Absorption of foliar-applied urea-15N and the impact of low nitrogen, potassium, magnesium and sulfur nutritional status in tea (Camellia sinensis L.) plants

Foliar urea application provides an alternative strategy for minimizing the risk of nitrogen (N) over-fertilization in green tea (Camellia sinensis L.) production. Solution and pot experiments were conducted with the objective to evaluate the utilization efficiency of foliar applied urea and the impact of plant nitrogen (N), potassium (K), magnesium (Mg) and sulfur (S) nutritional status. In the solution experiment, the dynamic absorption and translocation of foliar N was followed by scheduled samplings at 2 and 6 h, 1, 2, 3, 7 and 13 d after application. In the pot experiment, foliar urea-N absorption was measured for tea plants supplied with adequate nutrients (Control, CK) or low levels of respective nutrients (–N, –K, –Mg, and –S) in soil. The results showed that foliar- ¹⁵N uptake rate was the highest within the earliest 6 h and the ¹⁵N amount in treated leaves reached a maximum within 2 d after application in the solution experiment. The absorbed ¹⁵N was mainly transported to young shoots, and the export rate became significant after 1 d. The total ¹⁵N absorption in plants increased significantly within 2 d and increased slightly but constantly afterwards until 13 d. In the pot experiment, the low supply of nutrients except S significantly decreased their concentrations in mature leaves. The N dff values in urea-treated leaves and young shoots were 8.1–16.4% and 7.1–19.2%, respectively, and were the highest in the –N treatment compared to the other treatments, suggesting the most remarkable contribution of foliar N application to the N status of –N plants. Low N (–N) supply reduced the total ¹⁵N absorption and its allocation to young shoots. Low K (–K) supply significantly diminished the ¹⁵N amount in young shoots without affecting the total absorption. Total ¹⁵N absorption in tea plants was decreased by low S (–S) but unaffected by low Mg (–Mg) status. The amount of ¹⁵N in the young shoots correlated closely (r = 0.94, p < 0.001) with its biomass and also with the biomass of whole above-ground plants, to a lesser extent (r = 0.51, p < 0.05). It was concluded that foliar urea-N was rapidly absorbed and exported mainly to the young shoots, representing the major sink. The utilization efficiency of foliar-N was reduced by depleted N, K and S nutritional status which weakened the N sink strength resulting from poor young shoot growth.