Morphology, pore size distribution, and nutrient characteristics in biochars under different pyrolysis temperatures and atmospheres

To evaluate the agronomic potential of biochar, we prepared a series of biochars using rice straw waste under the limited oxygen cracking condition (CO₂ or N₂) and the different pyrolysis temperatures including 300, 400, 600, and 800 °C. The results showed that morphology structure, specific surface area, pore size distribution, and element contents of the biochars were superior to the biochars prepared under traditional inert atmosphere (N₂) and the same four pyrolysis temperatures. In comparison with the rice straw, pore structure of biochars was mainly mesoporous and more developed, average pore size decreased, and BET-specific surface area increased with the increase of temperature from 300 to 800 °C. Biochars distributed abundant mesopores and macropores under 400 and 600 °C; the maximum macropores of the biochar were shaped under 600 °C. Concentration of phosphorus (P) and potassium (K) increased significantly with increasing temperature, while that of nitrogen (N) first increased and then decreased and reached a maximum at 400 °C. In addition, taking these physiochemical properties into consideration, we drew a conclusion that the optimum quality biochar could acquire under the work conditions of 400 °C and CO₂ atmosphere, which was supposed to provide theoretical guidance for biochar returning to soil.