The spatial and seasonal variability of nutrient status in the seaward rivers of China shaped by the human activities

Understanding the riverine nutrient status is of paramount importance to the management of coastal ecosystems. This study investigated the spatial and seasonal variability of nutrient status in the seaward rivers of China at a national scale, using the latest observational data (2020-2023). A total of 126 rivers were considered, covering nearly all rivers in China that enter adjacent seas. The average flux of total nitrogen (TN) to the sea amounts to about 8968 tons d-1, while the total phosphorus (TP) and ammonium (AN) flux are about 329 tons d-1 and 276 tons d-1, respectively. In general, the river nutrient fluxes are higher in summer, with the Southern China (SC) showing higher levels than the Northern China (NC). This pattern is largely regulated by the spatio-temporal characteristics of runoff. By contrast, the spatio-temporal pattern of riverine nutrient concentration differs from it slightly. Riverine TN/TP concentration typically shows higher levels in NC, peaking in winter/summer, while the spatio-temporal variations of AN are minimal. The coal-fired heating prevalent in NC during winter causes severe atmospheric nitrogen pollutions, contributing to the heightened TN concentration in local rivers. At the meantime, the vast cropland in NC creates a substantial demand for the phosphorus fertilizer, responsible for the elevated TP concentration in northern rivers. The seasonal variation of TP concentration is mainly driven by rainfall, which exacerbates the loss of phosphorus from croplands into nearby rivers, leading to a simultaneous increase in riverine TP and turbidity levels during summer. This process is particularly pronounced in NC, due to elevated phosphorus levels in croplands and more concentrated summer rainfall. The present study suggests the crucial role of human activities in shaping the riverine nutrient status in China, which will be of practical significance for the future environmental management.

Automated summary

Understanding the variation in nutrient status in rivers is crucial for the management of coastal ecosystems. This study investigated the spatial and seasonal variability of nutrients in rivers entering adjacent seas in China. The results showed that nutrient fluxes were higher in southern rivers, especially during summer. Total nitrogen/total phosphorus concentration in rivers were generally higher in northern regions, while ammonium concentration showed minimal variations. Coal-fired heating in winter and the demand for phosphorus fertilizer in croplands contributed to the elevated nutrient concentrations in northern rivers. Rainfall exacerbated the loss of phosphorus from croplands, leading to increased total phosphorus and turbidity levels in rivers during summer.