title: Enhanced nitrogen loss from rivers through coupled nitrification-denitrification caused by suspended sediment.
abstract: Present-day estimations of global nitrogen loss (N-loss) are underestimated. Commonly, N-loss from rivers is thought to be caused by denitrification only in bed-sediments. However, coupled nitrification-denitrification occurring in overlying water with suspended sediments (SPS) where oxic and anoxic/low oxygen zones may coexist is ignored for N-loss in rivers. Here the Yellow and Yangtze Rivers were taken as examples to investigate the effect of SPS, which exists in many rivers of the world, on N loss through coupled nitrification-denitrification with nitrogen stable (15N) isotopic tracer simulation experiments and in-situ investigation. The results showed even when SPS was surrounded by oxic waters, there were redox conditions that transitioned from an oxic surface layer to anoxic layer near the particle center, enabling coupled nitrification-denitrification to occur around SPS. The production rate of15N2from15NH4+-N (R15N2-production) increased with increasing SPS concentration ([SPS]) as a power function (R15N2-production=a·[SPS]b) for both the SPS-water and bed sediment-SPS-water systems. The power-functional increase of nitrifying and denitrifying bacteria population with [SPS] accounted for the enhanced coupled nitrification-denitrification rate in overlying water. SPS also accelerated denitrification in bed-sediment due to increased NO3-concentration caused by SPS-mediated nitrification. For these two rivers, 1gL-1SPS will lead to N-loss enhancement by approximately 25-120%, and the enhancement increased with organic carbon content of SPS. Thus, we conclude that SPS in overlying water is a hot spot for nitrogen loss in river systems and current estimates of in-stream N-loss are underestimated without consideration of SPS; this may partially compensate for the current imbalance of global nitrogen inputs and sinks.
authors.affiliation: School of Environment, Beijing Normal University/State Key Joint Laboratory of Environmental Simulation and Pollution Control, Beijing 100875, China. Electronic address: xiaxh@bnu.edu.cn.
authors.forename: Xinghui
authors.initials: X
authors.lastname: Xia
authors.affiliation: School of Environment, Beijing Normal University/State Key Joint Laboratory of Environmental Simulation and Pollution Control, Beijing 100875, China.
authors.forename: Ting
authors.initials: T
authors.lastname: Liu
authors.affiliation: School of Environment, Beijing Normal University/State Key Joint Laboratory of Environmental Simulation and Pollution Control, Beijing 100875, China. Electronic address: zfyang@bnu.edu.cn.
authors.forename: Zhifeng
authors.initials: Z
authors.lastname: Yang
authors.affiliation: Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, IN 47907, United States. Electronic address: gmichals@purdue.edu.
authors.forename: Greg
authors.initials: G
authors.lastname: Michalski
authors.affiliation: Department of Geography, National University of Singapore, 1 Arts Link, Kent Ridge 117570, Singapore.
authors.forename: Shaoda
authors.initials: S
authors.lastname: Liu
authors.affiliation: School of Environment, Beijing Normal University/State Key Joint Laboratory of Environmental Simulation and Pollution Control, Beijing 100875, China.
authors.forename: Zhimei
authors.initials: Z
authors.lastname: Jia
authors.affiliation: School of Environment, Beijing Normal University/State Key Joint Laboratory of Environmental Simulation and Pollution Control, Beijing 100875, China.
authors.forename: Sibo
authors.initials: S
authors.lastname: Zhang
keywords: Coupled nitrification-denitrification; Nitrogen budget; Nitrogen loss; Rivers; Suspended sediment