Comparative analysis of the properties of biochars produced from different pecan feedstocks and pyrolysis temperatures

Pecan is an important woody oil and nut tree and it generates a large amount of residues during cultivation and management. However, the utilization of pecan residues remains largely unexplored. In this study, three types of pecan feedstocks (branches, leaves and nut shells) were selected as feedstocks to produce biochars at four pyrolysis temperatures (300, 400, 500 and 600 degrees C), and their properties were evaluated for agricultural and environmental applications. The biochar yield decreased with increasing temperature, and the highest yield was generally obtained from leaf biochars. As the pyrolysis temperature increased from 400 to 600 degrees C, specific surface area and total pore volume increased sharply while with an obvious decrease in the average pore diameter as a whole. The pH of different biochars presented an upward trend with increasing temperature, while the electrical conductivity and cation exchange capacity (CEC) showed a complex trend, among which the CEC of the biochars produced at 500 degrees C was the highest. The application of pecan biochars improved the water holding capacity of saline-alkali soil overall (except for the biochars produced from nut shells at 500 and 600 degrees C). The element analysis suggested that the organic carbon contents in the branch and nut shell biochars were significantly higher than those of leaf biochars; the branch and leaf biochars contained significantly higher total phosphate, available phosphate (AP) and zinc contents; the total potassium and available potassium (AK) levels were highest in the branch biochars, and the highest levels of total nitrogen and other metallic elements were obtained in the leaf biochars. Notably, the levels of some elements (AP, AK, calcium, magnesium, iron, manganese and copper) generally increased with increasing temperature. Pearson's correlation analysis showed the associations among the biochar properties, and principal component analysis demonstrated that the properties of pecan biochars were generally determined by the interactions of feedstock type and pyrolysis temperature. In conclusion, biochar with known properties produced from different feedstocks and pyrolysis temperatures could be chosen as a suitable soil conditioner or adsorbent applied to agricultural and environmental fields.