Comprehensive assessment of phytotoxic effects, morphology, chemical compositions, and water retention capacities of biochars

The exponential increase in global organic waste production necessitates the development of efficient, economically viable, and environmentally friendly recycling methods. Pyrolysis is among the modern techniques based on the conversion of organic waste to biochars. This research focuses on evaluating the chemical compositions, water retention capabilities, phyto-toxicity, and morphology of six distinct organic biochars. Phytotoxicity assessments were conducted by subjecting lettuce and barley to varying concentrations of each biochar to evaluate chemical toxicity, while watercress was utilized to test for gaseous phytotoxic substances. Notably, no adverse effects on the germination and productivity of cress or barley were observed. However, germination and fresh lettuce weight experienced significant reduction at a 6% concentration of biochar derived from tomato waste, olive pomace biochar, municipal sewage sludge biochar and a mixture of biochar from date seeds and tomato waste. Conversely, two biochars, namely biochar from industrial agri-food sewage sludge and biochar from date seeds, exhibited no negative impact on fresh weight and lettuce germination. Furthermore, the combination between biochars decreases phytotoxic effects. Interestingly, biochar derived from tomato waste exhibited the highest water holding capacity compared to other biochars and substrates, with an average exceeding 5 g H₂O per g DW. Electron microscope visualization revealed a partial inverse correlation between pore diameter and water retention. However, this relationship is not solely determined by pore size. Other critical factors, such as impurities lodged within the pores, significantly influence the water retention capacity, highlighting the complex interplay of multiple parameters in determining this property. This study emphasizes that the type of biomass and pyrolysis conditions play pivotal roles in determining the chemical, morphological, and phytotoxic properties of biochar. To harness these findings effectively it is recommended to apply these biochars at varying doses on multiple plant species.