Synthetic nitrogen fertilizers alter the soil chemistry, production and quality of tea. A meta-analysis

The intensive use of synthetic nitrogen fertilizers over the last century has both increased agricultural productivity and modified biogeochemical cycles in terrestrial ecosystems, causing severe negative environmental impacts. Tea (Camellia sinensis L.) plantations usually receive high levels of synthetic fertilizers, which strongly affect plant and soil properties. However, there is no quantitative study to assess how synthetic N additions affect soil chemistry and the production and quality of tea shoots. Here, we conducted a comprehensive meta-analysis to evaluate the effects of experimental synthetic N fertilizers. Our main findings are (1) N additions in tea plantations acidify soils (-0.41 pH unit in average) and produce soil nutrient imbalance. Soil acidification commonly exacerbates the accumulations of toxic aluminum ions. (2) Synthetic N fertilizer additions may strongly increase tea production by almost 70% but alter tea shoot quality by increasing the concentrations of free amino acids (+ 16%), caffeine (+ 14%), and water extracts (+ 5%) while decreasing those of soluble sugars (-8%) in the tea shoots. The responses of soil chemistry, tea production, and quality to N additions can vary among experimental conditions, tea tree species, and N fertilizer forms. Because there is statistical limitation in this meta-analysis, our findings recommend performing additional field studies to explore the potential mechanisms of nutrient cycling and ecosystem functioning under synthetic N additions. The development of a sustainable N management strategy in tea plantations is also urgently needed to enhance N use efficiency and reduce environmental risks.