Abstract: Background, aim, and scope In recent years, with the rapid development of industry and agriculture, a large amount of nitrate has been brought into the soil through human activities. Nitrate pollution in rivers and groundwater has become a ubiquitous environmental problem. Material and methods In September 2017, a variety of samples were taken from the source of the Chanhe river to the confluence of Chanhe with the Bahe river. The collected samples include 6 river water, 7 surface soils and 7 plant tissues. All the samples were located by GPS. Cl-, SO42- and NO3- -N concentrations were measured on river water using ion chromatography (Dionex ICS-1000), and nitrogen isotope analysis was performed using a Flash EA coupled to Delta Plus continuous flow isotope ratio mass spectrometry system. Results Soil NO3- -N content ranges from 0.4 to 55.9 mg·kg-1. We found that the soil NO3- -N content in farming areas is significantly higher than those in other areas due to the use of fertilizers and manure in agriculture practice. Except for the farming areas, nitrate content in soil tends to decrease from the upstream source to the downstream urban areas. In forest areas, the soil δ15N-NO3- is significantly more negative, which are -5.7‰ and -5.9‰, respectively. In farming areas, soil δ15N-NO3- values reach 38.0. With the intensification of anthropogenic activities, the changes in soil nitrogen isotope values display a positive trend. The δ15N values in plants range from -2.1 ‰ to -4.6 ‰, showing a negative trend with the increase of altitude. The content of NO3- -N in Chanhe river varies from 3.2 to 6.4 mg · L-1 and δ15N-NO3- values in river water ranges from -1.4 ‰ -2.1 ‰. DiscussionThe changes in the upstream areas may be related to the rapidly developing tourism in the source area in recent years. The tourism industry will bring more man-made disturbances and increase the discharge of domestic sewage, which will affect the nitrate content of the river and δ15N-NO3- . Increasing usage of chemical fertilizers and manures in the farming areas year by year will inevitably input more nitrogen. After the natural environment changes, it will exert a certain influence on the nitrate content and the δ15N-NO3- in the Chanhe River. Unlike the previous study in 2008 and 2011, we did not find any paper mills and factories that could produce pollution and untreated sewage outlets along the Chanhe River. Therefore, this may be the reason why the nitrate content in the downstream areas has not increased. The reduced industrial pollution makes the water δ15N-NO3- significantly negative in our study. This suggests that the industrial pollution that has been neglected in previous years caused great impact on the nitrate of the Chanhe River. In recent years, remediation of the Chanhe River Basin has played an important role in the changes of nitrogen. ConclusionsThrough the study of the nitrate nitrogen isotope composition of river-soil-plant in the Chanhe river in September 2017 and comparing the 2008 and 2011 study on the water quality of the Chanhe river, the following conclusions are drawn: (1) Different types of land uses account for the change of soil nitrate content and δ15N-NO3- values. Soil nitrate content decreases as the following order: farming area> forest area> urban area. By contrast the soil δ15N-NO3- values change in a slightly different order: farming area> urban area> forest area. (2) The study combined with the concentration of nitrate and δ15N-NO3- in the river can find that the rapid development of tourism in the upper reaches of the Chanhe river pollute the river. In the middle reaches of the agricultural areas, the river is polluted by the factors of fertilization, the main source of pollution in downstream areas may be domestic sewage. Comparing the study on Chanhe river in 2008 and 2011, we find that the nitrate concentration in the upper reaches tends to increase obviously, while the lower reaches decrease. This can be attributed to the rapid development of the upstream tourism industry and the result of remediation of the downstream urban areas. Recommendations and perspectives This study is possible to identify and analyze the water-soil-plant nitrogenous matters which provides a theoretical basis for ecological monitoring and management in Chanhe river.
Keywords: river water; soil; plant; nitrate; nitrogen isotope