Effect of Fertilization Timing on Nitrogen Uptake in Spring Tea of Different Sprouting Phenological Cultivars: A Field Trial with ¹⁵N Tracing

Applying a top dressing of nitrogen fertilizer before harvesting spring tea is vital for producing high-quality spring tea. However, the interaction between the sprouting phenological characteristics of various cultivars and the timing of top dressing remains unclear. A field trial was conducted to investigate such interaction. Urea enriched with ¹⁵N was applied to soil of the early-sprouting cultivar Jia-ming-1 (JM1) and the late-sprouting cultivar Tie-guan-yin (TGY) on 29 January (early application, EApp) or 10 March (late application, LApp), respectively. The bud density and yield of young spring shoots were significantly decreased in LApp compared to EApp. Such differences were more remarkable in the early-sprouting cultivar (JM1) than in the late-sprouting cultivar (TGY). The N_dff (N derived from ¹⁵N-enriched urea) in mature leaves and young spring shoots as well as the amount of ¹⁵N in young spring shoots were all higher in EApp than in LApp. N_dff in both mature leaves (R² = 0.99, p < 0.001) and young spring shoots (R² = 0.61–0.89, p < 0.01) could be well predicted by the growing degree days of the duration between the N fertilization and sampling. N_dff and ¹⁵N concentrations in mature leaves were significantly correlated with the content of nitrate and the ratio of ammonium to total inorganic nitrogen. Partial least squares path modeling revealed that thermal condition directly affected soil N supply and soil pH and thereby affected N_dff in mature leaves and young spring shoots. Our findings highlight the importance of early pre-spring topdressing of N fertilizer to improve the yield and N use efficiency of spring tea in both early- and late-sprouting tea cultivars. The work identified a synergistic effect of N uptake by tea plants, N transformation, and soil pH related to the thermo-conditions of early and late N topdressing.