Spatial-temporal variation of nitrogen and diffusion ﬂ ux across the water-sediment interface at hydro- ﬂ uctuation belt of Danjiangkou reservoir in China

Based on overlying water and sediment samples collection from ﬁ fteen sites during July, September, November 2018 and January 2019 in hydro- ﬂ uctuation belt of Danjiangkou reservoir China – the variation of nitrogen (N) was studied. And the concentrations of NH 4 þ -N, NO 3 (cid:2) -N and NO 2 (cid:2) -N in the sediment, pore water and overlying water were determined to evaluate the diffusion ﬂ ux across the water-sediment interface. The results showed that the lowest sediment N concentrations was 36.54 mg/L in July, and the highest one was 145.93 mg/L in November. Spatially, the sediment N concentrations were higher in tidal soil and loam than sandy soil. According to diffusion ﬂ uxes of NH 4 þ , NO 3 (cid:2) and NO 2 (cid:2) , sediments at all sites tend to release N to the overlying water except sampling month of November, where sediment act as a sink of NO 3 (cid:2) . The highest release rates of NH 4 þ -N and NO 3 (cid:2) -N were 17.66 mg m (cid:2) 2 ·d (cid:2) 1 and 80.15 mg m (cid:2) 2 ·d (cid:2) 1 , respectively, which is much higher than the release rate of NO 2 (cid:2) -N (0.29 mg m (cid:2) 2 ·d (cid:2) 1 ). The ﬁ ndings indicate that hydro- ﬂ uctuation belt sediment contributes a lot to the nitrogen contents in the overlying water, and internal pollution is a main reason for the water quality deteriorate even eutrophication.


INTRODUCTION
Eutrophication is one of the most concerns in the field of water environment and water ecology. Excess nutrients accumulation (especially nitrogen and phosphorus) is of critical importance in the eutrophication of aquatic ecosystem.
Due to eutrophication, problems such as algae blooms, oxygen depletion, decreasing species biodiversity, aquatic ecosystem deterioration may occur (Smith & Schindler ; Chislock et al. ). Under the influence of human activities, eutrophication and hypoxia of aquatic ecosystem has become a problem of water pollution control worldwide. Nitrogen (N) play a dominant role in the biogeochemical cycle of aquatic ecosystem, the change of its content and proportion will affect the community structure of aquatic vegetations, the sediment nutrients distribution and energy transformation (Zhao et al. ). Many rivers and lakes worldwide have changed the trophic status due to large amount of nitrogen entering the water bodies (Xu et al. ), and some studies have estimated that anthropogenic N from grey footprints could contribute to 32.6 million tons annually to aquatic systems (Mekonnen & Hoekstra ). As one of the main limiting factors for eutrophication, N mainly comes from atmospheric deposition, nitrogen fixation and sediment retention et al. (Holland et al. ; Herridge et al. ). On the one hand, sediment can act as an important retention tank for nitrogen and purified the overlying water. On the other hand, take agricultural fertilization as an example, only 30%-40% of the fertilizer applied to farmland can be absorbed and utilized by crops, the excess nitrogen accumulated in the sediment are mobilized and released and result in eutrophication of the water body, even groundwater pollution (Luederitz et al. ). Thus, sediment play an important role in eutrophication process as well as restoration and water quality control.
Due to the uneven spatial-temporal distribution of water resource in China, plenty of water conservancy and hydropower projects have been constructed to achieve the optimal allocation of water resources and promote the sustainable development of social economy. However, water level fluctuation caused by reservoirs for electric power generation, irrigation and flooding control has been a common phenomenon worldwide. Under the long-term periodic fluctuation of water level, a large area of hydro-fluctuation belts has been formed. Hydro-fluctuation belt refers to the interface between terrestrial and aquatic ecosystems formed in the basin of reservoir due to water level fluctuation caused by hydrologic regulation (Gregory et al. ).

Hydro-fluctuation belts play a critical role in buffering and
filtering non-point source pollution, such as reservoir sediments, organic matter, agricultural fertilizer and wastewater. It is the last barrier to ensure the safety of reservoir water quality (Gregory et al. ; Zhang et al. a).
According to the characteristics of hydro-fluctuation belts, a large area of land is in the alternated state of drying-wetting annually, which has a change in soil physical-chemical characteristics, soil oxidation-reduction state, soil anaerobic-aerobic state and microbial environment (Ma et al. ; Erakhrumen ). However, after inundation, the problem is that nutrients, heavy metals and pesticides accumulated in the sediments are mobilized, released across the water-sediment interface and transported to overlying water, resulting in water environment deterioration (Ma et al. ). The change of water quality and sediment properties in hydro-fluctuation belts is closely related to the periodic fluctuation of water level, the highest values of total phosphorus, soluble reactive phosphorus, nitrate and chlorophyll a were found during minimum water level phase of reservoir (Geraldes & Boavida ).
Reservoirs, act as an indispensable role in water supply and water resources management, reservoir water quality protection is one of the most concern with the development of urbanization in China (Shi et al. ). Due to long water residence time, nutrients and mineral elements could accumulate in reservoir more easily than in running water (Szarek-Gwiazda & Mazurkiewicz-Boroń ). To date, some studies have examined the distribution characteristics and load of soil organic and nutrients in the hydrofluctuation belts, and gradually hydro-fluctuation belts have become a hot topic in the water environment domain.
The aim of this study was to study the spatial-temporal distribution of overlying water and sediment and investigate the diffusion process of nitrogen in the reservoir hydrofluctuation belt, which will provide a theoretical basis for managing the water environment of Danjiangkou reservoir.

Study area description
Danjiangkou reservoir is one of the world's largest water conservancy and hydropower projects. It is located in According to the soil type, the 15 sampling sites can be categorized into loam, sandy soil and tidal soil (4 sites for loam, 6 for sandy soil and 5 for tidal soil) ( Table 1). Field sampling campaigns were performed in July, September, November 2018 and January 2019 which represented two stages (drying and inundation). Also, the seasonally variation (summer, autumn and winter) can be studied. There was no obvious rain before sampling. Overlying water samples were collected in a 500 ml polyethylene bottles by using 60 ml syringes at approximately 10 cm below the water surface. Water samples were stored at 4 C in a car refrigerator for further process and analysis. Additionally, during each sampling campaign, the overlying water temperature, dissolved oxygen (DO, mg·L À1 ), pH and electrical conductivity (EC, μs·cm À1 ) were measured simultaneously in situ by a multi-parameter water quality meter (HORIBA, Japan).
Calibration of sensors was performed before measurement.
The sediment-water column samples were collected at each plot by a gravity sampler, the sediment samples were sealed in polyethylene plastic bags and kept at 4 C for further analysis. The sediment samples were separated for two parts, one part for the measurement of sediment physiochemical properties, which were freeze-dried, homogenized and ground to fine powder for analyzing TN, NH 4 þ -N, NO 3 À -N and NO 2 À -N, and the other one for sediment porewater analysis. Porewater was extracted from bulk sediment by centrifugation at 3,000 rpm for 30 minutes (TDZ4-WS, Shanghai Incorporation) and filtered through 0.45-μm cellulose acetate filters by low pressure vacuum.
Pore water could be extracted from greater volumes of sediment more rapidly, especially from sandy sediments.
The index of nutrient elements in overlying water and sediment of Danjingkou hydro-fluctuation belt includes the following: total nitrogen (TN), ammonium nitrogen (NH 4 þ -N), nitrate nitrogen (NO 3 À -N) and nitrite nitrogen (NO 2 À -N). And the index of nutrient elements in pore water includes NH 4 þ -N, NO 3 À -N as well as NO 2 À -N. Analysis methods were as follows. TN were determined by alkaline potassium persulfate digestion-UV spectrophotometry method. After sample filtering by a 0.45-μm cellulose acetate filters, NH 4 þ -N was determined by Nessler's reagent colorimetric method, NO 3 À -N was determined by UV spectrophotometry method (the chromogenic agent was phenol disulfonic acid) and NO 2 À -N was determined by UV spectrophotometry method (the chromogenic agent was sulfanilamide and n-(1-naphthyl)-ethylenediamine dihydrochloride). Sediment moisture content (W) was determined by a gravimetric method (dried 6 h at 105 C) until a constant weight was reached, the sediment porosity was calculated as follows: where W is the moisture content, d s is the sediment average densities (2.65 g cm À3 ) and the d w is the density of overlying water (1 g cm À3 ). Three replicates were done for each parameter and from each plot.

Diffusion flux
It's important to recognize the exchange processes of nutrients between the sediments and the overlying water in order to understand the impacts of polluted sediments on the aquatic environments. Generally, nutrients released from sediments and maintained in the pore water first, and then they are transformed and diffused into overlying water under the concentration gradient (Zhao et al. ). In the static water, the transport process across the sediment-water interface is dominant by the direct diffusion process. The diffusion flux can be estimated based on the measured concentration gradient between sediment pore water and over- where F is the flux of a solute with concentration C at depth x, φ is the sediment porosity on the surface, D w is θ is the tortuosity and @C=@x is the concentration gradient of chemical species between pore water and overlying water, which was calculated from sediment porewater at the depth of 1 cm minus the overlying water in the sediment surface.

RESULTS AND DISCUSSIONS
Physicochemical characteristics of sediments and overlying water The physical-chemical characteristics in the sediments and the overlying water from the hydro-fluctuation belt of Danjiangkou reservoir are shown in Table 2. As seen in Figure 2(a), we can find that the DO of overlying water in SS, TS and LS decreased with the increasing temperature except for the sampling in July, which has the highest DO concentrations at all sites. This may be attributed to the longer day time in summer, the photosynthesis of phytoplankton can increase more DO in the water column.
Additionally, since the monitoring time is daytime, with the phytoplankton photosynthesis stopped at night, the DO concentrations will be reduced significantly. However, despite that, the DO concentrations are much higher than water quality criteria (criterion V, 2 mg L À1 ) (GB3838-2002). contents at all sites were found in September (Figure 2(b)).
Because of the long duration inundation and sufficient contact between the sediment and overlying water, as well as the biogeochemical processes across the sediment-water interfaces, the sediment porosities in SS, TS and LS were fairly accepted.

Temporal and spatial variation of nitrogen in sediment
Many forms of nitrogen exist in sediment, which include NH 4 þ -N, NO 3 À -N, NO 2 À -N and some small-molecular organic       Table 3. The top surface area is an active zone for the sediments, the average porosities for three sites were: 51% for SS, 63% for TS and 64% for LS, which are less than 70%, thus the parameter m, Fr and θ 2 can be determined, the actual diffusion coefficients of nutrients were acquired.

Sediment-water interface nitrogen dynamics and fluxes
which include transformation (ammonification, nitrification and denitrification), net movement into sediments (adsorption) and release into overlying water (desorption) via physical and biogeochemical processes are influenced by a wide range of factors. Hydro-fluctuation belt, due to its drying-rewetting regime and microorganism activity, is a place of intense accumulation and recycling of nutrients.
The concentration ratio between sediment pore water and overly water can reflect the releasing of internal pollutants due to these closely relationship (Zhao et al. ). Based on the calculated diffusive fluxes across the sedimentwater interface of NH 4 þ -N, NO 3 À -N and NO 2 À -N (

CONCLUSIONS
In this study, the physiochemical characteristics of the sediment and the overlying water in the hydro-fluctuation belt of Danjiangkou reservoir were determined, the spatial-temporal variation of nitrogen in the sediment and surface water were studied, and the diffusive fluxes of nitrogen nutrients across the sediment-water interface were calculated.
(1) Due to the high concentration of NO 3 À -N and NH 4 þ -N in the sediment, the hydro-fluctuation belt sediment of Danjiangkou reservoir has become a potential source of inner pollution, and the lowest sediment nitrogen concentration (36.54 mg L À1 ) were observed in July, and the highest concentration (145.93 mg L À1 ) was in November, which was 3 times higher than the lowest value. These indicate that water quality deteriorate and eutrophication are more likely to occur in the reservoir in November.
(2) The main nitrogen components in the overlying water is (3) According the Fick's first law, the diffusive fluxes of NH 4 þ -N, NO 3 À -N and NO 2 À -N across the sediment-water interface were calculated. Sediments at all sites were sources of nitrogen nutrients to the overlying water except sampling month of November, where both sites were a relatively large sink of NO 3 À -N. NH 4 þ -N and NO 3 À -N, the release rates reach up to 17.66 mg m À2 ·d À1 and 80.15 mg m À2 ·d À1 , respectively, which is much higher than the release rate of NO 2 À -N (0.29 mg m À2 ·d À1 ).
The sediment of hydro-fluctuation belt contributes a lot to the nitrogen contents in the overlying water.

DISCLOSURE STATEMENT
The authors declare that they have no competing interests.