## 3 Results and Discussion
### Effect of Soil pH on Volatile Compounds in Tea Leaves  
The results of the analysis of the effect of soils with different pH values on the volatile compounds in tea leaves (Figure 1A, Table S3) showed that soil pH could significantly affect the amount and content of volatile compounds in tea leaves. When the soil pH was 3.29, 52 volatile compounds were detected in tea leaves at 22.966 ug/kg. When the soil pH was 4.13, 50 volatile compounds were detected at 31.142 ug/kg. When the soil pH was 4.74, 69 volatile compounds were detected at 46.238 ug/kg. When the soil pH was 5.32, 73 volatile compounds were detected at 60.084 ug/kg. A further analysis showed that the same 36 volatile compounds were detected in tea leaves grown in soils with different pH levels. Secondly, when the soil pH levels were 3.29, 4.13, 4.74 and 5.32, there were 7, 0, 2 and 11 compounds unique to the tea leaves, respectively (Figure 1B). It has been reported that when plants were subjected to external environmental stress, short-term stress stimulated the synthesis and accumulation of volatile substances in plants, while long-term stress was detrimental to plant growth and reduced the synthesis and content of volatile substances in plants [32; 33; 34; 35]. It is evident that acidification of tea-tree-planting soils has long affected the growth of tea trees, leading to decreases in the type and content of volatile compounds in tea leaves which, in turn, has affected the quality of tea leaves. A further analysis showed (Figure 1C) that the volatile compounds of the tea tree leaves could be mainly divided into seven categories, namely terpenoids, hydrocarbons, heterocycles, alcohols, ketones, aldehydes and esters, and the volatile compounds behaved most similarly to tea trees planted in soil with a pH of 5.32 and in soil with a pH of 4.74, followed by soils with pH levels of 4.13 and 3.29. Tea trees planted in soils with different pH levels had the highest contents of terpenoids in their leaves, with terpenoids accounting for 52.83% (pH 3.29), 65.98% (pH 4.13), 63.70% (pH 4.74) and 65.60% (pH 5.32) of the total volatile compounds, respectively. [PERSON] et al. [36] studied the terpenoid contents in the leaves of different tea tree varieties and found that terpenoids accounted for the highest proportion of volatile compounds in tea leaves, and these terpenoids played an important role in the formation of tea aroma. [PERSON] et al. [37] also found that the main aromat-contributing compounds in tea leaves were terpenoids when they studied the composition of tea aroma compounds. Many scholars have also concluded that aroma intensity was significantly related to the terpenoid contents in tea leaves and that the higher the content, the higher the quality [38; 39; 40]. [PERSON] et al. [41] found that the expression of 7 out of 10 terpenoid-synthesizing genes in tea leaves showed a trend of increasing with an increase in the soil pH. This suggests that the increase in the soil pH was favorable to the synthesis of terpenoids in tea leaves. In summary, the quantity and content of different types of volatile compounds in tea leaves changed significantly with changes in the soil pH, with the most obvious change occurring in the contents of terpenoids. Moreover, the terpenoid contents decreased significantly with the decrease in soil pH which, in turn, led to a decrease in the aroma intensity of the tea leaves.