Characteristics of biochars from crop residues: Potential for carbon sequestration and soil amendment

• Published in: Journal of environmental management 2014-12, Vol.146, p.189-197
• Main Authors: Windeatt, Jayne H., Ross, Andrew B., Williams, Paul T., Forster, Piers M., Nahil, Mohamad A., Singh, Surjit
• Format: Article
• Language: English
• Subjects: Animal, plant and microbial ecology; Applied ecology; Biochar; Biological and medical sciences; Carbon; Carbon Sequestration; Charcoal; Charcoal - chemistry; Conservation, protection and management of environment and wildlife; Crop residue; Crop residues; Crops, Agricultural; Fundamental and applied biological sciences. Psychology; General aspects; Humans; Plant growth; Pyrolysis; Recalcitrance; Sequestration; Soil - chemistry; Soils; Temperature
• ISSN: 0301-4797; 1095-8630
• DOI: 10.1016/j.jenvman.2014.08.003

Biochar has potential to sequester carbon in soils and simultaneously improve soil quality and plant growth. More understanding of biochar variation is needed to optimise these potential benefits. Slow pyrolysis at 600 °C was undertaken to determine how yields and characteristics of biochars differ when produced from eight different agricultural residues. Biochar properties such as carbon content, surface area, pH, ultimate and proximate analysis, nutrient and metal content and the R50 recalcitrance index were determined. Significant variations seen in biochar characteristics were attributed to feedstock variation since pyrolysis conditions were constant. Biochar yields varied from 28% to 39%. Average carbon content was 51%. Ash content of both feedstocks and biochars were correlated with biochar carbon content. Macronutrients were concentrated during pyrolysis, but biochar macronutrient content was low in comparison to biochars produced from more nutrient rich feedstocks. Most biochars were slightly alkaline, ranging from pH 6.1 to pH 11.6. pH was correlated with biochar K content. Aromaticity was increased with pyrolysis, shown by a reduction in biochar H/C and O/C ratios relative to feedstock values. The R50 recalcitrance index showed biochars to be either class 2 or class 3. Biochar carbon sequestration potential was 21.3%–32.5%. The R50 recalcitrance index is influenced by the presence of alkali metals in the biochar which may lead to an under-estimation of biochar stability. The residues assessed here, at current global availability, could produce 373 Mt of biochar. This quantity of biochar has the potential to sequester 0.55 Pg CO2 yr−1 in soils over long time periods. •Biochars from eight crop residue feedstocks have been characterised in detail.•Variation in biochar recalcitrance and carbon sequestration potential is noted.•High alkali content catalysed the degradation of biochars.•21.3%–32.5% of the feedstock carbon would be stored long-term.•150 Mt C yr−1 (0.55 Pg CO2 eq yr−1) could be sequestered using these crop residues.