Cascade damming alleviates imbalanced particulate C: N: P stoichiometry to regulate methane accumulation in the upper...

Cascade damming and changes in land use are significant human activities that alter natural flow patterns and the biogeochemical cycles of carbon (C), nitrogen (N), and phosphorus (P). However, there is limited understanding of how damming and other terrestrial human activities affect stoichiometric imbalances, contributing to uncertainties in the processes and mechanisms of methane (CH₄) emissions from river-reservoir systems. Here, we discuss the spatiotemporal patterns of particulate C: N: P mole ratios and dissolved CH₄ in the upper Yangtze River, which covers 11 large dams along its main stem. The particulate C: N ratio increased with particle size, but C: P and N: P ratios were synchronous and showed adverse trends. Autochthonous (A-POC) and terrigenous (T-POC) particulate organic carbon to the bulk POC accounted for 44 % and 54 %. The POC/PON close to the Redfield ratio was predominantly influenced by coupling effects between damming and terrestrial anthropogenic activities. An increase in terrestrial anthropogenic activities tended to balance POC/PON but unbalance POC/PP and PON/PP. The cascade damming effect can alleviate stoichiometric imbalances and contribute about 15 %, 61 %, and 98 % for POC/PON, POC/PP, and PON/PP close to balance. The imbalances of POC/PP and PON/PP would be gradually amplified downstream in the cascade reservoir. A-POC can lead to short-term CH₄ accumulation, but T-POC with higher POC/PON, lower POC/PP, and PON/PP could potentially regulate long-term CH₄ accumulation. Particulate C: N: P close to balance and CH₄ oxidation before the dams reduced CH₄ accumulation in the upper Yangtze River. These insights have substantial potential implications for the adaptive management of cascade reservoir systems.