Biochar - synergies and trade-offs between soil enhancing properties and C sequestration potential

Abstract

The characterization of biochar has been predominantly focused around determining physicochemical properties including chemical composition, porosity and volatile content. To date, little systematic research has been done into assessing the properties of biochar that directly relate to its function in soil and how production conditions could impact these. The aim of this study was to evaluate how pyrolysis conditions can influence biochar's potential for soil enhancing benefits by addressing key soil constraints, and identify potential synergies and restrictions. To do this, biochar produced from pine wood chips (PC), wheat straw (WS) and wheat straw pellets (WSP) at four highest treatment temperatures (HTT) (350, 450, 550 and 650 °C) and two heating rates (5 and 100 °C min-1) were analysed for pH, extractable nutrients, cation exchange capacity (CEC), stable-C content and labile-C content. Highest treatment temperature and feedstock selection played an important role in the development of biochar functional properties while overall heating rate (in the range investigated) was found to have no significant effect on pH, stable-C or labile-C concentrations. Increasing the HTT reduced biochar yield and labile-C content while increasing the yield of stable-C present within biochar. Biochar produced at higher HTT also demonstrated a higher degree of alkalinity improving biochar's ability to increase soil pH. The concentration of extractable nutrients was mainly affected by feedstock selection while the biochar CEC was influenced by HTT, generally reaching its highest values between 450-550 °C. Biochar produced at ≥550 °C showed high combined values for C stability, pH and CEC while lower HTTs favoured nutrient availability. Therefore attempts to maximize biochar's C sequestration potential could reduce the availability of biochar nutrients. Developing our understanding of how feedstock selection and processing conditions influence key biochar properties can be used to refine the pyrolysis process and design of 'bespoke biochar' engineered to deliver specific environmental functions.