Biochar is produced as a charred material with high surface area and abundant functional groups by pyrolysis, which refers to the process of thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. The carbon component in biochar is relatively stable, and, hence, biochar was originally proposed as a soil amendment to store carbon in the soil. Biochar has multifunctional values that include the use of it for the following purposes: soil amendment to improve soil health, nutrient and microbial carrier, immobilising agent for remediation of toxic metals and organic contaminants in soil and water, catalyst for industrial applications, porous material for mitigating greenhouse gas emissions and odorous compounds, and feed supplement to improve animal health and nutrient intake efficiency and, thus, productivity. This article provides for the first time an overview of the multifunctional values and unintended consequences of biochar applications.

Biochar can influence native soil organic carbon (SOC) mineralisation through “priming effects”. However, the long-term direction, persistence and extent of SOC priming by biochar remain uncertain. Using natural 13C abundance and under controlled laboratory conditions, we show that biochar-stimulated SOC mineralisation (“positive priming”) caused a loss of 4 to 44 mg C g−1 SOC over 2.3 years in a clayey, unplanted soil (0.42% OC). Positive priming was greater for manure-based or 400°C biochars, cf. plant-based or 550°C biochars, but was trivial relative to recalcitrant C in biochar. From 2.3 to 5.0 years, the amount of positively-primed soil CO2-C in the biochar treatments decreased by 4 to 7 mg C g−1 SOC. We conclude that biochar stimulates native SOC mineralisation in the low-C clayey soil but that this effect decreases with time, possibly due to depletion of labile SOC from initial positive priming, and/or stabilisation of SOC caused by biochar-induced organo-mineral interactions.