Monthly nitrogen concentration anomaly dataset (2018-2024) at key monitoring sections of the Middle Route of the South-to-North Water Diversion Project of China

Due to climate change and human activities, the risk of water pollution, particularly nitrogen pollution, in large-scale water transfer projects such as the Middle Route of the South-to-North Water Diversion Project (MR-SNWDP) in China, has emerged as a critical environmental concern. Accurate monitoring and scientific assessment of nitrogen concentrations, along with an exploration of their variation characteristics and driving mechanisms, are curitial for effective water quality management. However, comprehensive datasets on nitrogen concentrations have been scarce. In this study, monthly total nitrogen (TN) and ammonia nitrogen (AN) concentration data were collected across 125 monitoring sections along the MR-SNWDP from 2018 to 2024 for the first time. Based on these data, the temporal and spatial characteristics as well as driving factors of TN and AN concentrations were revealed. Temporally, TN concentration showed a V-shaped seasonal anomaly pattern, while AN exhibited higher positive anomalies from 2018 - 2020 with peak shifts. Spatially, TN concentration anomalies were prominent in industrial and agricultural areas of Hebei province, and AN concentration anomalies were notable in specific cities. TN concentrations were highly sensitive to extreme temperatures (TXx, TNx, TXn, TNn) with a 6-month lag. Precipitation extremes (RX1day, RX5day) and streamflow discharge exerted contrasting effects on TN and AN concentrations, with lagged responses revealing multi-phase mechanisms (decline-rebound-drop for TN and rise-decline-resurgence for AN). Agricultural fertilizer application was a primary pollution source significantly correlated with AN concentrations. During 2018–2019, atmospheric nitrate and ammonia deposition both declined synchronously with TN/AN concentrations, with nitrate deposition showing strong spatial correspondence with TN. These nitrogen concentration data provide critical insights for understanding nitrogen pollution dynamics and managing water transfer systems.The monthly nitrogen concentration anomaly dataset consists of three files: Section Information.xlsx (4 columns and 125 rows; 4 columns: Serial Number, Section Code, Longitude, Latitude; 125 rows: 125 monitoring sections), Monthly Total Nitrogen Concentration Anomaly Sequences from 2018 to 2024.txt (85 rows and 127 columns; 85 rows: Information + 84 months of total nitrogen anomaly values; 127 columns: Year, Month, 125 monitoring sections), and Monthly Ammonia Nitrogen Concentration Anomaly Sequences from 2018 to 2024.txt (85 rows and 127 columns; 85 rows: Information + 84 months of total nitrogen anomaly values; 127 columns: Year, Month, 125 monitoring sections).