Study on nitrogen discharge from farmland surface sources in Changzhou City based on improved output coefficient approach

Quantitatively analyzing the spatial and temporal distribution tendencies of non-point source (NPS) pollution is of vital significance for the management and control of NPS pollution in China. Taking the total nitrogen (TN) loss of farmland in Changzhou City, a representative city in the plain river network area, as an example, the TN inflow coefficient has been calculated based on the traditional output coefficient model by introducing rainfall erosion factor, terrain influence factor, surface runoff factor, soil leaching factor, and vegetation interception factor. Based on the classification of farmland planting patterns, the TN loss and flow generated were calculated, and the main pollution types of farmland planting patterns in Changzhou City were identified. The results showed that the inter-annual variation of TN inflow coefficient in Changzhou City was not significant, but the spatial and temporal variation was obvious. In 2010 and 2015, the TN inflow coefficients in western Changzhou City were higher than in eastern Changzhou City, while in 2020, the TN loss coefficient in western Changzhou City was approximately the same as in eastern Changzhou City. The planting patterns were divided into 27 categories, with the top three in terms of TN loss being rice-oil rotation in plain paddy fields, rice-wheat rotation in plain paddy fields, and other types of planting in plain paddy fields. The three planting patterns in plain paddy fields accounted for 80.54%, 81.67%, and 77.62% of the TN loss in Changzhou City in 2010, 2015, and 2020, respectively. From the annual variation of TN loss, the rice-wheat rotation pattern in plain paddy fields produced the highest TN loss in 2015, at 1 220.98 t. The TN loss and river discharge in Liyang City and Jintan District were significantly higher than those of other districts in Changzhou City, and these two areas contributed an average of 64.61% of the TN loss and 68.85% of the TN river discharge in 2010, 2015, and 2020. Therefore, Liyang City and Jintan District should be the priority targets for controlling agricultural NPS pollution in Changzhou City.