The stability of low- and high-ash biochars in acidic soils of contrasting mineralogy

Abstract

The potential of biochar as a tool for long-term soil carbon (C) storage has led to an increasing interest in its use as a soil amendment. While much research has been conducted on the stability of high C and low ash biochars, the stability of low C (with relatively high inorganic C) and high ash biochars has been largely neglected. In light of this, an incubation experiment was conducted to compare and assess the stability of a high ash and low C biochar produced from tomato green waste and low ash and high C biochar produced from blue mallee biomass. The two biochars were applied at 2% and 4% (w/w) to two acidic soils of contrasting mineralogy, a Ferralsol and a Solonetz. The soil–biochar mixtures were incubated at 20 °C for 120 days. The CO2–C mineralised was captured in NaOH traps and the source of C mineralisation determined by isotope analysis. The tomato biochar was mineralised (1.4–3.7%) to a greater extent than the blue mallee biochar (0.28–0.77%), possibly due to dissolution of the large quantity of inorganic C. In biochar amended soils, with the exception of the Solonetz applied with 2% blue mallee biochar, greater cumulative mineralisation (positive priming) of native SOC occurred as compared to their respective controls. Mean residence time for the two biochars suggests much greater potential of the blue mallee biochar for long-term soil C storage than the tomato biochar. However, the tomato biochar may have greater agronomic value, in particular a high liming potential, although field studies are required to confirm these results.