Contrasting effects of ecological restoration and sewage input on the carbonate system variability in an urban river on the Loess Plateau

Abstract

Intensive human disturbances have led to complex internal variations in the riverine CO₂ system, especially in the urbanized rivers in semi-arid areas. This study examines the factors controlling carbonate system in a river draining an urban ecological restoration area on the Loess Plateau, North China, where riverine dissolved inorganic carbon (DIC) and CO₂ partial pressure (pCO₂) exhibited pronounced variations influenced by land use changes. The most notable changes included a decrease in DIC (24.0 mg L⁻¹) and pCO₂ (280 μatm) in the urban restored area with damming, driven by strong aquatic photosynthesis, and a sharp increase in DIC (55.7 mg L⁻¹) and pCO₂ (6,222 μatm) in the urban downstream, attributed to sewage input. In contrast, in the upstream of the urban restored area, DIC (33.8 mg L⁻¹) and pCO₂ (723 μatm) were mainly controlled by carbonate weathering. Overall, this river acted as a strong atmospheric CO₂ source, with an average CO₂ emission of 450.1 mmol m⁻² d⁻¹, slightly higher than the global riverine average (359.4 mmol m⁻² d⁻¹), and exhibited pronounced spatial variability. A CO₂ sink (−11.2 mmol m⁻² d⁻¹) developed in the urban restoration area, whereas sewage input at urban downstream transformed the river into a strong CO₂ source (1,668.5 mmol m⁻² d⁻¹), highlighting the impact of anthropogenic perturbations on riverine CO₂ source - sink dynamics. Therefore, enhancing urban ecological restoration while effectively reducing CO₂ emission from sewage is crucial for achieving CO₂ neutrality in urban rivers, particularly in arid and semi-arid climates.