Abstract: [Objective] In studies of nitrogen cycling of forest system, it is important to determine nitrate nitrogen, nitrite nitrogen, ammonia nitrogen and total nitrogen of tree stem-flow accurately, the aim of this paper was to develop a method to measure the contents of these chemical properties with UV spectrophotometer.[Method] Stem flow of 8 canopy tree species from Shenlongjia Mountain were selected to measure nitrate nitrogen (F) content in the stem-flow using 4 common methods, including UV spectrophotometry, nitrate electrode, phenol disulfonic acid and thymol, and to decide which method was the best for detecting nitrate nitrogen (F) content; the stem-flow was oxidize at 125℃ for 45 minutes by using refined potassium persulfate solution and total nitrogen (T) was then detected using UV spectrophotometer and nitrate cathode respectively. The stemflow was oxidized with potassium permanganate solution and the total content of nitrate and nitrite (B) was s then measured with UV spectrophotometer. The content of nitrite was calculated as B-F, and ammonia as T-B, then the content of ammonia was compared with that detected by the method of Nessler's reagent.[Results] The UV spectrophotometry was the best method among the 4 methods in detecting nitrate nitrogen, with an accuracy and a precision of 106.19%±8.26% and 3.0%, respectively; the accuracy and precision of nitrate cathode, thymol and phenoldisulfonic acid were 102.5%±12.08% and 2.0%, 107.44%±9.26% and 4%, 43.62%±26.39% and 12%, respectively. The accuracy and precision of total nitrogen (T) measured with UV spectrophotometry were 94.62%±2.19% and 2.0%; In contrast, the accuracy of nitrate electrode method was 98.93%±37.64%%, indicating that it was not applicable to some tree species. Accuracy and precision of nitrate and nitrite(B) measured with UV spectrophotometry were 100.97%±2.85% and 4.0%, indicating that it was perfect in detecting nitrate and nitrite. The precision of ammonia and nitrite calculated from T, F and B were up to 3.0% and 4.0% respectively, suggesting that our method was suited for detecting ammonia and nitrite.[Conclusion] Tree stemflow could be oxidized with potassium persulfate and potassium permanganate, then nitrate nitrogen (F), total nitrogen (T) and total content of nitrite and nitrate all can be measured directly with UV spectrophotometer, and nitrite and ammonia of stemflow could be calculated with them.

Key words: UV spectrophotometric, nitrate nitrogen, nitrite nitrogen, ammonia nitrogen, total nitrogen