Abstract: In order to understand the situation of surface water nitrogen pollution in Tianjin, this paper analyzed the nitrogen (nitrate, ammonium, and nitrite) concentrations in the river, reservoir, salt water, and sewage, and the isotopic composition of nitrate and ammonium nitrogen in most water samples. The NO^-₃-N concentration in the waters ranged from 0.01 mg·L^-1 to 16.51 mg·L^-1, with a mean value of 2.25 mg·L^-1. Some waters had the NO^-₃ concentration >50 mg·L^-1 (11.29 mg·L^-1 of NO^-₃-N), showing their high nitrate pollution. The NH⁺₄-N concentration in the waters was from 0 to 10.60 mg·L^-1, with a mean value of 1.15 mg·L^-1. One fourth of the waters had the NH⁺₄-N concentration >1.5 mg·L^-1. About 1/12 of the waters had higher NO^-₂-N. In most water samples, nitrate was the dominant species of inorganic nitrogen. Ammonium was the main nitrogen species in some waters, especially in sewage. The δ¹⁵NO^-₃-N value ranged from -5.5‰ to +28.6‰ (n=49, mean +9.0‰, median +8.5‰), and the δ¹⁵NH⁺₄-N value ranged from -8.8‰ to +34.3‰ (n=11, mean +11.6‰, median +13.6‰). All the results suggested that domestic sewage was the main source of surface water nitrogen pollution in Tianjin, and the nitrogen isotopic composition was obviously affected by nitrification, ammonia volatilization, and denitrification.

Key words: Larch plantation, Rhizosphere soil, Organic phosphorus forms, Availability