Spectral, Chemical and Physical Characterisation of Mustard Stalk Biochar as Affected by Temperature

Conversion of agricultural residue to biochar and its application to soil has been accepted as a novel approach to sequester carbon, improve soil fertility and reduce environmental pollution. Application and impact of biochar depends on its intrinsic properties which in turn depend on the pyrolysis conditions. Mustard stalk was pyrolysed at 400°C (MSB4), 500°C (MSB5), and 600°C (MSB6) through slow pyrolysis process to examine the effect of temperature on biochar properties. Mass recoveries of biochars decreased while pH, EC, calcium carbonate equivalent (CCE), ash content, bulk density and particle density increased with increase in pyrolysis temperature. MSB4, MSB5, and MSB6 had pH 8.7, 10.1 and 11.0, respectively and the EC values of the biochar were 2.41dS m⁻¹, 3.25 dS m⁻¹ and 4.76 dS m⁻¹ in the same order. The CCE of the biochar increased from 19.06 for MSB4 to 34.38 for MSB6. The C recovery efficiency of biochar was around 50%. Though the total P content in biochar increased with increase in pyrolysis temperature, the water and acid soluble P content decreased. The major inorganic minerals present in the biochar identified by X-ray diffraction analysis were quartz (SiO₂), sylvite (KCl) and calcite (CaCO₃). Infrared spectroscopy analysis of biochars samples indicated the presence of CO₃^2-, aromatic C=O ring and hydroxyl group (^–OH). The scanning electron micrograph clearly indicates the presence of highly porous structure in biochar. The low temperature mustard stalk biochar could be used as a good source of P and K in P and K deficient soil, and high temperature biochar (MSB6) is recommended as a liming material for reclamation of acid soil.