Biochar and high-carbon wood ash effects on soil and vegetation in a boreal clearcut

Abstract

Additions of fire residues in the form of charcoal and wood ash may better emulate natural disturbance processes in managed boreal forests. We examined the effects of a poplar (Populus) wood biochar and a high-carbon wood ash on soil and vegetation in a 3-year experiment in northwestern Ontario, Canada. Both soil amendments increased soil pH and soil Ca levels; high-carbon wood ash also increased soil Cu, Zn, B, S, and Pb. Amendments had large effects on plant community composition, favoring a subset of ruderal species including raspberry (Rubus idaeus L.) and goldenrod (Solidago canadensis L.). The addition of high-carbon wood ash resulted in declines in growth of planted white spruce (Picea glauca (Moench) Voss); a path analysis suggests that this was due to the effects of toxic elements rather than the indirect effects of competition. We conclude that high-carbon wood ash, while qualifying as a type of biochar and having some beneficial effects on soil properties, can enhance toxic metals in boreal forest soils, with negative consequences to early tree growth. Differences in plant species responses to biochars and the potential for toxicity effects and indirect effects mediated by plant competition will require screening and field trials of potential biochars prior to their use in operational forestry and forest restoration.