Temperature has an enhanced role in sediment N2O and N2 fluxes in wider rivers

Abstract

Riverine N₂O and N₂ fluxes, key components of the global nitrogen budget, are known to be influenced by river size (often represented by average river width), yet the specific mechanisms behind these effects remain unclear. This study examined how environmental and microbial factors influenced sediment N₂O and N₂ fluxes across rivers with varying widths (2.8 to 2,000 m) in China. Sediment acted as sources of both N₂O and N₂ emissions, with both N₂ (0.2 to 20.8 mmol m^-2 d^-1) and N₂O fluxes (0.7–54.2 μmol m^-2 d^-1) decreasing significantly as river width increased. N₂ fluxes were positively correlated with denitrifying bacterial abundance, whereas N₂O fluxes, when normalized by the abundance of denitrifying bacteria, were negatively correlated with the abundance of N₂O-reducing microbes. Water physicochemical factors, particularly temperature and nitrate, were more important drivers of these fluxes than sediment factors. Nitrate significantly increased denitrifying bacterial abundance, whereas higher temperatures enhanced cell-specific activity. Lower N₂O and N₂ emissions in wider rivers were attributed to decreased denitrifying microbial abundance and lower denitrification rates, in addition to the commonly assumed reduction in exogenous N₂O and N₂ inputs. Rolling regression analysis showed that nitrate concentration had a stronger effect on sediment N₂O and N₂ fluxes in narrower rivers, whereas temperature was more influential in wider rivers. This difference is attributed to more stable nitrate concentrations and decreased nitrogen removal efficiency in wider rivers, while temperature variation remained consistent across all river widths. Beyond sediments, temperature had a greater effect on excess N₂O concentrations than nitrate in the overlying water of wider rivers (>165 m), highlighting its broader impact. This study provides new biogeochemical insights into how river width influences sediment N₂O and N₂ fluxes and highlights the importance of incorporating temperature into flux predictions, particularly for wider rivers.