Study on Nitrogen and Phosphorus Distribution Characteristics by Spectrophotometry and Quantitative Source Analysis of Rivers With Different Land Use Types in Different Water Periods

Nitrogen and phosphorus are essential nutrients for the growth and reproduction of aquatic organisms and affect the primary productivity of the water body. The eutrophication level of the water body is closely related to the fractions of nitrogen and phosphorus. With the change in the water environment, the sediments will release nitrogen and phosphorus into the water body, causing secondary pollution. At the same time, quantitative identification of the contribution of external nitrogen and phosphorus pollution sources can effectively manage and control the nitrogen and phosphorus pollution load in the water body. Pihe River and Shiting River are important tributaries of Tuojiang River and affect the water quality of the Mother River of the Yangtze River. In this paper, the distribution characteristics of total nitrogen (TN), total phosphorus (TP) and various fractions of nitrogen and phosphorus in the water body and surface sediments of Pihe River and Shiting River in the upper reaches of Tuojiang River in the dry season and wet season were studied by molybdenum blue/ascorbic acid spectrophotometry and continuous extraction method. The behavior characteristics and release risk of nitrogen and phosphorus in rivers with different land-use types were compared. The APCS-MLR receptor model was used to identify and quantify the sources of nitrogen and phosphorus pollution. The results showed that: (1) nitrogen and phosphorus in the water and surface sediments in the study area were at different pollution levels. The main contributors of TP in the dry season were particulate inorganic phosphorus (PIP) and particulate organic phosphorus (POP), while in the wet season, it was particulate inorganic phosphorus (PIP) and dissolved inorganic phosphorus (DIP). The main contributors of TN in the two water periods were nitrate-nitrogen (NO3-N) and organic nitrogen (ON). In surface sediments, the main contributor of TP was calcium-bound phosphorus (HC1-P), and the main contributor of TN was acidolysis nitrogen (HN). In dry season and wet season, the average ratio of bioavailable phosphorus (BAP) in TP of surface sediments of Pihe River (19. 7% and 23. 0%) was higher than that of Shiting River (11. 0 % and 12. 5%), indicatinga a higher risk of phosphorus release. It was found that the nitrogen and phosphorus pollution degree in the dry season was higher than that in the wet season, and the nitrogen and phosphorus pollution degree in Shiting river was higher than that in Pihe River. (2) APCS-MLR model extracted four pollution source factors in the Pihe River, including urban domestic sewage, leachate generated by domestic garbage accumulation, decomposition of animal and plant residues and aquaculture wastewater. Among them, urban domestic sewage contributed the most to nitrogen and phosphorus pollution in the Pihe River (50. 9% in dry season and 54. 8% in wet season). At the same time, wastewater generated in industrial production, degradation of animal and plant residues and the weathering of agricultural waste, agricultural wastewater from farmland drainage channels and an unreasonable application of pesticides and fertilizers were five pollution source factors, among them, the wastewater produced in industrial production contributed the most to the nitrogen and phosphorus pollution of Shiting river (58. 7% in dry season and 55. 8% in wet season). Therefore, the relevant local departments should strengthen the management and control of high contribution pollution sources to reduce the basin's nitrogen and phosphorus pollution load.