Limited effect of wood ash application on soil quality as indicated by a multisite assessment of soil organic matter attributes

Abstract

In Canada, the combustion of forest biomass for bioenergy production has been increasing with an associated increase in residual wood ash. Wood ash is typically landfilled as waste but there is growing interest in applying wood ash to the soils of commercial forests. Ideally, wood ash supplies nutrients that may have been removed through biomass harvesting, increases soil pH, which improves nutrient availability, and potentially improves site productivity, but there is also potential for detrimental effects, such as toxicity, that impair soil functions. The objective of this study was to investigate the effects of wood ash application on soil organic matter attributes at eight experimental sites across Canada that are examining the effects of wood ash application on site fertility, productivity, and soil biodiversity. Wood ash application had an effect on total carbon (TC) and total nitrogen, microbial biomass carbon (MBC), hot water extractable carbon (HWEC), mineralizable C, sand size C, and HWEC and MBC normalized to TC, but changes were typically restricted to single sites or differed in their direction, that is, positive or negative. Based on the limited and inconsistent effects of ash on established indictors of soil quality measured in this study, there does not appear to be any advantageous or detrimental effects of adding wood ash to forest soil quality.

Forest soil biotic communities show few responses to wood ash applications at multiple sites across Canada

“…High calcium and magnesium concentrations in ash have more potential to alleviate metal stresses as many of these ions are in the forms of oxides and hydroxides, which increase pH and lower metal availability 4 . Observed responses were also in line with the known soil chemical response to ash addition 18 . In general, chemical response to ash addition is variable, with reports of increased soil quality after ash addition 6 , 21 , 48 , some where minimal or no changes were detected or, the changes were limited to specific ash and amendment rates 6 , 59 .…”

“…The magnitude of Bray–Curtis or Jaccard distance differences and low explanatory power of constrained RDA axes also supports that ash amendment did not result in particularly large shifts in community structure. The infrequent effects as seen in this study and others within the AshNet network suggest that negative effects on biological community structure and function are minimal 18 , 58 , 59 . Other studies imply that the risk of damaging soil biotic function associated with heavy metal increases are low, and likely to be mitigated by the high calcium and magnesium concentrations in ash 6 , 60 .…”

“…As eukaryotic DNA signatures are more susceptible to random sampling biases due to their multicellular nature, and varied sizes (i.e., cells are attached, and less likely to be evenly distributed in samples), this is not particularly strong evidence of community shift. As there was also little evidence of changes to soil quality or pH in these sites, the lack of community response to ash addition is likely because the changes to soil characteristics were minor to non-detectable 18 . It has been demonstrated that harvesting can cause detectable community changes 17 .…”

“…Precipitation seasonality was included because it gave an indication of whether there was variation in precipitation across the year. Climate variables were produced by the climate models of McKenney et al 43 Total soil carbon and total soil nitrogen data were calculated via TOC/TON analysis as described in Joseph et al 18 Differences between control and treatment soil samples were used as explanatory parameters within models.…”

“…While harvest disturbance alters moisture, pH and calcium which results in stark changes to microbial community composition from uncut forest systems, increases to harvesting intensity can be small and undetectable 17 . The effects of wood ash amendment are likely also small; a recent study of sites across Canada showed inconsistent responses in soil quality metrics due to wood ash amendment 18 . In general, reported microbial responses to ash application are inconsistent, and changes in mineral layers take longer than those in organic layers 19 – 22 .…”