Wood ash in boreal, low-productive pine stands on upland and peatland sites: Long-term effects on stand growth and soil properties
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 wood-based 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 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, Saarsalmi et al., 2012, Ludwig et al., 2002, Moilanen et al., 2002, Moilanen et al., 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, Saarsalmi et al., 2012, Moilanen et al., 2002).
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 (⩽13 years) (Sikström, 1992, Jacobson, 2003, Saarsalmi et al., 2004, Saarsalmi et al., 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, Moilanen et al., 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, Moilanen et al., 2012, Perkiömäki and Fritze, 2002, Perkiömäki and Fritze, 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, Saarsalmi et al., 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, Saarsalmi et al., 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.
Section snippets
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 acetate-extractable nutrient concentrations at the time of
Tree growth
In the southernmost upland Exp. 402, no essential growth response to wood ash was detected during the first years of the study (Fig. 2, Fig. 3). However, wood ash significantly increased stem volume growth during the third (25%) and fourth (11%) 5-year 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,
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
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
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.
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