sustainability Article Effect of High Concentrations of Wood Ash on Soil Properties and Development of Young Norway Spruce (Picea abies (L.) Karst) and Scots Pine (Pinus sylvestris L.) 1,2 1 1 1 Baiba Jansone , Valters Samariks , Modris Okmanis ,Darta¯ Kl, avin, a 1, and Dagnija Lazdin, a * 1 Latvian State Forest Research Institute Silava, Rigas 111, Salaspils LV-2169, Latvia; [email protected] (B.J.); [email protected] (V.S.); [email protected] (M.O.); [email protected] (D.K.) 2 Forest Faculty, Latvia University of Life Sciences and Technologies, Akademijas 11, Jelgava LV-3001, Latvia * Correspondence: [email protected] Received: 29 September 2020; Accepted: 7 November 2020; Published: 14 November 2020 Abstract: Wood ash recycling can be a reasonable method for energy producers to decrease waste problems. Using wood ash as a fertilizer or liming material could improve soil macro and micronutrient content in peat soils. Therefore, the effect of wood ash on Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris L.) juvenile growth and nutrient content in the soil after spreading wood ash in medium to high doses before and after planting seedlings was investigated in peat forests in the Eastern part of Latvia. The aim of the study was to evaluate the effect of high doses of wood ash on soil properties and the growth of planted Norway spruce and Scots pine seedlings up to 10 years after experiment establishment. Wood ash was applied a year before planting seedlings in doses 1 1 of 5 and 10 t ha− and right after planting in concentrations of 5, 10, 15, and 20 t ha− . Changes in macronutrient content (i.e., phosphorus [P], and potassium [K]) and tree height and diameter at breast height were measured at one and 10 years after establishing the experiment. Fertilization one year prior to planting the seedlings exhibited a positive response on tree height and diameter growth compared to fertilization after the seedlings were planted. Soil samples from fertilized plots one year after establishment contained more P and K in the soil than the control plots. Wood ash application of the highest doses did not reach the overdose limit, as the tree growth (height and diameter at breast height) results of fertilized plots were similar to those of the control fields; therefore, no significant negative effect on tree growth was discovered. Keywords: fertilization; forest regeneration; liming; seedling growth; wood ash 1. Introduction Wood ash application intensifies the increase of available nutrients in forest soils and improves forest production, however the effects of wood ash on tree growth differ between peat and mineral soils [1–5]. Wood ash fertilization has a long-lasting pH neutralizing effect on the soil organic layer and wood ash application counteracts soil acidity after forest harvesting of whole tree biomass (including logging residues and, in some cases, even roots) for energy production [4–6]. Therefore, wood ash could be used as an alternative mineral fertilizer and liming material because it primarily consists of metal oxides, hydroxides, carbonates, salts, and soil minerals [6,7]. Recycling ash may contribute to sustainable forest management, and it can be an environmentally acceptable solution to the waste problems of energy producers [8]. In Latvia, wood chip ash is deposited on waste disposal sites; thus, only a minor portion is used for fertilization and liming in farms. However, according to Sustainability 2020, 12, 9479; doi:10.3390/su12229479 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 9479 2 of 13 the State Plant Protection Service registry data, interest in wood ash as a fertilizer has grown in the last few years, and currently, 24 companies have registered wood ash as a liming material. Studies on the effect of wood ash application and soil chemistry of forest ecosystems have been carried out in Fennoscandia [8,9], and several small-scale experimental trials of wood ash usage for forest soil fertilization and liming were established in Latvia [10–13]. These studies have found that the utilization of ash by moving it back to forest mineral elements after the burning of organic matter for energy production could be defined as the “recycling of nutrients”, “fertilizing”, and “liming”. These methods are effective for stabilizing soil pH, enriching the soil with phosphorus (P), potassium (K), and micronutrient elements (zinc, boron, copper, etc.), especially in peat soils (due to the fact that wood ash does not contain nitrogen, however peat soils are nitrogen rich in comparison to mineral soils where nitrogen is the growth limiting element), and decreasing the need to deposit wood ash in waste disposal sites or landfills [14–24]. In general, no short-term growth effect of wood ash application has been found on mineral soils; however, recycling wood ash and using it as a fertilizer with other macronutrients or fertilizers may affect stand growth in different ways over a longer period [5]. On nitrogen (N) rich organic soils, a substantial growth increase is usually seen [25,26]. However, wood ash application can deplete organic carbon (C) in locations with N-rich soil [27]; therefore, experiments and investigations are needed to determine the potential and ecological consequences of wood ash recycling. The application of wood ash leads to changes in mycorrhizal fungi communities, increased species richness, and higher Shannon diversity index in fertilized plots compared to control plots [28]. The share of wood as a renewable fuel for energy and the increase in pellet production are factors that re-actualize investigations about the effect of wood ash as a source of nutrient elements for forest trees in the long term, therefore such investigations were done on experimental fields established several years ago. If forests within the Nordic and Baltic regions functioned as a “deposit” for the wood ash coming from the total harvest of wood for energy within the region, then the forests would receive 1 an average of 0.2 to 0.5 t ha− of dry wood ash within one rotation period [8]. Recommended wood 1 1 ash doses in the rotation period are 1–2 t ha− and 0.7–1.5 t ha− for Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris L.), respectively [29]. In the trial, high doses of wood ash are used to determine both positive and negative effects on plant development at an early stage and to assess the effects on the nutrient elements in the soil. Therefore, the aim of this study is to characterize the influence of high doses of wood ash on soil properties and the growth of Norway spruce and Scots pine seedlings up to 10 years after planting. 2. Materials and Methods 2.1. Study Site The research site is located in the Kalsnava district in the central part of Eastern Latvia (56◦42’ N. latitude, 25◦50’ E. longitude), approximately 150 km from the Baltic Sea. The research area is located on the border between continental and marine climate conditions. The average air temperature is +5 ◦C to +6 C (January 5 C to 6 C, July +16 C to +18 C). The average precipitation in previous years before ◦ − ◦ ◦ ◦ ◦ planting was 797 mm [10]. The experiment was established on restoration site on homogenous drained fertile (grass) peat soil in the forest type Myrtillosa mel. [30]. Experimental plots were divided into 4 random blocks and total study site area was 0.2 ha, thus a single experimental plot size on average was 2.7 m2. The study site design is presented in Klavina et al. [31]. Soil was prepared manually in each plot, the understory plants were removed, and the top soil layer was overturned. The study site consists of 46 experimental plots (with 50 cm buffer zones between the plots) established with two tree species: Norway spruce and Scots pine. Altogether 18 spruce plots and 28 pine sample plots were established with two years old spruce containerized seedlings and one year old pine containerized seedlings obtained from JSC “Latvia’s State Forests” tree nursery. The seedlings met basic operational standards in terms of seedling morphological parameters. In each experimental Sustainability 2020, 12, 9479 3 of 13 Sustainability 2020, 12, x FOR PEER REVIEW 3 of 14 plot, approximately 70 containerized seedlings were planted in rows with 0.5 0.5 m spacing (Figure1). spacing (Figure 1). The experimental plots of the Norway spruce consisted× of randomly assigned The experimental plots of the Norway spruce consisted of randomly assigned plots with six replications plots with six replications for control and twelve replications with fertilization. Wood ash in doses of for control and twelve replications with fertilization. Wood ash in doses of 0 (control group) and 0 (control group) and 5 and 10 t ha−1 (medium dose before) was applied in May 2003—one year before 5 and 10 t ha 1 (medium dose before) was applied in May 2003—one year before the spruce stand the spruce stand− establishment. For Scots pine, the experimental plots consisted of randomly assigned establishment. For Scots pine, the experimental plots consisted of randomly assigned plots with 6 plots with 6 to 8 replications for each type, where wood ash was applied a year prior to planting (in to 8 replications for each type, where wood ash was applied a year prior to planting (in May 2003) May 2003) in doses of 0 (control group) and 5 and 10 t ha−1 (medium dose before). In May 2004, in 1 in doses of 0 (control group) and 5 and 10 t ha− (medium dose before). In May 2004, in each plot each plot seedlings were planted and the wood ash with doses of 0 (control group), 5 and 10 t ha−1 1 seedlings(medium were dose planted after), or and 15 the and wood 20 t ashha− with1 (high doses dose of after) 0 (control was group),applied 5 afterwards.