Both microbial abundance and community composition mattered for N₂ production rates of the overlying water in one...

Overlying water is another potential hotspot of nitrogen removal through anammox and denitrification reactions in river systems. However, N₂ production and the controlling factors have rarely been investigated in the overlying water of high-elevation rivers. This study analyzed the abundance and community of denitrifying and anammox bacteria as well as their effects on N₂ production rates in the overlying water of the Yellow River source region (elevation range: 2687-4223 m). Higher suspended particle concentrations remarkably promoted functional gene abundances of both denitrifying and anammox bacteria (r > 0.9, p < 0.01). N₂ production rates in overlying water samples ranged from 0.25 to 4.22 μmol N₂ L^-1 d^-1. The overlying water was estimated to contribute to 36.8% (on average) of riverine N₂ emission flux. Higher temperatures markedly accelerated N₂ production rates (p = 0.051). Moreover, N₂ production rates were positively related to both anammox and denitrifying bacterial abundances (p < 0.05), and such relationships were markedly affected by corresponding community compositions. The explanatory power of denitrifier abundance (R² = 0.56) for N₂ production rate variations was greatly elevated when it was integrated with community composition (R² = 0.92). This study highlights the significance of overlying water nitrogen removal in the Yellow River source region; moreover, the effects of both microbial abundance and community composition on riverine N₂ production rates should be considered in future research.