Effects of input of terrestrial materials on photodegradation and biodegradation of DOM in rivers: The case of Heilongjiang River

The input of terrestrial materials is an essential source of dissolved organic matter (DOM) in rivers. The mechanism of the influence of terrestrial materials on the DOM cycle in the water column is still unclear. In this study, microbial degradation and photodegradation processes of DOM were evaluated after the input of terrestrial materials into the water column. The input of terrestrial materials promotes the biodegradation and photodegradation process of DOM. PARAFAC results indicated that terrestrial sources of DOM contained more humic-like fractions. C2 and C3 exhibited higher fluorescence intensity after the input of terrestrial materials. The results indicate that more humic fractions are photodegraded. Based on the 16S rRNA analysis results, the input of terrestrial material shifted the microbial community and altered microbial abundance. The network analysis results showed that the microorganisms exhibited completely different degradation mechanisms under light avoidance and exposure conditions. Terrestrial humic fractions can be photodegraded into bioavailable substrates to stimulate microbial metabolism. After the input of terrestrial material, random forest modeling and structural equation modeling screened and validated the critical environmental variables affecting the DOM cycle. The above discoveries may be beneficial for identifying the fate of terrestrial DOM in the water column and predicting future DOM cycling processes in the water column.