Biochar addition to tea garden soils: effects on tea fluoride uptake and accumulation

Long-term consumption of tea with high fluoride (F) content has a potential threat to human health. The application of different amounts of biochar to reduce F accumulation in tea leaves has been little studied. In this study, a pot experiment was conducted to investigate the effect of biochar amounts (0, 0.5%, 2.5%, 5.0%, 8.0%, and 10.0%, w/w) on tea F content during the tea plant growth. Changes in tea quality, soil F fraction, and soil properties caused by biochar and the relationship with tea F accumulation were also considered. The results showed that the application of biochar amendment significantly reduced water-soluble F contents in tea leaves compared to CK (without biochar), especially in the 8.0% treatment (72.55%). Overall, biochar contributed to improving tea polyphenols and caffeine, but had no significant impact on free amino acids and water leachate. Compared with CK, 5.0-10.0% biochar significantly increased soil water-soluble F content due to the substitution of F- with OH- under high pH. Additionally, biochar applied to tea garden soil was effective in decreasing the soil exchangeable aluminum (Ex-Al) content (46.37-91.90%) and increasing the soil exchangeable calcium (Ca2+) content (12.02-129.74%) compared to CK, and correlation analysis showed that this may help reduce F enrichment of tea leaves. In general, the application of 5.0-8.0% biochar can be suggested as an optimal application dose to decrease tea F contents while simultaneously improving tea quality.