As a main agronomic intervention in tea cultivation, nitrogen (N) application is useful to improve tea yield and quality. However, the effects of N application on the formation of tea quality-related metabolites have not been fully studied, especially in long-term field trials. In this study, a 10-year field experiment was conducted to investigate the effect of long-term N application treatments on tea quality-related metabolites, their precursors, and related gene expression. Long-term N application up-regulated the expression of key genes for chlorophyll synthesis and promoted its synthesis, thus increasing tea yield. It also significantly increased the contents of total free amino acids, especially l-theanine, in fresh tea leaves, while decreasing the catechin content, which is conducive to enhancing tea liquor freshness. However, long-term N application significantly reduced the contents of benzyl alcohol and 2-phenylethanol in fresh tea leaves, and also reduced (E)-nerolidol and indole in withered leaves, which were not conducive to the formation of floral and fruity aroma compounds. In general, an appropriate amount of N fertilizer (225 kg/hm²) balanced tea yield and quality. These results not only provide essential information on how N application affects tea quality, but also provide detailed experimental data for field fertilization.

Land-use change is one of the most important driving factors of change in soil microbial diversity. Deforestation for tea plantations has transformed large areas of forestland in hilly areas of Southeast China. However, its impact on the soil fungal community structures and functions is still understudied. We compared the soil fungal communities and their functions in forestland (FD), a 3-year-old tea plantation (ZC3) and a 30-year-old tea plantation (ZC30) at 0–20 cm and 20–40 cm soil depths. The soil fungal community compositions and potential functions were analyzed using high-throughput sequencing techniques coupled with FUNGuild analysis. The results showed that the initial conversion from forestland to tea plantations significantly decreased soil fungal diversity. With an increase in the tea plantation age, the soil fungal diversity rebounded. The dominant phyla included Ascomycota, Basidiomycota, unclassified_k_fungi and Mortierellomycota, which were identified in all soil samples and accounted for 90% of all fungal communities. Non-metric multidimensional scaling analysis (NMDS) indicated that the soil fungal community was more responsive to the duration of tea planting than to the soil depth. FUNGuild analysis showed that the relative abundance of pathogenic and pathotrophic-saprotrophic fungi was higher while saprotrophs were lower in ZC30 compared with FD. Among the analyzed soil properties, soil available nitrogen, available phosphorus, available potassium and bulk density were the prime factors affecting the abundance and diversity of soil fungal community compositions. Network analysis showed that fungal microbial taxa increased positive interactions to enhance the adaptability of fungal microorganisms to long-term tea planting. Collectively, our results provide a clear view on the dynamic yet differential responses of fungal communities to land-use changes, and further emphasizes the need for long-term conventional tea plantations to adopt sustainable agricultural practices to reduce soil pathogenic fungi.