Fertilizer Effects on the Nitrogen Isotope Composition of Soil and Different Leaf Locations of Potted Camellia sinensis over a Growing Season

The nitrogen-stable isotopes of plants can be used to verify the source of fertilizers, but the fertilizer uptake patterns in tea (Camellia sinensis) plants are unclear. In this study, potted tea plants were treated with three types of organic fertilizers (OFs), urea, and a control. The tea leaves were sampled over seven months from the top, middle, and base of the plants and analyzed for the delta N-15 and nitrogen content, along with the corresponding soil samples. The top tea leaves treated with the rapeseed cake OF had the highest delta N-15 values (up to 6.6 parts per thousand), followed by the chicken manure, the cow manure, the control, and the urea fertilizer (6.5 parts per thousand, 4.1 parts per thousand, 2.2 parts per thousand, and 0.6 parts per thousand, respectively). The soil treated with cow manure had the highest delta N-15 values (6.0 parts per thousand), followed by the chicken manure, rapeseed cake, control, and urea fertilizer (4.8 parts per thousand, 4.0 parts per thousand, 2.5 parts per thousand, and 1.9 parts per thousand, respectively). The tea leaves fertilized with rapeseed cake showed only slight delta N-15 value changes in autumn but increased significantly in early spring and then decreased in late spring, consistent with the delivery of a slow-release fertilizer. Meanwhile, the delta N-15 values of the top, middle, and basal leaves from the tea plants treated with the rapeseed cake treatment were consistently higher in early spring and lower in autumn and late spring, respectively. The urea and control samples had lower tea leaf delta N-15 values than the rapeseed cake-treated tea and showed a generalized decrease in the tea leaf delta N-15 values over time. The results clarify the temporal nitrogen patterns and isotope compositions of tea leaves treated with different fertilizer types and ensure that the delta N-15 tea leaf values can be used to authenticate the organic fertilizer methods across different harvest periods and leaf locations. The present results based on a pot experiment require further exploration in open agricultural soils in terms of the various potential fertilizer effects on the different variations of nitrogen isotope ratios in tea plants.