Interaction and variability of silicon and carbon associated with particulate matter in the Changjiang and Yellow Rivers

The interrelated biogeochemical cycles of silicon and carbon (Si-C) are essential for the effective functioning of aquatic ecosystems. However, there exists a notable gap in methodological research addressing the quantitative transport dynamics of Si-C within riverine systems. This study examines the spatiotemporal variations, sources, and export mechanisms of Si-C in the Changjiang and Yellow Rivers. Our results reveal that biogenic silica (BSi) concentrations are significantly affected by dam operations, while dissolved silicate (DSi) concentrations exhibit a modest downstream decline. Moreover, BSi associated with suspended particulate matter (SPM) shows an inverse relationship with SPM concentration. Particulate organic carbon (POC) in the Changjiang River predominantly originates from terrestrial C3 plants, whereas in the Yellow River, there is a greater contribution from C4 plants. Variations in the stoichiometric ratios of POC to nitrogen (C/N_bulk) and BSi suggest that carbon degradation during river transport is more pronounced than in estuarine environments. Additionally, the δ¹³C of bulk POC (δ¹³C_bulk) in the river exhibits greater sensitivity to environmental changes compared to the δ¹³C associated with BSi (δ¹³C_BSi). We propose a normalization-based methodology to assess the potential decay rates of POC and organic carbon associated with BSi within river systems. Carbon is more readily mineralized from particulate matter than from that associated with silicon, and the coupled Si-C dynamics can be employed to explore their differential behaviors and the role of silicon in carbon preservation. This study provides valuable insights into Si-C dynamics in river-estuary systems, particularly under the increasing influence of anthropogenic activities.