Biochar and biodiversity: A meta-analysis of biochar effects on vegetation in restoration trials

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Sean C. Thomas, Jillian M.H. Bieser, Mark Horsburgh, Melanie Sifton, Leeladarshini Sujeeun and Juliana Vantellingen, Institute of Forestry and Conservation, University of Toronto, Toronto, ON, Canada, Jasmine M. Williams, Insitute of Forestry and Conservation, University of Toronto, Toronto, ON, Canada

Background/Question/Methods
Biochar (charcoal created by pyrolysis of organic waste biomass and used as a soil amendment) has been widely promoted in a restoration context as a means to enhance soil organic matter, increase soil water and nutrient retention, and reduce bioavailability of heavy metals and other toxicants. Broad implementation of biochar could also contribute to enhanced ecosystem carbon sequestration and even result in net negative carbon emissions by converting waste biomass into recalcitrant carbon, and by enhancing net primary productivity. However, it has commonly been observed that fertilizer additions may enhance competitive interactions and thus reduce local species diversity, favoring more competitive species and species that can tolerate or take advantage of enhanced resources. Alternatively, biochar might enhance biodiversity by reducing allelopathic effects, by favoring fire-adapted species, by enhancing facilitation effects, or by increasing the range of environmental conditions in adverse conditions. We compiled results from our own studies and from published literature on vegetation responses to biochar-based restoration trials in forest, grassland, and wetland ecosystems and used meta-analysis techniques (random effects models based on the log response ratio metric) to examine biochar effects on vegetation cover, on metrics describing local species diversity (richness, evenness, and diversity), and on aspects of species composition.
Results/Conclusions
Biochar additions resulted in marked increases in vegetation amount across the pooled set of 18 studies, with an average 14% increase in vegetation cover values. Although there was heterogeneity among studies, biochar additions resulted in an average increase in local species richness and diversity indices (Shannon and inverse Simpson), but no detectable effects on species evenness. Similar patterns were found in analyses that included only native plant species. In general, studies also showed pronounced effects of biochar additions on plant species composition. Where present, N-fixing species (mainly legumes) increased in relative abundance; no average effect on the proportion of native species was detected, and insufficient information was available on fire adaptation to evaluate predicted increases in fire-adapted species.
In conclusion, biochar additions in the context of restoration resulted in favorable overall responses in terms of most common restoration objectives, increasing vegetation cover and most biodiversity metrics, with no obvious adverse effects on species composition.