In-depth investigation of the spatial variations of DOM components in sediments under the influence of cascade dams and their interactions with microbial communities.

Small cascade dams drive spatial divergence in the composition of dissolved organic matter (DOM) in local sediments. Taking Xixi River in the southeast of China, a representative small cascade-dammed watershed, as an example, this study explored the spatial variations of DOM components and its interactions with microbial communities under the influence of cascade dams. Results revealed that DOM composition differed significantly, i.e., reservoirs were enriched in microbially derived protein-like components (C1: 21.5 ± 3.2 %; C3: 34.1 ± 8.5 %), while riverine regions were dominated by terrestrial humic-like material (C2: 31.1 ± 7.5 %). Bacterial α-diversity positively correlated with DOM freshness indices, i.e., β/α and BIX during the non-flood season (p < 0.05), with riverine sediments exhibiting 6-fold higher connectivity between bacterial OTUs and DOM fractions than reservoirs, indicating efficient carbon processing. Bacterial communities in reservoir areas exhibited marked enhancement of amino acid metabolism and lipid synthesis pathways (p < 0.001), alongside suppressed xenobiotic degradation gene expression, indicating that DOM components regulated functionality by modulating metabolic strategies. Structural equation modeling results highlighted the roles of TN and NO-N in stimulating primary productivity and microbial biomass, thereby increasing precursor substrates for humic substance condensation and abiotic humification. The phenomenon in turn suppressed microbial activity in reservoir areas by reducing DOM bioavailability, while protein-like DOM (C1/C3) exerted competitive inhibition on riverine communities. These results deciphered the diversity characteristics of bacterial communities and DOM and their biogeochemical interplay in cascade-dammed watersheds, providing theoretical foundations for ecological management of dam operations.