EFFECTS OF FORMATION CONDITIONS OF BIOCHARS on WATER EXTRACTS

Biochar is a recalcitrant high carbon product that results from thermal degradation of biomass under reduced oxygen conditions. Application of biochar to soil has been proposed both as a means for long-term atmospheric carbon sequestration and to improve soil fertility and crop production. Adding biochar to soil increases its organic carbon content, and reduces the need for addition of energy-intensive synthetic ammonium nitrate-based fertilizers. How adding large amounts of biochar to soil might affect nearby surface waters has not been well characterized. In recent studies we found that char formation conditions (time and duration of heating) have a substantial effect on elemental composition, distribution of aliphatic and aromatic carbon, surface area, acid functional groups, and water sorption. The effects of large-scale biochar incorporation into agricultural soils on surface and ground waters were assessed by analyzing water extracts of chars from cellulose, lignin, pine, and switchgrass using a wide range of formation conditions. The pH of water extracts of the chars increased from acidic to basic (pH 3 to over 10) as the chars were produced at higher temperatures, and correlated with the ash content of the chars. Dissolved organic carbon of the water extracts from chars was highest from low temperature chars, and correlated with the higher aliphatic content and higher acid functional groups of the low temperature chars. Dissolved inorganic carbon increased as charring temperature increased and is correlated with higher ash content. As charring temperature increases, the acid content decreases and the base content increases. With further increases in char formation temperature, the type of base changed from predominantly bicarbonate to bicarbonate plus hydroxyl/carbonate, for both pine and switchgrass chars. The range of heating times and temperatures produce very complex changes in biochar, and have dramatic effects on other water extractable species from the biochar as well.