Comparative analysis of physicochemical, nutrient, and spectral properties of agricultural residue biochars as influenced by pyrolysis temperatures

The objectives of this study were to assess the physicochemical, nutrient, and spectral properties of biochar prepared from four major agricultural residues of India [rice straw (RSB), wheat straw (WSB), maize stover (MSB), and pearl millet stover (PSB)] at three (400, 500, and 600 °C) pyrolysis temperatures. Pyrolysis temperatures and residue types profoundly influenced biochar properties, for instance, PSB biochar had the greatest pH (10.75 ± 0.01), calcium carbonate equivalent (CCE) (47.8 ± 0.5), and carbonate (CO 3 = ) content (432 ± 17 meq kg−1). Irrespective of residue, greater pyrolysis temperature improves the biochars’ acid-neutralizing capacity by increasing pH in water (pHw), CCE, and CO 3 =  content. The CCE of biochar showed a significant positive correlation with pHw (R 2 = 0.51, p < 0.001) and ash content of biochar (R 2 = 0.54, p < 0.001). A great amount of water-soluble potassium (20.6–29.5 g kg−1) in all the biochars made them suitable for supplying potassium to plants. Infrared spectroscopy explained the functional group formation, while XRD revealed mineral formation in the biochar. Thus, depending on the requirement, diverse properties of biochar can be prepared by designing residue type and pyrolysis temperature suitable for application in a specific soil to alleviate nutrient deficiency and improve soil productivity.