Hierarchical microbial distribution organized by ordered river structure explains why GHG emission favors small streams

Abstract

In river networks, small streams usually emit the largest proportion of greenhouse-gases (GHGs). However, the associated microbial mechanisms affected by the hierarchy of river systems are unclear. To address this, we measured N 2 O, CH 4 and CO 2 fluxes at 22 sites selected from 3rd- to 6th-order tributaries in the middle Yellow River Basin, and analyzed microbial communities, functional genes, aquatic chemistry and river network transport characteristics at different stream orders. Despite the higher richness of the overall microbial community in high-order (5th- and 6th-order) streams, GHG-producing taxa ( Nitrosomonas , Gemmatimonas , Pseudomonas , Trichococcus and Methanothrix ) were more abundant in low-order (3rd- and 4th-order) streams, as were the corresponding functional genes mediating N 2 O production ( hao and nirS genes), methanogenesis ( fmdA and ACS genes), and CO 2 conversion ( amyA gene). Our conceptual river network model indicates that dissolved solutes such as ammonium and nitrite accumulate in high-order streams due to sharp discrepancies in bifurcation ratio and watershed area ratio between high- and low-order streams, which inhibit the growth of GHG-producing bacteria.