Biochar Carbon Stability In Contrasting Soils As Influenced by Pyrolysis and Incubation Temperatures.

Biochar production and application to soil is increasingly discussed as a tool to sequester carbon (C) over centuries to millennia. Biochar may be highly stable in soil environment but the effects of soil mineralogy, soil C content and incubation temperature on biochar stability are not known. Moreover, there are conflicting reports about biochar effect on native soil C mineralisation, and the contrasting nature of biochar and soil types may explain some of the conflicting results. Our study is aimed at assessing the impacts of biochar–mineral interactions, soil C content, biochar chemical composition and incubation temperature on the stability of biochar-C and ‘native’ C in soils of contrasting properties.

We have employed a novel depleted-13C labelling approach to quantify decomposition of biochar and ‘native’ C in soils. Biochars were produced at two pyrolysis temperatures (450°C, 550°C) from woody biomass of 2 year-old Eucalyptus salinga that was depleted in 13C (δ13C -36.4‰). The biochars were incubated in four soils (collected from New South Wales, NSW; Queensland, Qld; South Australia, SA; Western Australia, WA) of contrasting properties (such as clay content and compositions, soil C content, pH) at three incubation temperatures (20 °C, 40 °C and 60 °C) for 6 months. The δ13C values of soils ranged from -17.5‰ to -27.7‰. The CO2 gas produced from biochar-amended and control soils was trapped in NaOH and these traps were sampled at regular intervals for respired-C and δ13C analyses.

The CO2-C mineralisation rate was initially high across all treatments, and then decreased and stabilised 30 days after incubation. The early results showed that the decomposition of carbon in biochar-amended soils increased with increasing soil temperature and varied with soil type, with NSW’s Oxisol produced the highest cumulative CO2-C followed by SA’s Entisol, WA’s Inceptisol and Qld’s Vertisol. The cumulative C produced from biochar450 amended soils was generally higher for the 450°C biochar than the 550°C biochar. The isotopic (δ13C) and spectroscopic analyses are being performed on gas, soil and/or biochar samples to quantify the decomposition rate of biochar-C and ‘native’ soil C, and the effect of soil mineralogy and soil carbon content on biochar C stability; these results will be presented at the conference.