Comparisons of Biochar Properties from Wood Material and Crop Residues at Different Temperatures and Residence Times

Abstract

Biochar has been increasingly used as a method for C sequestration and soil improvement. To understand how feedstock and pyrolysis conditions affect biochar characteristics, we investigated two wood-based biochars (bamboo and elm) and five crop-residue-based biochars (wheat straw, rice straw, maize straw, rice husk, and coconut shell), which were pyrolyzed at 500 or 700 °C and remained at that temperature for 4, 8, and 16 h under oxygen-limited conditions. For a given feedstock, increasing pyrolysis temperature from 500 to 700 °C resulted in increases in ash content, BET surface area, pH, and total P and Ca contents ( P < 0.05) and decreases in yield, cation exchange capacity (CEC), total acid, and total N ( P < 0.01). Prolonging residence time (from 4 to 8 or 16 h), the BET surface area and ash content of biochars increased ( P < 0.05), whereas the yield decreased ( P < 0.01). Fourier-transform infrared spectroscopy (FTIR) analysis showed that more recalcitrant and aromatic structures were formed in the biochars with increased temperature. The three straw-based biochars consistently exhibited far greater ash percentage (14.5–40.3 wt %), CEC (14.1–34.8 cmol kg –1 ), and the contents of total N (0.24–2.81 wt %), P (0.60–8.41 wt %), Ca (0.63–1.48 wt %), and Mg (0.24–0.63 wt %) and generally had higher yield (19.0–37.6 wt %), pH (9.2–11.1), and contents of total acid (0.15–0.53 mmol g –1 ), C (41.7–55.1 wt %), Na (0.27–6.72 wt %), and K (6.56–28.1 wt %) than the two wood-based biochars. The BET surface area of straw-based biochars with 700 °C pyrolysis temperature could be mostly as high as 112–378 m 2 g –1, a comparable level with that of wood-based biochars. Despite the high variability in biochar properties, these results demonstrate that biochars from crop straw may be more effective and desirable for improving soil fertility and C sequestration in Chinese vast soils.