Sediment nitrogen form occurrence associated with its ecological risk in the Yangtze River Delta plain-river network

The Yangtze River Delta, renowned for its intricate waterway system and fertile soil, is a crucial ecological zone in China. However, anthropogenic activities have led to significant changes in sediment nitrogen (N) forms and associated ecological risks in this region. This study aims to comprehensively investigate the occurrence of various N forms in the sediment of the Yangtze River Delta plain-river network through field sampling and laboratory analysis and to assess their ecological implications. The results revealed that the concentration of total N in river water (TN_w) ranged from 1.24 to 5.96 mg/L, decreasing from northwest to southeast. The nitrate in river water (${\text{NO}}_3^{-}–N_w$) was the major species of TN_w, accounting for 67.97% of the total. The average values of TN, total carbon (TC), and total sulfur (TS) in the sediment were 1.97, 18.36, and 1.35 mg/g, respectively. The predominant N fractions in the sediment were residual nitrogen (RN) (72.42%) and hydrolyzable nitrogen (HN) (22.44%). The ammonia in river water (${\text{NH}}_4^{+}–N_w$) was significantly affected by the sediment N (p < 0.05). Residential density (RD) and industrial density (ID) were significantly positively correlated with the concentration of TN_w (p < 0.01), and ID was significantly positively correlated with increased RN (p < 0.05), which suggests that industrial wastewater may be a source of RN. The risk assessment revealed that 36% of the sediment samples were above the moderate TN pollution level. Furthermore, 94% of the sediment was contaminated with organic matter, with 66.7% specifically affected by organic N contamination. High riverine RN and ${\text{NO}}_3^{-}–N_w$ contents indicate serious ecological pollution risks in the complex river network area in the Yangtze River Delta. This study provides valuable insight into N cycling and associated ecological risks in the Yangtze River Delta plain-river network, which is crucial for developing effective management strategies to safeguard the ecological health of this vital region.