Both microbial abundance and community composition mattered for N2 production rates of the overlying water in one high-elevation river

Abstract

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.