VARIATIONS IN THE PROPERTIES OF LABORATORY-PRODUCED BIOCHARS: SURFACE CHEMISTRY, LABILITY AND INTERACTION WITH SOIL ORGANIC MATTER

Examinations of the chemical properties and interactions of pyrogenic carbon are needed to understand its effects on soil and sediment chemistry and ecology following fires or intentional amendment (biochar) for fertility or C sequestration enhancement. Biochar produced from a range of parent biomass types (hardwoods, softwoods and grasses) and under a range of combustion conditions (250 to 650 ^oC) were analyzed for surface area (by N₂ and CO₂ sorptometry), CEC, surface charge and functional group content (Boehm titration). In addition, examinations of biochar lability were carried out by microbial incubations (both alone and mixed with soil) and of organic matter (OM) interaction using batch adsorption equilibrium experiments with catechol (a natural OM monomer) and a humic acid mixture.

With increasing combustion temperature, biochar surface area (found mainly within micropores < 2 nm) and pH increased, while volatile matter (VM) content, acid functional group content and CEC decreased. These relationships suggest that char micropores are filled with variable amounts of VM which carries most of biochar’s acidity, negative charge, and cation complexation ability. The VM content was also directly correlated to biochar lability with C half-lives on orders ranging 10²-10⁷ years, but was indirectly correlated to catechol adsorption affinity and capacity. While catechol exhibited specific sorption within micropores (correlated with CO₂-surface area), humic acid sorption capacity was much lower and only within nanopores, indicating exclusion from the majority of biochar surfaces. Both the lability and OM adsorption of biochar played a role in the pattern of CO₂ evolution from incubations of soil-biochar mixtures. Both increased native OM mineralization (positive priming) and biochar-OM protection (negative priming) effects were observed, depending upon the char type and incubation stage. These results are of practical value to those considering biochar as a tool for soil contaminant remediation, amelioration, or atmospheric C sequestration.