Positive effects of nitrogen fertilization on the flavor ingredients of tea (Wuniuzao), soil physicochemical properties, and microbial communities

Nitrogen fertilizer is crucial in tea plantation management, but its effects on tea quality, soil environment, and microbial communities under different fertilization schemes remain unclear. This study evaluated the impacts of six nitrogen application levels on tea quality, soil physicochemical properties, and microbial community structures, aiming to reveal the optimization effects of nitrogen application on the soil environment and its significance for sustainable agriculture. Results showed that nitrogen application (especially under N3 and N4 treatments) increased total free amino acid (TA) content, total catechin content, and caffeine content in tea leaves, but reduced the TP/TA ratio. Significant changes in soil physicochemical properties were observed, including a decrease in pH, an increase in electrical conductivity (EC), higher levels of available potassium (Ava K) and available phosphorus (Ava P), an increase in soil organic carbon (SOC) content, and a reduction in available nitrogen (Ava N) content. These changes indicate that nitrogen application optimized soil nutrient status to some extent but may also contribute to environmental issues such as soil acidification. Regarding soil microbial communities, higher nitrogen application led to an increase in beneficial bacteria abundance, a more complex microbial community structure, and tighter interaction networks. Mantal Test analysis indicated that the changes in tea leaf biochemical components under nitrogen fertilization were the result of the combined effects of soil physicochemical properties and key microbial communities. Furthermore, soil bacterial characteristics were more strongly correlated with changes in tea quality. Based on random forest model analysis, p__Proteobacteria (associated with total catechin and caffeine) and p__Firmicutes (associated with TP/TA) were the key microbial communities influencing tea flavor components, while soil EC (affecting total catechin and TP/TA) and SOC content (affecting caffeine) were the key soil physicochemical properties influencing tea flavor components. Overall, nitrogen application not only improved tea quality but also significantly impacted soil environment and microbial community structure, indicating that appropriate nitrogen application (194.58-223.61 kg N ha⁻¹) plays a vital role in enhancing tea quality and promoting sustainable management of soil ecosystems.