Thermo Condition Determines the Uptake of Autumn and Winter Applied Nitrogen and Subsequent Utilization in Spring Tea (<i>Camellia sinensis</i> L.)

The effect of thermal condition on the uptake of autumn and winter applied N and its subsequent utilization in spring tea (Camellia sinensis) was investigated by applying 15N enriched urea as single or split applications between October and February in two commercial plantations at Xingyang of Henan province and Yongchuan of Chongqing with different thermal conditions. The proportion of N derived from 15N-labeled urea (Ndff%) in fibrous root and mature leaves 15 days after application at Xingyang and the Ndff% of mature leaves on the day of the first spring tea harvest at both sites were the highest in the single October application. The Ndff% of the following spring tea was also the highest in the single October application at both sites. The results showed that application of N fertilizer in October relative to other later months most significantly improves the accumulation of plant N reserves and consequently contributes more significantly to the early spring tea. Such timing effect was related to the thermal condition, i.e., the growing degree days (°C•d, T > 8 °C) between the dates of fertilization and harvest of young shoots, which represents the combining effect of the temperature and the residence time of N fertilizer in the soil