The Dominant Role of the Water Column in Nitrogen Removal and N2O Emissions in Large Rivers

Rivers are important sites for nitrogen (N) cycling processes. N removal is thought to occur predominantly in the benthic zone and the role of the water column is frequently neglected. Here, we find that the water-air and sediment-water fluxes of N-2 and N2O decreased, while the water column production increased with stream order through 4-year observations across six river networks in China. The water column contribution increased with stream order and accounted for more than 50% in rivers above fifth order. The increase in the contact area of suspended sediment (SPS)-water caused by higher SPS concentrations and river depths resulted in the shift as river size increased. N removal and N2O emissions would be underestimated approximately by 50% if neglecting water column processes for the six river networks. This study highlights the necessity and provides a pathway to incorporate water column processes into models to balance the global N budget.