Biochar and forest restoration: Soil processes and plant community response
Ecological forest management in many regions has embraced a guiding ecological principle that harvests and other management activities should “emulate” natural disturbances. In most temperate forest ecosystems the main agent of natural disturbance is fire – an important physio-chemical legacy of which is charcoal produced by incomplete combustion. The term “biochar” has been adopted to denote charcoal designed for use as a soil amendment, generally to enhance ecosystem C sequestration and improve soil properties and productivity. Prior biochar research has focused on agricultural applications. I present an overview of my lab’s recent work on forest soil and ecosystem responses to biochar additions, including both lab/greenhouse trials and field experiments, with special attention to a stand-level trial (30 x 30 m blocks) initiated in a regenerating Northern Hardwood forest site in central Ontario in 2012.
Field trials indicate that biochar additions to forest soils result in increased availability of non-nitrogenous nutrients, in particular K, P, and Ca. Data from lab incubations suggest a transient increase in soil CO2 efflux following biochar additions, but no such “priming” effect was detected in field trials. In contrast, litter decomposition trials exhibit a “negative priming” effect in which biochar additions acted to reduce litter mass loss. A meta-analysis of experimental biochar effects on woody plants suggests that trees show mean growth responses somewhat larger than those reported for agricultural systems, with an average ~40% increase in biomass. However, both greenhouse and field trials are characterized by large variability in growth responses, with analyses of variance showing large species x biochar interaction terms. Two-year whole-forest growth responses to biochar additions (at 5t/ha) were not statistically significant, but related work on forest liming suggests that stem growth responses may be delayed by up to 5-10 years post-treatment. No negative effects of biochar were found on understory vegetation, soil invertebrates, or terrestrial amphibians, but certain species show large positive responses not shown by others. In conclusion, biochar may be a viable substitute for other forms of organic materials, non-N fertilizers, and liming agents in a forest restoration context. However, given large variation in species responses, we can expect large changes in forest community composition. A better understanding of mechanisms underlying variable responses will be an essential scientific foundation for broad-scale operational use of biochar in a forest restoration context.