Acceleration of Denitrification in Turbid Rivers Due to Denitrification Occurring on Suspended Sediment in Oxic Waters

Abstract

High suspended sediment (SPS) concentration exists in many rivers of the world. In the present study, the effects of SPS concentration on denitrification were investigated in airtight chambers with sediment samples collected from the Yellow River which is the largest turbid river in the world. Results from the nitrogen stable ((15)N) isotopic tracer experiments showed that denitrification could occur on SPS in oxic waters and the denitrification rate increased with SPS concentration; this was probably caused by the presence of low-oxygen microsites in SPS. For the water systems with both bed-sediment and SPS, the denitrification kinetics fit well to Logistic model, and the denitrification rate constant increased linearly with SPS concentration (p < 0.01). The denitrification caused by the presence of SPS accounted for 22%, 38%, 53%, and 67% of the total denitrification in systems with 2.5, 8, 15, and 20 g L(-1) SPS, respectively. The activity of denitrifying bacteria in SPS was approximately twice that in bed-sediment, and the denitrifying bacteria population showed an increasing trend with SPS concentration in both SPS and bed-sediment, leading to the increase of denitrification rate with SPS concentration. Furthermore, the denitrification in bed-sediment was accelerated by increased diffusion of nitrate from overlying water to bed-sediment under agitation conditions, which accompanied with the presence of SPS. When with 8 g L(-1) SPS, approximately 66% of the increased denitrification compared to that without SPS was attributed to denitrification on SPS and 34% to agitation conditions. This is the first report of the occurrence of denitrification on SPS in oxic waters. The results suggest that SPS plays an important role in denitrification in turbid rivers; its effect on nitrogen cycle should be considered in future study.

Denitrification occurring on suspended sediment in a large, shallow, subtropical lake (Poyang Lake, China)

“…SPS may support enhanced bacterial activity (heterotrophic and autotrophic) (Crump and Baross, 1996;Ochs et al, 2010;Plummer et al, 1987) and play an important role in the reactivity, transport and biological impacts of substances in aquatic environments (Turner and Millward, 2002). Therefore, SPS and the water column may provide similar essential conditions for denitrification occurring on suspended sediments (DSS), such as low-oxygen microsites, SPS-associated denitrifying bacteria, organic matter and nitrate (Garneau et al, 2009;Liu et al, 2013;Michotey and Bonin, 1997;Turner and Millward, 2002). Compared with bottom-sediments, SPS may be more advantageous for denitrification due to access to nitrate for denitrifying bacteria on SPS .…”

Responses of bacterial community structure and denitrifying bacteria in biofilm to submerged macrophytes and nitrate

“…Biological denitrification is the major pathway for nitrogen removal in most wetlands through the reduction of nitrate or nitrite to gases (nitrous oxide or dinitrogen)7, as nitrogen stored in sediment systems and macrophytes, algae and microorganisms might subsequently be released again8. Denitrification uses nitrogen compounds as alternative electron acceptors for energy production9.…”

The varying role of water column nutrient uptake along river continua in contrasting landscapes

“…However, a few studies of water column nutrient processing have shown that water column P uptake increases with stream order in a forested watershed (Corning et al 1989), and phytoplankton uptake constituted 16 % of total P uptake in a eutrophic river (Barlow-Busch et al 2006). In addition to assimilatory nutrient uptake, denitrification can occur within anoxic microsites associated with suspended sediments in river water (Liu et al 2013), providing another mechanism for water column nutrient uptake in rivers. These individual studies suggest that water column nutrient dynamics may contribute significantly to riverine nutrient uptake and supplement benthic nutrient processing in certain ecosystems, but fieldbased research across stream size gradients is needed to provide a mechanistic understanding of where and when water column processes constitute a substantial nutrient uptake pathway.…”