Basin characteristics determine longitudinal distribution and source composition of nitrogen pollution in a multi-tributary mountain river basin

• MixSIAR showed that cropland contributed nearly 50% of nitrate in Hanjiang River. • Contribution rates of wastewater and forest & grassland ranged from 20% to 30%. • BC indirectly affected nitrate source distribution by directly affecting LU and HA. • Contribution rate of each LU type was related to area proportion & pollutant loads. Longitudinal distribution and source identification of nitrogen pollution are important for the basin delicacy management and pollution source control. The upstream Hanjiang River Basin was divided into 95 sub-basins based on topography, then 190 riverine water samples in two seasons and end-member samples of nitrate pollution sources were collected to conduct a spatial analysis of various forms of riverine nitrogen and source composition of nitrate along the river course. The concentrations among ammonium nitrogen, nitrate nitrogen and nitrate nitrogen isotope in the mainstream gradually increased from the headwater to riverine sections of Hanzhong City and the Huangjinxia Reservoir. However, concentrations did not increase continuously from the Huangjinxia Reservoir but fluctuated until the river entered the estuary in Danjiangkou Reservoir. Results of MixSIAR in April and August indicated that cropland contributed nearly 50% of the nitrate, while the contribution rates of both wastewater and forest & grassland ranged from 20% to 30%. Results of the random forests model and structural equation modeling showed that basin characteristics indirectly affected the distribution of nitrate sources by directly influencing land uses and human activities. Watershed area, perimeter and total length of river network were positively correlated with the contribution rate of wastewater, while altitude and slope were positively correlated with the contribution rate of cropland and forest & grassland because basin characteristics such as topography, slope and altitude influenced land uses by affecting human activities. This study provided insight for understanding the cumulative and changing process of nitrate pollution along the course of rivers.