Mechanistic study of the influence of pyrolysis conditions on potassium speciation in biochar preparation-application process

Biochar samples produced from rice straw by pyrolysis at different temperatures (400 degrees C and 800 degrees C) and under different atmospheres (N-2 and CO2) were applied to lettuce growth in a 'preparation-application' system. The conversion of potassium in the prepared biochar and the effect of the temperature used for pyrolysis on the bio-availability of potassium in the biochar were investigated. Root samples from lettuce plants grown with and without application of biochar were assayed by X-ray photoelectron spectroscopy (XPS). The optimal conditions for preparation of biochar to achieve the maximum bio-availability of potassium (i.e. for returning biochar to soil) were thus determined. Complex-K, a stable speciation of potassium in rice straw, was transformed into potassium sulfate, potassium nitrate, potassium nitrite, and potassium chloride after oxygen-limited pyrolysis. The aforementioned ionic-state potassium species can be directly absorbed and used by plants. Decomposition of the stable speciation of potassium during the pyrolysis process was more effective at higher temperature, whereas the pyrolysis atmosphere (CO2 and N-2) had little effect on the quality of the biochar. Based on the potassium speciation in the biochar, the preparation cost, and the plant growth and rigor after the application of returning biochar to soil, 400 degrees C and CO2 atmosphere were the most appropriate conditions for preparation of biochar. (c) 2017 Published by Elsevier B.V.