Combined Application of Magnesium Fertilizer and Biochar Boosts Tea (Camellia sinensis) Quality Through Soil Quality and Photosynthetic Traits Improvement

The application of biochar and magnesium (Mg) fertilizer can alleviate acidification, improve soil fertility, and enhance tea quality. However, the effects of different biochar application rates on soil properties, tea photosynthesis, and quality remain unclear. To elucidate their effects, a 90-day pot experiment was conducted with nine treatments: conventional fertilization (CK), improved conventional fertilization (CF), Mg fertilizer (MF), low/medium/high biochar (LBCF, MBCF, HBCF), and their combination with Mg (LBMF, MBMF, HBMF) in a controlled growth chamber, and soil properties, tea plant photosynthesis, and tea quality were measured. The results showed that higher application rates of biochar with Mg significantly alleviated soil acidification, increasing pH by 0.20–0.24 units. HBMF significantly enhanced key soil nutrients: total carbon (+198.11%), total nitrogen (+116.43%), and ammonium nitrogen (+73.89%). Ex-Mg increased with Mg addition and was further promoted by biochar, and photosynthetic rates increased by 71.69–101.41% in biochar and Mg application. These improvements increased free amino acids, caffeine, and soluble sugars in tea leaves by 39.56%, 46.88%, and 30.79% respectively under HBMF. Additionally, the relative contents of alcohols and esters increased by 93.29% and 62.20%, with marked rises in linalool and methyl salicylate. The increased tea quality is linked to higher activities of β-glucosidase and lipoxygenase observed in HBMF. In conclusion, the combined application of high biochar (2% of soil weight) and Mg fertilizer (15% of fertilizer weight) effectively improved soil quality, enhanced photosynthesis, and ultimately elevated both the taste and aroma quality of tea.