Biochar production and use: environmental risks and rewards

Abstract

Biochar is a carbon dense solid that is produced via the pyrolysis of organic materials for application to soil. Organic carbon structures, such as those found in animal manures or wood, are modified by exposure to elevated temperatures in an oxygen-limited environment. This processing causes a loss of mass through a preferential loss of hydrogen along with a loss of volatile carbon molecules. The carbon remaining in the solid forms stable, conjugated aromatic structures, which are more recalcitrant to degradation in the natural environment than the pre-processed feedstock material. The surface area of the biochar material is also greatly increased compared to the feedstock, especially at the micropore scale. The mineral ash content of the original organic materials is preserved in the biochar, increasing in concentration with the loss of volatile components of the feedstock. Biochar production and use offers many opportunities for enhancing soil properties and sequestering carbon. The evidence to support the application of biochar as a soil amendment to enhance crop productivity is growing. Its potential as a carbon reservoir, able to sequester carbon in soils, and hence reduce atmospheric concentrations of greenhouse gases is a relatively new area of scientific investigation, on which lies significant sociopolitical interest due to the agenda to find cost effective climate change solutions. This thesis makes a significant contribution to the field of biochar production and use in developing several aspects of its physical characterization, environmental sustainability, risks, and opportunities. The investigation builds the evidence and methods required to understand the potential contribution biochar can make to atmospheric greenhouse gas reduction and agricultural sustainability. Key findings include a comprehensive review of the physical characteristics of biochars and how they are influenced by processing conditions and feedstocks choice. Also, the discovery and investigation of Terra Preta Australis sites, which provide an Australian analogy to the Terra Preta de Indio soils, that provide a case study demonstrating the long-term impact on soil properties and potential for carbon sequestration in that environment. Other major outcomes from the work include the development of risk assessment and greenhouse gas accounting methodologies that can be applied to biochar production and use. Extensive collaborative work has been included, particularly concerning the agronomic assessment of a number of biochars derived from commercially relevant feedstocks.