Pyrolysis temperature affects pore characteristics of rice straw and canola stalk biochars and biochar-amended soils

Abstract

Soil physical properties are potentially affected by the addition of biochar. The effect is largely determined by the pore characteristics of biochar, especially the biochar’s feedstocks and pyrolysis processes. This study examined the pore characteristics of rice straw biochars (RSB) and canola stalk biochars (CSB) produced at pyrolysis temperatures of 250, 350, 450, 550, and 650 °C, and evaluated the effects of biochars on soil pore properties. The pore characteristics of biochars were characterized by nitrogen adsorption isotherm (NAI), mercury intrusion porosimetry (MIP), and scanning electron microscope (SEM). Each biochar was mixed into two soils (Ultisol and Alfisol) at the rate of 1% (w/w), then the soil-biochar mixtures were incubated for 90 days. The specific surface area (SSA) and total pore volume (TPV) measured by NAI technique increased with increasing pyrolysis temperature of biochar. Biochars pyrolyzed at 450 °C had the highest porosity and TPV measured by MIP. The largest pore class for RSB was the macropore (>75 μm), while the main pore classes for CSB were micropore (5–30 μm) and ultramicropore (0.1–5 μm). Incubation experimental results showed that biochar addition altered the pore size distribution of the Alfisol and Ultisol. RSB enhanced the total porosity and microporosity of soils, whereas CSB decreased total porosity, macroporosity, and mesoporosity. The water-holding capacity of soil was increased by increasing the amount of soil storage pores caused by biochar. Heatmap analysis on the correlation among pyrolysis temperature of biochar, pore characteristic and water retention capacity of biochar-amended soils illustrated that the pore properties of the Ultisol amended with CSB and the water properties of the Alfisol with CSB were highly correlated with pyrolysis temperature of biochar. Our results suggested the meso- and micro-pores in biochars played an important role to affect the soil response to biochar addition. Based on pore properties, pyrolysis temperature around 450 °C was found to be the optimal condition for producing porous biochar with larger porosity.