Low-temperature pyrolysis around 250 °C represents a mild carbonization that differs from conventional high-temperature biochar production, and the role of pyrolysis duration under mild thermal conditions remains insufficiently understood. In this study, plant residues, including rice straw, sorghum leaves and stems, barley straw, and mixed woodchips, were converted into charred materials under low-temperature pyrolysis at 250 °C (4 h, 12 h) and compared with those produced at 500 °C (4 h). Pyrolysis at 250 °C (4 h) resulted in higher solid yields (51.9–72.8%) and higher recovery of carbon and nitrogen, whereas yields declined to 27.2–31.6% at 500 °C. Materials produced at 250 °C preserved abundant oxygen-containing functional groups, exhibited lower pH, and showed significantly higher cation exchange capacity (up to 93.68–119.91 cmolc/kg at 12 h). Prolonged treatment at 250 °C enhanced humification, increasing the carbon extracted from humic acid by 25.3–237.9%, whereas humic substances were largely decomposed at 500 °C. Structural analyses indicated that low-temperature chars maintained reactive surface chemistry, while high-temperature chars showed greater aromaticity and porosity, particularly for wood-derived materials (378.5 m2/g). Overall, low-temperature pyrolysis produces functionally active carbon materials suitable for saline-sodic soil amendment and nutrient management, whereas 500 °C pyrolysis generates more aromatic and porous materials better suited for long-term carbon stability and physical soil conditioning.
Chu et al. (Mon,) studied this question.