Cadmium accumulation in organisms from a spruce plantation amended with wood ash - an environmental risk?

Large-scale liming and wood ash addition are common practices to mitigate soil and water acidification in temperate and boreal forests. In addition, wood ash recycles nutrients removed at harvest to the forest ecosystem. Both liming and wood ash applications typically increase soil pH by 1–2 units. Therefore, they affect a range of soil processes and organisms including ectomycorrhizal (EM) fungi which are vital for the nutrition of many tree species. Here we review field studies reporting the effects of lime and wood ash amendments on EM fungi. We systematically compiled studies where known amounts of ash or lime were distributed to plots paired with comparable control plots, and where mycorrhizal variables were recorded. For a subset of studies meeting explicit criteria, we performed meta-analyses using overall mycorrhizal abundance, species richness or the abundance of specific fungal taxa as response variables. Guided by availability of data, the focus is on Nordic coniferous forests. Although the reviewed field studies varied widely in dosage and experimental setup they clearly demonstrated that liming and wood ash amendments influence EM fungal species composition. Across studies, species belonging to the lineages Cortinarius and Russula-lactarius (Basidiomycota), particularly Russula ochroleuca, decreased in relative abundance, while species within the Tuber-helvella (Ascomycota) increased. Particular species within the Amphinema-tylospora lineage responded in opposite directions; Tylospora fibrillosa decreased in relation to the control, while Amphinema byssoides increased. The significant changes in species or clade abundances were in the range of 5–20% compared to non-treated plots. In contrast, neither the belowground mycorrhizal biomass nor species richness responded to liming or wood ash applications. We conclude that liming and wood ash amendments cause consistent EM fungal species dominance shifts, but that a high EM fungal biomass and species and phylogenetic richness is maintained on the tree roots. Given the large dispersal potential of many EM fungi, we therefore suggest that these treatments at normal recommended dosages do not pose any immediate threats to EM fungal biodiversity, at least not when applied at relatively small spatial scales. Whether the observed dominance shifts among EM fungal clades have consequences for the functioning of the EM fungal guild, e.g. in relation to nutrient cycling or tree nutrition, is an important question that should be further investigated.

Return of wood ash from power plants to plantations makes it possible to recycle nutrients, counteract acidification, and to reduce economically costly waste deposition of the wood ash. However, current legislation restricts the amount of wood ash that can be applied and it is desirable to increase the allowed application dose, if possible, without negative effects on the plantation ecosystems. Here, we applied wood ash in levels corresponding to 0, 3, 9, 15, 30, and 90 t ash ha−1 and monitored the effect of the different ash doses on bryophytes in a Norway spruce (Picea abies) plantation with a dense bryophyte cover dominated by Hypnum jutlandicum, Dicranum scoparium, and Pleurozium schreberi. We used two complementary methods, image analysis, and pinpoint registration during a three-year period. To our knowledge, we are the first to apply this combined effort, which provides a much more exhaustive description of the effects than the use of each method separately. Moreover, the inclusion of a wide range of different wood ash levels enabled us to establish detailed dose-response relationships, which previous authors have not presented. The bryophyte cover decreased with increasing ash level with concomitant changes in species composition. At ash doses above the currently allowed 3 t ha−1, the ash significantly reduced the bryophyte cover, which only re-established very slowly. With increasing wood ash dose, the dominating species changed to Brachythecium rutabulum, Ceratodon purpureus, and Funaria hygrometrica. We conclude that application of more wood ash in spruce plantations than currently allowed will reduce total cover of bryophytes and cause a pronounced change in bryophyte species composition. These changes will in particular harm bryophyte species with specific environmental requirements and generally impair the bryophyte cover as habitat for invertebrates and its economic value for moss harvesting.

The complexity of wood ash fertilization disentangled: Effects on soil pH, nutrient status, plant growth and cadmium accumulation

Wood ash decreases cadmium toxicity to the soil nematode Caenorhabditis elegans

Effects of crushed wood ash on soil chemistry in young Norway spruce stands

Nutrient concentrations in current and 1-year-old needles were analyzed annually for 5 years after application of hardened wood ash in 1–4-year-old Norway spruce (Picea abies (L.) Karst.) stands within a range of climate and fertility gradients. At each site, 3000 kg ha−1 hardened wood ash of two types, Nymolla and Perstorp, was applied in a randomized block design. Wood ash Nymolla contained 12 kg ha−1 P, 30 kg ha−1 K, 891 kg ha−1 Ca, 72 kg ha−1 Mg and wood ash Perstorp contained 12 kg ha−1 P, 60 kg ha−1 K, 486 kg ha−1 Ca, and 60 kg ha−1 Mg. The ash was intended to compensate for nutrients removed at the preceding harvest when logging residues were collected and removed from the site (whole-tree harvesting). The climate gradient included four climate zones throughout Sweden and each of these included a fertility gradient of three sites classified according to their ground vegetation type. There were no effects on nutrient concentrations in the needles 1 year after the application of wood ash. Five years after ash application, the concentrations of P, K and Ca in current and 1-year-old needles were higher than in the control plots. The results were consistent over all stands, irrespective of climate zone and fertility status. P and K concentrations were higher in spruce needles from plots treated with Perstorp wood ash, whereas Ca concentrations were higher in those of Nymolla treated plots. Analyses across all study sites revealed a treatment effect in terms of increased ratios of P:N, K:N and Ca:N in 1-year-old needles. The ratio P:N tended to increase with time in the Perstorp wood ash treatment compared with the control. The needle concentrations of Mg and S were not affected by the ash applications. The increase in needle nutrient concentrations after application of hardened wood ash suggests that wood ash recycling could be used in order to replace nutrients removed at whole-tree harvesting.

Wood ash for application in municipal biowaste composting

The impact of forest residue removal and wood ash amendment on the growth of the ectomycorrhizal external mycelium

Intensive harvesting of forest residues for energy production may lead to the depletion of organic matter and mineral nutrients in the forest floor. In order to restore nutrient content wood ash has been suggested as a fertiliser. Ectomycorrhizal (EM) fungi are involved in the nutrient uptake of forest trees and this study investigates the influence of intensive harvesting and wood ash fertilisation on the external EM mycelium in forest soil. Nylon mesh bags filled with sand were buried in September 1997 in field plots which had or had not been intensively harvested. The effect of wood ash on the production of external EM mycelium was studied in mesh bags amended with wood ash. Mesh bags were retrieved in May and October 1998. The relative amount of fungal mycelia in the mesh bags was estimated with phospholipid fatty acid analysis. The fungi colonising the mesh bags were mainly (>90%) ectomycorrhizal. Fungal biomass in the mesh bags was low in the spring but high in the autumn. No significant effect on EM fungal biomass was observed in the mesh bags collected from intensively harvested plots compared with those from control plots, but wood ash amendment resulted in 2.4 times more EM fungal biomass (P<0.05). The effect of external EM mycelium on the dissolution of wood ash was studied in mesh bags filled with wood ash, using mesh bags buried in soil isolated from roots as EM-free controls. The external EM mycelium had no effect on the dissolution rate of the wood ash. 80% of the potassium was lost from the wood ash within a month, whereas no phosphorus was lost during the experimental period (up to 13 months).

Wood ash amendment to forest soils contributes to the sustainability of the growing bioenergy industry, not only through decreased wood ash waste disposal in landfills but also by increasing soil/site productivity and tree growth. However, tree growth studies to date have reported variable responses to wood ash, highlighting the need to identify proper application rates under various soil/site conditions to maximize their benefits. We explored the influence of tree species, wood ash nutrient application rates, time since application, stand development stage, and initial (i.e., before wood ash application) soil pH and N on short‐term tree growth response to wood ash amendment across eight unique study sites spanning five Canadian Provinces. Jack pine (Pinus banksianaLamb) had the most positive response to wood ash amendment compared to white (Picea glaucaMoench), hybrid (Picea engelmanniixglaucaParry), and black spruce (Picea marianaMiller), where increasing nutrient application rates increased height growth response. In comparison, black spruce had the most negative response to wood ash amendment, where increasing nutrient application rates slightly decreased height growth response. Site as a random effect explained additional variation, highlighting the importance of other unidentified site characteristics. By examining trends in short‐term growth response across multiple studies with variable site characteristics, we found growth response differed by tree species and nutrient application rates, and that jack pine is a promising candidate for wood ash amendment. These results contribute to our knowledge of optimal wood ash amendment practices and environmentally sustainable bioenergy production.

Wood ash effects on growth and cadmium uptake in Deschampsia flexuosa (Wavy hair-grass)

Wood ash recycling to forests after logging residues harvest is important to ensure long-term sustainable forest management, however, it is not recycled in Sweden at the level required to compensate for current logging residue out-take. A problem in this context is wood ash contamination through co-incineration of waste wood with forest fuels, a practice driven by the political goal of a circular bioenergy system. We performed a case study of co-incineration at a typical Swedish district heating (DH) plant, which showed that the forest fuel ash alone could be recycled to forests due to high nutrient levels. Co-incineration with waste wood resulted however in such high levels of contaminants that the ash was landfilled as hazardous waste. Our assessment of the Swedish DH sector showed that wood ash contamination through co-incineration is common, and that only a minor proportion of the ash from forest fuels is recycled to the forest. It also revealed a lack of reliable data regarding ash production and management, making implementation and evaluation of effective countermeasures difficult. Practical measures to enable wood ash recycling, such as removal of waste wood from the fuel mix, incineration of separate fuels, and ash after-treatment, are hampered by technical and economic barriers. Furthermore, no comprehensive policy tools currently exist on either a national or EU level that facilitate wood ash recycling. Thus, we conclude that comprehensive and efficient policy tools are urgently needed to overcome current barriers, and stimulate large-scale recycling of wood ash for long-term sustainable forest fuel utilisation.

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 of 5 and 10 t ha−1 and right after planting in concentrations of 5, 10, 15, and 20 t ha−1. 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.

Wood ash was applied to a forest ecosystem with the aim to recycle nutrients taken from the forest and to mitigate the negative effects of intensive harvesting. After two years, the application of 8,000 kg ha−1 of wood ash increased soil exchangeable Ca and Mg. Similarly, an increase in Ca and Mg in the Norway spruce fine roots was recorded, leading to significant linear correlations between soil and root Ca and soil and root Mg. In contrast to these macronutrients, the micronutrients Fe and Zn and the toxic element Al decreased in the soil exchangeable fraction with the addition of wood ash, but not in the fine roots. Only Mn decreased in soil and in fine roots leading to a significant linear correlation between soil and root Mn. In soil, as well as in fine roots, strong positive correlations were found between the elements Ca and Mg and between Fe and Al. This indicates that the uptake of Mg resembles that of Ca and that of Al that of Fe. With the wood ash application, the pH increased from 3.2 to 4.8, the base saturation from 30% to 86%, the molar basic cations/Al ratio (BC/Al) of the soil solution from 1.5 to 5.5, and the molar Ca/Al ratio of the fine roots from 1.3 to 3.7. Overall, all below-ground indicators of soil acidification responded positively to the wood ash application within two years. Nitrate concentrations increased only slightly in the soil solution at a soil depth of 75–80 cm, and no signs of increased heavy metal concentrations in the soils or in the fine roots were apparent. This suggests that the recycling of wood ash could be an integral part of sustainable forest management because it closes the nutrient cycle and reverses soil acidification.

ABSTRACTRecycling of wood ash from energy production may counteract soil acidification and return essential nutrients to soils. However, wood ash amendment affects soil physicochemical parameters that control composition and functional expression of the soil microbial community. Here, we applied total RNA sequencing to simultaneously assess the impact of wood ash amendment on the active soil microbial communities and the expression of functional genes from all microbial taxa. Wood ash significantly affected the taxonomic (rRNA) as well as functional (mRNA) profiles of both agricultural and forest soil. Increase in pH, electrical conductivity, dissolved organic carbon and phosphate were the most important physicochemical drivers for the observed changes. Wood ash amendment increased the relative abundance of the copiotrophic groups Chitinonophagaceae (Bacteroidetes) and Rhizobiales (Alphaproteobacteria) and resulted in higher expression of genes involved in metabolism and cell growth. Finally, total RNA sequencing allowed us to show that some groups of bacterial feeding protozoa increased concomitantly to the enhanced bacterial growth, which shows their pivotal role in the regulation of bacterial abundance in soil.