Effects of nitrogen fertilizer on fluorine content in tea garden soil and tea shoots

A field experiment was conducted to determine the effects of nitrogen (N) fertilization on fluorine (F) contents in tea garden soils and tea shoots. The rates of nitrogen application were respectively 0 kg(N)hm^-2 (CK), 360 kg(N)hm^-2 (T1) and 720 kg(N)hm^-2 (T2). The dynamic changes of soil pH and contents of NH₄⁺-N, NO₃^--N and F in different?forms in tea garden soil were studied and F contents of shoots with one bud and four leaves, and shoots with one bud and five leaves were measured in spring, summer and autumn, respectively. The results showed that: 1) N application significantly decreased the content of water soluble F (F_W) and slightly decreased the content of exchangeable F (F_Ex) and F_Fe/Mn oxide-bound F (F_Ex/Mn) in tea garden soil in 20 to 30 days after N application. In 4550 days after N fertilization, the reduction in F_W slowed down while the contents of F_Ex and F_Ex/Mn increased. Compared with CK, the contents of various forms F in the 020 cm soil layer decreased under T1 treatment, while they increased under T2 treatment at the end of the experiment 164 days after N application. 2) Correlation analyses showed that for the 020 cm soil layer, NH₄⁺-N content was significantly negatively correlated with F_W and positively correlated with F_Fe/Mn (P < 0.01). Then for the 2040 cm soil layer, NO₃^--N content was significantly positively correlated with F_W and negatively correlated with F_Ex (P < 0.01). Soil pH had a significantly negative correlation with F_W (P < 0.01), but no significant correlation with the other three forms of F. F_Fe/Mn was significantly positively correlated with F_Ex and organic matter-bound F (F_Or) at P < 0.01, but no significantly correlated with F_W. 3) N application before and after spring tea harvest reduced F content in new shoots with one bud and four leaves and shoots with one bud and five leaves in spring, summer and autumn. However, the effect was not significant among different treatments. Under T1 condition, the largest reduction in F content in tea shoots was in summer tea (25.1527.95 mgkg^-1), followed by autumn tea (21.0624.31 mgkg^-1) and spring tea (18.5821.03 mgkg^-1). Under T2 condition, the largest reduction in F content in tea shoots was in autumn tea (18.6422.34 mgkg^-1), followed by summer tea (7.7914.14 mgkg^-1) and then spring tea (3.527.30 mgkg^-1). F content in tea shoots was mainly affected by soil inorganic N in the 020 cm soil layer and by soil pH in the 2040 cm soil layer. N application influenced tea root absorption of F and tea leaf accumulation of F, which regulated F content in new tea shoots. The study laid the theoretical basis for N fertilizing management in order to reduce F content in tea garden soils and in tea shoots.