L.) stands on dry upland sites (Table 1 and Fig. 1). The organic layer was mor, with a thickness of 1–3 cm. The mineral soil texture was sorted fine sand. The sites are rather infertile, with C/N ratios of 44, 47 and 49 in Experiments 402, 407 and 408, respectively (Saarsalmi et al., 2004). According to the ammonium acetateextractable nutrient concentrations at the time of the experiments were established, the organic layer in Exp. 408 was clearly the most nutrient-poor. It was also the most acidic. At the time the experiments were established, the needle N concentrations were 12.5, 11.3 and 10.8 g kg 1 in Experiments 402, 407 and 408, respectively (Saarsalmi et al., 2004). The average stem volume was 91, 47 and 54 m3 ha 1 in Experiments 402, 407 and 408, respectively. For more details of stand characteristics, soil chemical properties and needle nutrient concentrations, see Saarsalmi et al. (2004). In each experiment, a randomized block design was applied and the treatments were replicated three times on 0.09 ha plots. The amount of 3 Mg ha 1 of loose wood ash (Ca 209–232, K 18– 40, Mg 14–36 and P 7–15 g kg 1 depending on the experiment, see Saarsalmi et al., 2004), originating mainly from chip-wood fuel used in a thermal power plant, was applied manually in May 1991 in Experiments 402 and 407, and in May 1992 in Exp. 408. In Exp. 407, 120 kg N ha 1 as ammonium nitrate with lime (N 27.5%, Ca 4%, Mg 1%) was applied together with wood ash. A treatment with 120 kg N ha 1 as ammonium nitrate with lime, but no ash was also included in Exp. 407. In this treatment 2 kg B ha 1 as fertilizer borate (Na2B2O7x5H2O) was also applied. This was due to the fact that needle B concentrations in this experiment were low (6 mg kg 1) at the time of the establishment (see Saarsalmi et al., 2004). Moreover, low boron concentration in needles has been caused experimentally by repeated nitrogen fertilization, especially in the north (Möller, 1983; Jalkanen, 1993). In the control treatment no ash or nutrients were added.

2.1.2. Peatland site The peatland experiment (Exp. 251) was situated on a barren mire drained for forestry in 1976 with ditch spacing of 25 m (supplemented in 1982) (Fig. 1). Before drainage the pristine-mire site type was dwarf-shrub pine bog (IR, classification see Vasander and Laine, 2008), and the peat thickness ranged from 25 to 55 cm. After drainage, the site had transformed to dwarf-shrub forest site type (Vatkg) and with regard to the nutrient demands of trees, such as nitrogen, was classified as an oligotrophic fertility type (Table 1). A field experiment was established in an unthinned 30-year-old stand consisting mainly of Scots pine and a few downy birches (Betula pubescens Ehrh.) as a mixture of tree species, with a dominant tree height of 7–10 m, density of 1100–1890 stems per hectare and average stem volume of 35 m3 ha 1. The fertilization experiment was established in 1982 with 6 sample plots, each 0.08–0.10 ha. The two treatments were: an unfertilized control and a wood Ash treatment. The experimental layout followed a randomized block design with three replications. The loose wood ash, 4.8 Mg ha 1 (Ca 262, K 75, Mg 37 and P 24 g kg 1), originating mainly from chip-wood fuel used in a thermal power plant, was applied manually in April 1982.

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Table 1 Site characteristics at the time the experiments were established. Exp.

Site typea

Site index (m)b

Organic layer (cm)c

Mineral soil texture

Temp. sum (°C day)d

Annual precipitation (mm)e

402 407 408 251

CT MCClT ECT Vatkg

18 14 15 18

2.7 0.9 1.0 25–55

Sorted fine sand Sorted fine sand Sorted fine sand Sand

1440 905 1119 1152

577 518 546 413

a Site types (Cajander, 1949; Vasander and Laine, 2008) CT = Calluna vulgaris, MCClT = Vaccinium Myrtillus – Calluna vulgaris – Cladina, ECT = Empetrum nigrum – Calluna vulgaris, Vatkg = dwarf-shrub forest type. b Dominant height at an age of 100 years. c Peat layer in Exp. 251. d Mean annual effective temperature sum (threshold +5 °C) for the study period 1982–2006 (Exp. 251), 1991–2010 (Exps. 402 and 407) and 1992–2011 (Exp. 408) (Venäläinen et al., 2011). e Mean annual precipitation during the study periods (Venäläinen et al., 2011).

Fig. 1. Location of the experiments.

2.2. Tree stand measurements On the upland sites, tree stands were measured at the time the experiments were established and after that in the autumn 5, 10, 15 and 20 years after treatments. A fertilized buffer zone of 5 m was left at the edge of each plot. The breast-height diameter (d1.3) of each tree was measured from two directions with an accuracy of 1 mm. On each plot at least 30 permanent sample trees representing different size categories were chosen for tree height measurements using a hypsometer with an accuracy of 1 dm. The size categories were determined by first dividing all the trees into 5 diameter classes, each with the same basal area. From each size category, six trees were then randomly selected as sample trees. The sample trees were used for estimating height and stem volume. Increment cores at breast height were extracted on five trees from each plot in autumn of 1995/1996 and from 15 trees in autumn of 2005/2006, i.e. 5 and 15 years after the treatments. To represent different size categories, the trees (different trees each time) were chosen randomly. Annual radial growth during the pre-fertilization and post-fertilization years was then determined with an accuracy of 0.01 mm. On the peatland site, tree stand was measured 6, 16 and 25 years after treatments. A buffer zone of 5 m was left at the edge

of each plot. Each sample plot was measured in one circular subsample plot with a radius of 7–8 m, depending on the shape or size of the plot. On each subsample plot, all trees (17–38) exceeding 5 cm in diameter at breast height were counted by species in 1 cm diameter classes. From 17 to 25 randomly chosen pine sample trees the height and d1.3 were measured with an accuracy of 1 dm and 1 cm, respectively. The height increments of these sample trees were measured on five-year periods before and after fertilization. Increment cores at breast height were extracted from these sample trees 16 years after treatments. Annual radial growth during the pre-fertilization and post-fertilization years was then determined with an accuracy of 0.01 mm. 2.3. Soil sampling and chemical analyses Soil samples were taken separately for chemical and microbiological analyses with a volumetric cylinder (d = 60 mm) from upland Exp. 407 in September 2010, i.e. 20 years after the treatments. For both chemical and microbiological analyses, a composite sample from the organic layer (forest floor) of each sample plot consisted of 25 systematically collected subsamples. Samples of the organic layer did not include living plant material. In addition to chemical analyses, a composite sample from the 0–10 cm

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mineral soil layer was collected with a volumetric cylinder (d = 39 mm, h = 100 mm) at five spots, i.e. the plot center and corners. For chemical analyses, soil samples were air-dried at 40 °C. Samples of the organic layer were ground in a mill with a 2 mm bottom sieve, and the mineral soil samples were passed through a 2 mm sieve to remove stones and larger roots. For all samples the total C and N concentrations were determined with a Leco CHN 1000 instrument. pH was determined in a water suspension. Exchangeable acidity (EA) was determined on a KCl extract (Halonen et al., 1983). All samples were extracted with acid ammonium acetate. For organic samples, dry combustion was used. Ashed samples (Lego TGA 601 oven) were extracted with HCl. Elemental concentrations (Ca, K, Mg, P) in ammonium acetate and dry combustion analyses were determined with the ICP device Thermo Jarrell Ash Iris Advantage. For more details concerning methods used in soil chemical analyses, see Saarsalmi et al. (2010). Peat samples from Exp. 251 were collected in October 2008, i.e. 27 years after the treatments. One composite peat sample consisted of 15 subsamples uniformly distributed over the plot, each at a minimum distance of 3 m from the plot edge or from side of the ditch. The living vegetation and undecomposed plant material from the peat cores were discarded. Each subsample was cut vertically into two pieces, one for chemical analysis and one for microbiological analysis. Peat layers 0–5 cm, 5–10 cm and 10–20 cm were separated into plastic bags. For chemical analyses, the samples were ground in a mill with a 2 mm bottom sieve. The samples were dried for 40 h at 65 °C. After dry combustion and dissolution of the ash in HCl, the samples were analyzed for total N concentration (the Kjeldahl method), for Ca, K and Mg concentrations (atomic absorption spectrophotometer, AAS), and for P concentration (the vanado-molybdate method) (Halonen et al., 1983). pH was determined as explained above for upland soils. The cation exchange capacity (CEC) was calculated as the sum of equivalent concentrations of extractable Ca, Mg, K and Na and EA. Base saturation (BS) was calculated as the proportion of base cations (Ca, Mg, K, Na) out of CEC. 2.4. Determination of soil microbial biomass and processes related to C and N cycling To study soil microbial biomass and activities, fresh composite soil samples were taken to the laboratory, sieved through a 4 mm sieve and stored in plastic bags at 1–2 °C until analyzed. Soil water-holding capacity (WHC), dry matter and organic matter contents were measured as described by Priha and Smolander (1999). Concentration of organic matter, as percentage of dry matter was 61–68 and 83–92 in experiments 407 and 252, respectively. All the analyses described below were made on 3 laboratory replicates of each combined soil sample and, to describe the quality of organic matter, the results were calculated on organic matter basis. Microbial biomass and activities were measured as described previously (Smolander et al., 2010). Briefly, amounts of C and N in the microbial biomass were measured with the fumigationextraction method using K2SO4 as the extractant of the chloroform-fumigated and unfumigated samples. To describe the dissolved organic C (DOC) concentration in the soils, K2SO4extractable organic C in unfumigated control samples was also of interest. Net N and C mineralization and net nitrification were measured in a 6-week incubation experiment at constant moisture (WHC 60%) and temperature (14 °C). Aerobic mineralization of C was estimated by measuring CO2–C production with a gas chromatograph. Net N mineralization and net nitrification were determined, after KCl extraction, as accumulation of (NH4 + NO3)–N or NO3–N during incubation.

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2.5. Cellulose decomposition test On the peatland site, cellulose decomposition (see Unger, 1960; Moilanen et al., 2002, 2012) was studied in 2007–2008, i.e. 26 years after application of ash. Dried and stabilised cellulose sheets (40 mm  40 mm  1 mm) in nylon net bags (mesh size 1 mm) were placed on each plot at depths of 0–5, 5–10, 10–15 and 15–20 cm (four sheets in five rows across the plot, to give 20 sheets on each plot and at each depth). The sets of bags were kept in the soil from 8th June 2007 to 4th June 2008 (361 days) and then removed. The cellulose sheets were cleaned, dried (for 3–4 days at +50 °C) and then weighed. Results were calculated as the percentage of weight loss from the initial weight. 2.6. Calculation of the results Statistical significance of the differences in soil nutrient and microbiological parameters were tested using either t-test or analysis of variance (ANOVA) followed by Bonferroni’s test (chemical analyses) or LSD test (microbiological analyses). The stand characteristics were calculated using the KPL calculation programme (Heinonen, 1994). A function based on d1.3 and height was used for calculating the volume of the sample trees (Laasasenaho, 1982). Plotwise volume equations based on diameter at breast height were then calculated using data from the sample trees to estimate the volume of the other trees in the stand. Stem volume increment (Iv) for each study period was calculated as the difference between consecutive measurements (the same trees at the beginning and end of each period; trees that had died during each period were also included). The annual basal area increment (BAI) was estimated on the basis of data for tree diameter of all the trees, from different measurements and increment cores taken from sample trees 5 and 15 years (for upland sites) or 16 years (for peatland sites) after the treatments. Statistical significance of the differences in volume growth between the treatments was tested using either t-test or ANOVA. After ANOVA, Bonferroni’s test was used to test the equality of the treatment means. To remove the effects of possible differences in stand pre-treatment growth, basal area increment (BAI) of the remaining trees during either the 5-year or 10-year period before the treatments were tested as covariates in calculating Iv during different 5-year periods for the upland sites. Neither the BAI of the remaining trees during the 5-year period nor that of the 10year period before the treatment was significant. For the peatland site, in calculating Iv during the first two study periods (1982–1987 and 1988–1997) the pre-treatment Iv of the trees during the 4-year period before the treatments was used as covariate. In all statistical tests the level of significance was set at p < 0.05. All analyses were computed using the PASW Statistics 17.0 package. 3. Results 3.1. Tree growth In the southernmost upland Exp. 402, no essential growth response to wood ash was detected during the first years of the study (Figs. 2 and 3). However, wood ash significantly increased stem volume growth during the third (25%) and fourth (11%) 5year periods (Fig. 3). In Exp. 408, no essential growth response to wood ash was detected. During the 20-year study period, the cumulative Iv in the control and Ash treatment was 108 and 120 m3 ha 1, respectively, in Exp. 402 and 60 and 64 m3 ha 1, respectively, in Exp. 408. In Exp. 402, the difference was significant (p = 0.049).

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Fig. 2. Mean annual basal area increment (BAI) before and 15 years after treatments on upland sites. In Exp. 407, both N and Ash + N treatments increased growth significantly in 1992–1995, and Ash + N treatment also in 1996.

In Exp. 407, N and Ash + N gave significant (60–71%) increases in volume growth during the first 5-year period (Fig. 3). The response to N alone lasted about 7 years after application (Fig. 2). During the following three 5-year periods, no positive growth response to the N-alone treatment was detected (Fig. 3). In fact, during the two last 5-year periods, the volume growth in the N treatment was slightly below that of the control. In the Ash + N treatment, however, during the last three 5-year periods the Iv also tended to be higher than in the control. During the 20-year study period, the cumulative Iv was 73, 76 and 92 m3 ha 1 in the control, N, and Ash + N treatments, respectively. When the control and Ash + N treatments were compared with each other using a t-test, the difference in the cumulative Iv between the two treatments was significant (p = 0.037). On the peatland site, during the first 16 years after ash application, the growth response to wood ash seemed to have strengthened gradually (Figs. 4 and 5). During the first 6-year period after ash application, no essential difference in volume growth between the treatments was found. During the next 10-year period (7– 16 years after ash application), the Iv on the ash-treated plots was 25% higher than on the control plots, the difference between treatments being significant (p = 0.021). The increasing trend in the periodical volume growth was no longer seen after 16 years. During the last 9-year period, the Iv on the ash-treated plots was 22% higher than on the control plots. During the whole 25-year study period, the cumulative Iv in the control and the Ash treatments was 145 and 169 m3 ha 1, respectively. The difference between treatments was not significant (p = 0.127).

Fig. 3. Mean annual volume growth of tree stands on the upland sites during the first-, second-, third- and fourth 5-year period after treatments. Mean of three replicate plots. Mean values with different letters differ significantly from each other (p < 0.05). Standard error of the mean is marked on the columns by bars. Results for the first and second 5-year periods have been published earlier (Saarsalmi et al., 2004).

Fig. 4. Mean annual BAI (covariate adjusted values) during 16 years after wood ash application on the peatland site (Exp. 251). Ash treatment increased growth significantly in 1990–1997.

3.2. Chemical properties of soil In the upland Exp. 407, wood ash given together with N significantly increased the pH both in the organic layer and in the 0– 10 cm mineral soil layer (Table 2). In the organic layer the pH on the Ash + N plots was 0.6 and in the mineral soil layer 0.5 pH units higher than on the control. Nitrogen alone had no effect on soil pH. In both soil layers, the Ash + N treatment significantly increased concentrations of ammonium acetate-extractable Ca and Mg as well as BS. In the organic layer, total concentrations of Ca, K, Mg and P were also higher in the Ash + N treatment than in the control treatment. The response of the soil nutrients was due to wood ash.

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3.3. Microbial biomass and processes in soil

Fig. 5. Mean annual volume growth during three consecutive (I = 6 years, II = 10 years, III = 9 years) study periods on the peatland site (Exp. 251). Mean of three replicate plots. Mean values with different letters differ significantly from each other (p < 0.05). Standard error of the mean is marked on the columns by bars.

N alone had no detectable effect on nutrient concentrations in soil. Total N concentration in soil and C/N ratio were unaffected by both treatments. Thickness of the organic layer was 1.6, 1.5 and 1.8 cm in the control, N and Ash + N treatments, respectively, with no significant differences between treatments. On the peatland site, the effect of wood ash on chemical properties of soil was still seen 27 years after application (Table 3). In the surface peat (0–10 cm) the total Ca concentration on the ash plots was, on average, fourfold compared to the control plots; in the deeper peat the effect was not verified. Ash treatment significantly increased K concentration in the uppermost peat layer as well as Mg concentration in the 5–10 cm layer. On the ash plots K and Mg concentrations were also higher in the other peat layers, even though the differences were not significant. Ash significantly increased the P concentration of the uppermost peat layer (0– 5 cm). The decrease in soil acidity was most apparent in the 0– 5 cm and 5–10 cm peat layers, where pH values on the ash plots were 1.0 and 0.7 pH units higher, respectively, than on the control plots. There was also a tendency toward a decrease in the C/N ratio on the ash-treated plots in the 0–5 cm and 10–20 cm peat layers. Table 2 Exp. 407. Soil properties (on dry matter basis) in autumn 2010, i.e. 20 years after treatments. Mean values with different letters differ significantly from each other. Standard error of the mean is shown in parentheses. Variablea

Control

N

Ash + N

p valueb

Organic layer Ntot, g kg 1c C/N Catot, mg kg 1 Caext, mg kg 1 Ktot, mg kg 1 Kext, mg kg 1 Mgtot, mg kg 1 Mgext, mg kg 1 Ptot, mg kg 1 Pext, mg kg 1 pHH2O CEC, mmol kg 1 BS,%

13.5 (0.4) 46 (1) 1323a (102) 860a (54) 508a (10) 490 (8) 217a (13) 126 (7) 573a (9) 127 (6) 3.7a (0.04) 138a (4) 48a (1)

13.5 (0.6) 44 (1) 1300a (78) 821a (63) 408a (33) 381 (55) 233a (21) 126 (13) 493a (26) 99 (18) 3.6a (0.03) 135a (6) 46a (3)

13.8 (0.3) 45 (1) 4610b (718) 3040b (419) 745b (81) 582 (56) 507b (63) 242 (46) 708b (30) 153 (21) 4.3b (0.12) 215b (24) 87b (3)

.819 .534 .009 .001 .009 .055 .003 .040 .004 .140 .001 .012 .000

0.52 (0.01) 22 (1) 13.8a (2.0) 21 (1) 4.6a (0.8) 3.8 (0.2) 4.7a (0.0) 12 (0) 16a (2)

0.54 (0.04) 24 (2) 119b (16) 21 (1) 9.5b (0.6) 5.2 (0.6) 5.1b (0.1) 13 (1) 56b (3)

.245 .455 .000 .884 .002 .154 .005 .461 .000

Mineral soil (0–10 cm) Ntot, g kg 1 0.48 (0.01) C/N 22 (0) Caext, mg kg 1 11.6a (1.9) Kext, mg kg 1 20 (1) Mgext, mg kg 1 3.8a (0.5) 1 Pext, mg kg 4.1 (0.5) pHH2O 4.6a (0.2) CEC, mmol kg 1 12 (1) BS,% 14a (1) a b c

tot = dry combustion concentration, ext = acid ammonium acetate extraction. p Value from ANOVA. On the basis of organic matter.

In the upland Exp. 407, the concentration of K2SO4-extractable DOC and the rates of C mineralization and net N mineralization were all significantly higher in the Ash + N treatment than in the control or N treatments (Fig. 6). The same was true for the ratios describing the activity of the microbial biomass: Net N mineralization/microbial biomass N and C mineralization/microbial biomass C were both highest in the Ash + N treatment (results not shown). Control and N treatments did not differ from each other. The treatments did not significantly affect the amounts of C or N in the microbial biomass or the concentration of NH4–N, although with regard to the NH4–N concentration, the Ash + N treatment had the highest mean value. In all soils the rates of net nitrification and concentrations of NO3–N were negligible. In the peatland Exp. 251, in the uppermost 0–5 cm layer the concentration of K2SO4-DOC and C mineralization were both significantly higher in the Ash treatment than in the control treatment (Fig. 7) as was the ratio C mineralization/microbial biomass C (results not shown). The amount of N in the microbial biomass also tended to be higher in the Ash treatment (p = 0.095). For the other variables, in this layer there were no significant differences between treatments. In the lower 5–10 cm layer, there were no significant differences between treatments for any of the variables measured. Both nitrification and NO3–N concentrations were always very low, although on a few plots there was some activity. When the datasets for both upland and peatland sites were analyzed together, the correlation between the concentration of DOC and the rate of C mineralization was high and positive (r = 0.909, p = 0.000). DOC also correlated positively with microbial biomass N (p = 0.742, p = 0.000) and NH4–N concentration (r = 0.615, p = 0.003). 3.4. Decomposition of cellulose On the peatland site the rate of decomposition of the cellulose sheets gradually diminished with increasing peat depth (Table 4). In all peat layers studied, wood ash significantly accelerated the decomposition of cellulose. Depending on peat depth, the rate of decomposition on ash plots was 1.2–1.4 times that on the control plots. 4. Discussion In upland Exp. 407, N, both alone and together with wood ash, gave a significant increase in growth during the first 5-year period. In Nordic coniferous upland forests only N, as a single nutrient, considerably increases stand growth. In Finnish coniferous stands, application of N (150 kg ha 1) usually gives a volume increment of 12–20 m3 ha 1 (Saarsalmi and Mälkönen, 2001). Trees, other plants and microbes use fertilizer N rapidly, because N is a growth-limiting nutrient that circulates tightly within the boreal forest ecosystem (Nömmik and Larsson, 1989; Preston and Mead, 1994; Weetman et al., 1997; Tamm et al., 1999). The duration of the increase in stand growth after fast-release N fertilization is 6–8 years for pine (Saarsalmi and Mälkönen, 2001). In our study the response to N lasted about 7 years. Similarly to the results reported by Helmisaari et al. (2011), we also detected a slight negative growth response after the response to the N-alone treatment ended. This controversial phenomenon may be partly related to fertilization-induced changes in biomass allocation. After fertilization, trees have an abundance of available N and use it mainly to produce aboveground biomass rather than roots. Afterwards, when the easily available fertilizer N has been used, trees will balance the decreased availability by allocating more growth to roots, leading

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Table 3 Peatland site. Soil total nutrient concentrations (on the basis of dry matter), C/N ratio and pH in autumn 2008, i.e. 27 years after treatments. Standard error of mean is given in parentheses. p values from t-test. Variable

N, g kg 1a C/N Ca, mg kg 1 K, mg kg 1 Mg, mg kg 1 P, mg kg 1 pH a

0–5 cm

5–10 cm

10–20 cm

Control

Ash

p Value

Control

Ash

p Value

Control

Ash

p Value

11.7 (0.7) 50 (3) 2022 (170) 497 (91) 457 (30) 629 (21) 3.7 (0.1)

14.3 (0.9) 41 (2) 9407 (2551) 766 (31) 706 (104) 1296 (195) 4.7 (0.6)

.087 .075 .045 .049 .083 .027 .202

15.1 (2.0) 40 (5) 1661 (231) 350 (37) 472 (59) 817 (67) 3.4a (0.02)

15.1 (1.2) 39 (3) 6554 (867) 392 (30) 705 (65) 867 (69) 4.1b (0.1)

.983 .902 .005 .458 .034 .631 .004

20.2 (0.7) 29 (1) 974 (47) 152 (24) 208 (40) 849 (56) 3.6 (0.1)

16.6 (0.6) 35 (1) 1590 (762) 249 (14) 484 (232) 744 (113) 3.9 (0.1)

.062 .080 .504 .072 .362 .494 .168

On the basis of organic matter.

Fig. 6. Microbial biomass and activities, and the concentrations of DOC and NH4–N in the humus layer of upland Site 407 20 years after the treatments. Mean of three replicate plots. Mean values with different letters differ significantly from each other (p < 0.05). Standard error of the mean is marked on the columns by bars.

to a temporary decrease in growth of the aboveground parts of the trees (Fagerström and Lohm, 1977). A slight positive growth response to the Ash + N treatment was still apparent after the response to the N-alone treatment ended. Similarly, in a 60-year-old Scots pine stand Saarsalmi et al. (2012) detected a positive growth response to simultaneous addition of wood ash (2.5 Mg ha 1) and N (185 kg ha 1) after the response to N alone ended; this positive response continued for about 24 years. However, in younger Scots pine (31 year) and Norway spruce (45 year) stands, the mean annual growth in basal area after addition of wood ash (3 Mg ha 1) and N (150 kg ha 1) did not essentially differ from that with N alone during the 15 years after treatments (Saarsalmi et al., 2010). In upland Exp. 402, when it was added without N, wood ash also increased stem growth. This result differs from results reported in previous shorter studies on upland sites (Jacobson, 2003; Saarsalmi et al., 2004, 2005; Moilanen et al., 2013). In our study, the response was not seen until after about 8 years. In the other upland experiment in our study, wood ash alone had no noticeable effect on stem growth. On the peatland site, the stand growth response to the Ash treatment was slower and noticeably smaller than the results

obtained in previous studies on peatland sites (Silverberg and Huikari, 1985; Moilanen et al., 2002, 2005, 2013). Initially, the effect of ash increased in strength for about 15 years, after which the difference in growth to the unfertilized control remained constant for the next 10 years. The best increase in growth was 1.5 times higher than that of the control, whereas previous ash studies have shown 2- to 5-fold, or even higher, increases in growth. The reason for this modest response to wood ash is probably the scarcity of N on the site and partly the thinness of the peat layer. In previous studies those sites showing stronger responses have most commonly been N-rich, thick-peated drained peatlands, where the lack of mineral nutrients, especially P and K, has been a major growth-inhibiting factor (Moilanen et al., 2002, 2013). The site in the present study was originally a dwarf-shrub pine bog (IR) that had developed into a dwarf-shrub type drained peatland forest (Vatkg). According to the guidelines for an interpretation of needle analysis (Moilanen et al., 2010), the nutritional status of trees during the study period was appropriate with no severe deficiencies of any particular nutrient (results not shown here). On originally forested site types like this, deficiencies of P or K are noticeably less common than on N-rich drainage sites that were originally sparsely forested (Moilanen et al., 2010).

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Fig. 7. Microbial biomass and activities, and the concentrations of DOC and NH4–N on the peatland site (Exp. 251) at two depths 27 years after the treatments. Mean of three replicate plots. Mean values with different letters differ significantly from each other (p < 0.05). Standard error of the mean is marked on the columns by bars.

Table 4 Weight loss (%) of cellulose sheets in different peat layers in June 2007–June 2008 in control and Ash treatments (26 years after ash application) (Exp. 251) standard error of the mean is shown in parentheses.

b

Peat layer

Control

Ash

p Valueb

0–5 cm 5–10 cm 10–15 cm 15–20 cm

69 52 42 35

83 66 54 49

.000 .000 .005 .002

(3) (3) (3) (3)

(2) (3) (3) (4)

p Value from t-test.

The effect of the Ash treatment was visible in the nutrient status of the surface peat still 27 years after application. Acidity had decreased by almost one pH unit. The P from the ash seemed to have been bound in the upper layer of the surface peat, while Mg and K apparently leached deeper. These results agree with earlier findings concerning the leachability of the various elements in ash and the long-term effects of ash on peat (Moilanen et al., 2002, 2005; Piirainen et al., 2013). Both ash and ash together with N appeared to have very longlasting positive effects on the biological activity of soil. On the upland site, the Ash + N treatment increased rates of both C mineralization and net N mineralization, as well as the ratios describing the activity of the current microbial biomass with regard to both C and N cycling. This indicates that both C and N availability were highest in the Ash + N treatment compared to both the control and N alone. On the peatland site, ash affected on C mineralization and the ratio C mineralization/microbial biomass C in the uppermost 0–5 cm layer, but variables important in N cycling were not affected. The long-term positive effect of ash on C mineralization has been also observed on other sites (Perkiömäki and Fritze, 2005; Rosenberg et al., 2010; Saarsalmi et al., 2010, 2012).

Consistent with the results of C mineralization studies in the laboratory, on the peatland site the Ash treatment still accelerated cellulose decomposition 26 years after application. Decomposition increased at about the same rate as growth, and was 1.2–1.4 times that of the control. The effect was noticeably smaller than in previous studies on nitrogen-rich sites (Moilanen et al., 2002, 2012). Since the treatments were different, we can only speculate about the reasons for the different responses of N mineralization between the sites. The peatland site did not have the N treatment since, in peatlands, N is usually not the most growth-limiting nutrient. Upland Exp. 407 had no ash-alone treatment since N is usually needed to obtain a growth increase with ash. The results of Rosenberg et al. (2010) suggest that in N-rich soil wood ash may increase net N mineralization, but in N-poor soil the effect may even be opposite. It is possible that on the upland site the extra N with the fertilizer was needed to make possible the positive effect of ash. The fact that ash alone increased stem volume in Exp. 402 but not in Exp. 408 needs further study. Unfortunately, soil microbiological processes were not determined from the two N-poor upland experiments in this study. Ash increased the concentration of K2SO4-extractable DOC on both sites; furthermore, there was a highly positive correlation between the concentration of DOC and several microbial variables. Dissolved organic matter in soil consists of primary products leached from plants and litter and of secondary products that result from decomposition of litter. We do not know whether the longterm increase in pH due to ash addition was the primary factor that increased microbial activity and DOC concentrations, or whether addition of ash resulted in long-term improvement of the quality of litter. In any case, the highly positive correlation between concentration of DOC and C mineralization seems to be common in these northern soils (Smolander and Kitunen, 2011).

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With regard to N losses, wood ash did not seem to cause problems on these sites, at least not over the longer term since on neither of the sites were there signs of intensive net nitrification. Although ash fertilization may optimize pH for nitrifying bacteria, it does not necessarily lead to increased net nitrification over the long term, thus not increasing the risk of nitrate leaching. This was the case even when N was applied together with ash on the upland site. Other studies also suggest that wood ash alone or together with N treatment does not necessarily stimulate nitrification over the long term (Rosenberg et al., 2010; Saarsalmi et al., 2010, 2012). Accordingly, addition of wood ash did not increase the emissions of N2O (Maljanen et al., 2006). However, it should be borne in mind that the response to addition of ash is dependent on the time elapsed after the addition and on site properties. In a study made two years after application of wood ash seemed to stimulate net nitrification (Martikainen, 1984); and there are sites where indications of increased nitrification have been observed after additions of wood ash also over the longer term (Högbom et al., 2001). Increased tree growth due to the fertilization treatments coincided with better soil nutrient status and stimulated microbial activity. However, determination of the relationship between tree growth and e.g. microbial processes in N cycling requires longterm monitoring of both phenomena in the same forest stands. In this study, microbiological processes were measured in the autumn when tree growth had ceased and on only one occasion; and to describe the quality of organic matter, the results were expressed on the basis of organic matter. 5. Conclusions On low-productive upland sites, no growth response to wood ash has usually been found during the short study period. However, over the long term, wood ash added alone can increase stem growth also on N-poor upland sites. On these sites, wood ash may increase the duration of the growth response to N fertilizer, and also compensate for the growth loss, which is usually detected after the response to N alone ends. On a low-productive oligotrophic mire drained for forestry, wood ash increases stem volume more slowly and noticeably less than on mesotrophic and N-rich site types. The response, however, seems to be long lasting; and over the long term considerable growth increments can also be obtained on low-productive peatland sites. Long-term positive response of stem growth to addition of wood ash on peatlands and Ash + N on upland sites can be partly explained by changes in soil nutrient status and microbial processes related to C and N cycling. With regard to N losses, wood ash does not seem to cause problems on these sites, at least over the longer term since neither on the upland site nor peatland site were there signs of high levels of net nitrification. Acknowledgements We are grateful to Pekka Välikangas, Raino Lievonen; Hilkka Ollikainen and Timo Siitonen for measuring the tree stand and for soil sampling, Anneli Rautiainen for laboratory work, Anne Siika for making the figures and Dr. Joann von Weissenberg for checking the English language of this paper. References Brunner, I., Zimmermann, S., Zingg, A., Blaser, P., 2004. Wood-ash recycling affects forest soil and tree fine-root chemistry and reverses soil acidification. Plant Soil 267, 61–71.

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