Black C (or biochar) has been applied to soils from the dawn of civilization (think fire pits). Purposeful black carbon additions to the soil, along with natural deposition (forest fires, volcanoes), have resulted in the widespread presence of black carbon in many soils. The current application of biochar to soils is modeled after the Amazonian Terra Preta soils, which have higher soil fertility believed to be from the intentional use of biochar (from slash and char agriculture). However, not all biochar additions to soils have resulted in increased fertility. For example — the long-term application of pyrolysis products at historical U.S. pyrolysis plants from 1800–1960 raised soil organic chemical concentrations to such high levels that some sites are included on the U.S. EPA Superfund program. Yikes!
Because of the uncertainty in applying biochar to forest sites (you can’t take it back), we have been testing various biochar sources and a variety of soils to ensure that we ‘do no harm’ with our biochar applications. In many forest sites we’ve tested, there has been no change in tree biomass production attributed to biochar addition. However, we have seen a 10–20% increase in understory production on some sites, which means we are increasing C in biomass. We attribute this increase in production to increased water availability in the soil profile. In addition, biochar C is approximately 80% of the total we add to any given site. Therefore, when we apply 2 tons of biochar/acre, we are really adding about 3200 pounds of C to the soil.
Thinning operations can result in healthier forests that are more resistant or resilient to insects, disease, or wildfire. Remember — one reason we thin forests is so that the residual trees grow better (without biochar additions). The main reason biochar may be useful in forest soils is carbon sequestration and the potential to alter soil moisture regimes in light of climate change.
