Spatial Distribution and Coupling of Carbon, Nitrogen, Phosphorus, and Sulfur Functional Genes in a Highly Dam-regulated International River.

Cascade damming has profoundly altered river continuity and aquatic ecosystems; yet, its effect on the coupled cycling of nutrients, including carbon (C), nitrogen (N), phosphorus (P), and sulfur (S), remains unclear, despite their interrelations. Thus, we used multi-element analysis to elucidate the variation patterns, coupling relationships, and driving mechanisms of CNPS functional genes along the Lancang River, a representative cascade river, and to identify the core nutrients that warrant concern in the coupling cycle. The findings revealed that damming markedly enhanced microbial diversity, network complexity, and stability due to increasing resource availability, while also shifting communities from r- to k-strategists. Elevated levels of CNPS genes were detected in the reservoirs attributable to nutrient accumulation and hydraulic retention time (HRT), but an increase in functional genes in reservoirs correlated negatively with HRT. Besides, cascade damming expedited the growth of the top 30 genera and keystone species, which are the primary contributors to the nutrient cycling. In comparison to others, the C functional genes exhibited the greatest susceptibility to cascade damming, probably owing to their essential role in metabolism and the multiple metabolic pathways involved. The networks of functional genes exhibited that cascade damming markedly increased complexity and coupling, with C-related genes identified by Zi-Pi analysis serving as pivotal functions for the coupling of multiple nutrients, thereby enhancing connectivity and diversity interaction among genes. These findings offer novel insights into microbial functional responses crucial for understanding the biogeochemical cycling of CNPS in cascade-damming rivers.