Nitrous oxide emissions from cascade hydropower reservoirs in the upper Mekong River

Journal Pre-proof Nitrous oxide emissions from cascade hydropower reservoirs in the upper Mekong River Wenqing Shi, Qiuwen Chen, Jianyun Zhang, Dongsheng Liu, Qitao Yi, Yuchen Chen, Honghai Ma, Liuming Hu PII:

S0043-1354(20)30118-4

DOI:

https://doi.org/10.1016/j.watres.2020.115582

Reference:

WR 115582

To appear in:

Water Research

Received Date: 23 September 2019 Revised Date:

30 January 2020

Accepted Date: 1 February 2020

Please cite this article as: Shi, W., Chen, Q., Zhang, J., Liu, D., Yi, Q., Chen, Y., Ma, H., Hu, L., Nitrous oxide emissions from cascade hydropower reservoirs in the upper Mekong River, Water Research (2020), doi: https://doi.org/10.1016/j.watres.2020.115582. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

1

Nitrous oxide emissions from cascade hydropower reservoirs in the upper Mekong River

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Wenqing Shia,b, Qiuwen Chena,b,*, Jianyun Zhanga, Dongsheng Liub, Qitao Yic, Yuchen Chenb,

3

Honghai Mab, Liuming Hub

4 5

a

6

Hydraulic Research Institute, China.

7

b

Center for Eco-Environment Research, Nanjing Hydraulic Research Institute, China.

8

c

School of Earth and Environment, Anhui University of Science and Technology, Huainan

9

232001, China.

10

*

State Key Laboratory of Hydrology-Water Resources & Hydraulic Engineering, Nanjing

Corresponding Author: Qiuwen Chen ([email protected]).

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Abstract

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Nitrous oxide is a powerful greenhouse gas, and its emissions from single reservoirs have been

13

extensively studied; however, it still remains unclear about nitrous oxide emission patterns in

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cascade reservoirs. In this study, nitrous oxide emissions from cascade hydropower reservoirs

15

were investigated using the thin boundary layer model in the heavily dammed upper Mekong

16

River. Meanwhile, sediment denitrification for nitrous oxide production was analysed using the

17

stable isotope method and the quantitative polymerase chain reaction method. Our results

18

demonstrated that nitrous oxide emissions (0.47–1.08 µg m-2h-1) in the upper Mekong River were

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much lower than the global mean level (19.60 µg m-2h-1), but were increased by dam

20

constructions; nitrous oxide emissions exhibited an increase trend along the flow direction in the

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cascade reservoirs. Sediment accumulation by dams supplied sufficient nitrogen substrates and

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organic carbon, creating hotspots of denitrification at the transition zone in reservoirs. As the

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elevation decreased, the increase in temperature enhanced microbial denitrification at the active

24

zone, and thereby increased nitrous oxide production with the prolonged retention time. This

25

study advanced our knowledge on nitrous oxide emissions from cascade hydropower systems.

26 27

Keywords: Nitrous oxide, Mekong River, Cascade reservoirs, Denitrification, Elevation

2

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1. Introduction

29

Nitrous oxide is a potent greenhouse gas with a global warming potential nearly 300 times that

30

of carbon dioxide, greatly contributing to stratospheric ozone destruction and global warming

31

effects (Solomon 2007). Current concentration of atmospheric nitrous oxide reaches 324.2 ppb,

32

which is approximately 20% higher than the pre-industrial level (Allen et al., 2014). Rivers are

33

considered as important sources of nitrous oxide and have received considerable attention

34

(Beaulieu et al., 2011, Laverman et al., 2010, Yu et al., 2013). It was estimated that more than

35

0.68 Tg yr-1 of anthropogenic nitrogen inputs can be converted to nitrous oxide through

36

denitrification in global river networks, equivalent to 10% of the global anthropogenic nitrous

37

oxide emission rate (Beaulieu et al. 2011); a projected doubling of nitrate concentration by 2050

38

would cause riverine nitrous oxide emissions to further increase by about 40% (Turner et al.,

39

2016). River hydrology often affects environmental factors (water temperature, dissolved

40

oxygen), nutrient supply and hydraulic retention time, exerting a strong influence on

41

denitrification and thereby nitrous oxide emissions (Yu et al. 2013, Quick et al., 2016). Quick et

42

al. (2016) conducted column and large-scale flume experiments, and established a predictive link

43

between stream geomorphology, hydrodynamics, and nitrous oxide emissions. Hence, the

44

variation of river hydrology may alter nitrous oxide emissions from rivers.

45

In the past decades, many rivers worldwide have been intensively dammed for a variety of

46

purposes, including hydropower production, flood management, water supply, and navigation

47

(Moran et al., 2018). Over 70,000 large dams have been built, and more dams are under

48

construction, planned or proposed (Maavara et al., 2015). Many of these dams are built in a

49

cascade configuration, especially in large rivers (Grumbine and Xu 2011). Hydropower

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generated by these dams has been considered to be green energy, however, there has been an

3

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ongoing scientific debate over the role of hydropower in greenhouse gas emissions to the

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atmosphere. There is a prevailing viewpoint that greenhouse gas emissions tarnish the green

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credentials of hydropower (Giles 2006). Hu and Cheng (2013) urged to assess greenhouse gas

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budgets of hydropower reservoirs in China. After dam constructions, rivers are converted into

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lentic reservoirs, greatly modifying the fluvial regime by decreasing flow velocity, increasing

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hydraulic retention time and trapping sediments together with nitrogen (Maeck et al., 2013). This

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potentially alters nitrogen biogeochemical cycles and thereby nitrous oxide emissions along the

58

river continuum. Nitrous oxide emissions in dammed rivers have been extensively studied, but

59

they mainly focused on single reservoirs (Beaulieu et al., 2014, Musenze et al., 2014a, Zhu et al.,

60

2013, Guérin et al., 2008). For example, Beaulieu et al. (2014) studied nitrous oxide emissions in

61

a temperate reservoir and found that denitrification in the hypolimnion functioned as a small

62

nitrous oxide sink during the stratified period, while the reservoir was a nitrous oxide source on

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an annual time scale; Musenze et al. (2014a) demonstrated that there was a switch from weak

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nitrous oxide sinks in spring to strong sources for the rest of the year in three subtropical

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freshwater reservoirs in Australia. Nitrous oxide emissions from cascade reservoirs are being

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received concerns (Liang et al., 2019); however, the spatial patterns of nitrous oxide emissions in

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cascade reservoirs and the underlying mechanisms remain unclear.

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As flow velocity decreases after dam constructions, suspended particles settle down and

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potentially supply nitrogen substrates and carbon sources for nitrous oxide production.

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Meanwhile, nitrous oxide production is mainly mediated by microbes in aquatic systems

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(Kuypers et al., 2018). As the river system moves downstream, water temperature increases

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following the elevation decrease, which potentially enhance microbial nitrous oxide production.

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Hence, we hypothesized that nitrous oxide emissions exhibited an increase trend along the flow

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direction in cascade hydropower reservoirs. To test this hypothesis, we investigated nitrous oxide

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emissions from cascade hydropower reservoirs. The objectives were to (1) identify nitrous oxide

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emission patterns in cascade hydropower reservoirs; (2) reveal the main drivers for the nitrous

77

oxide emission patterns.

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2. Materials and methods

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2.1. Study area

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This study was conducted in the upper Mekong River, which has been heavily dammed for

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hydropower production. The Mekong River, one of the largest rivers in the world, originates

83

from the Tibetan Plateau and discharges into South China Sea. It has a length of 4,909 km, a

84

watershed area of 760,000 km2, and a mean annual discharge of 457 km3 at a rate of 14,500 m3

85

s-1 (Shi et al., 2017). In the upper Mekong basin, there is a small catchment with little

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disturbances from tributaries and human activities. As of 2016, six dams had been built for

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hydropower production on the mainstream of the upper Mekong River (Fig. 1); these dams are

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Gongguoqiao (GGQ), Xiaowan (XW), Manwan (MW), Dachaoshan (DCS), Nuozhadu (NZD)

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and Jinghong (JH). The locations of these dams are shown in Fig. 1, and the main features of

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these cascade reservoirs are presented in Table 1. Meanwhile, there is an unregulated river as the

91

control, Nujiang River, which flows in parallel to the upper Mekong River in the same region.

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The unique study site will be beneficial to studying nitrous oxide emission patterns in cascade

93

hydropower reservoirs (Fig. 1).

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2.2. Sample collection and analysis

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Two field surveys were conducted in September 2016 and September 2017. In the 2016

96

survey, samples were collected at 23 sites along the upper Mekong River, including 5, 3, 3, 3, 3,

5

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3 and 3 sites in the upstream channel (UC), GGQ, XW, MW, DCS, NZD and JH, respectively

98

(Fig. 1). Sampling in each reservoir was mainly carried out in the lacustrine zone. Meanwhile,

99

nitrous oxide emissions from the Nujiang River (N1−N9 sites in Fig. 1) were investigated for

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comparison with the upper Mekong River. According to the results of the 2016 survey, field

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survey in the single reservoir was conducted in JH Reservoir in 2017. Samples were collected at

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8 sites from the riverine zone at the tail of JH Reservoir to the lacustrine zone at the front of the

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dam, and the distance between sampling sites was about 8 km. At each site, sediment samples

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were collected randomly in triplicate using an Ekman grab sampler and homogenized

105

completely. Then, half of each homogenized sediment sample was kept frozen in the dark for the

106

analyses of total nitrogen, organic carbon, grain size and microbial abundance; and the other half

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was treated immediately to measure denitrification potentials. In the laboratory, part of each

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sample was taken randomly for analyses in triplicate. Sediment total nitrogen and organic carbon

109

were analysed using a Vario MACRO cube elemental analyser (Elementar Inc., Germany) after

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the fresh sediment was freeze-dried and ground. For the grain size analysis, raw sediment

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samples were dispersed by 2-min sonication using an ultrasonic oscillator (PS-60A, Shenzhun

112

Shenghuatai ultrasonic equipment Co. Ltd., China), and then measured using a laser particle size

113

analyser (Mastersizer 2000, Malvern Co., Untied Kingdom).

114

2.3. Sediment denitrification rate measurements

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Nitrous oxide is a byproduct of nitrification and denitrification (Kuypers et al. 2018).

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Nitrification is likely slow relative to denitrification in the upper Mekong River (Fig. S1). Hence,

117

we analysed denitrification process here to clarify nitrous emission patterns in cascade

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reservoirs. Sediment denitrification potentials were measured using the

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membrane inlet mass spectrometer (Zhang et al., 2012, An et al., 2001). Fresh sediment samples

6

15

N-tracer method with

120

were mixed and homogenized with field water at a ratio of 1:5, and then bubbled with pure

121

helium for 20 min. The mixtures were filled into 12-mL Exetainer® vials (839 W, Labco, UK)

122

without bubbles, which was then immediately sealed with butylrubber septa and screw caps

123

tightly to prevent solution leakage. Prior to incubation experiments, the prepared vials were

124

pre-incubated for 24 h to eliminate background nitrate, and then spiked with

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concentration, ~100 µmol L-1). The vials were shaken (200 rpm) at room temperature (25°C),

126

and three vials were randomly yielded and preserved with 0.1 mL of saturated HgCl2 solution at

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the time of 0, 2, 4, 6 and 8 h, respectively. The 30N2 gas in vials was analysed using a membrane

128

inlet mass spectrometer. The rates of denitrification were calculated according the following

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equation (1): × ×

=

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15

NO3− (final

+ 2 ×

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where DR is the denitrification rate (µmol kg-1h-1); T30 is the measured production rates of

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during the incubation; Fn is the fraction of 15N in NO3− after pre-incubation (%).

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2.4. Nitrous oxide flux analysis

(1) 30

N2

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Nitrous oxide fluxes across air-water interfaces were analyzed in triplicate using the thin

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boundary layer model (2), and the results are presented as mean values (Musenze et al., 2014b,

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Whitfield et al., 2011):

137

=

∙

−

∙ M

(2)

138

where F is the nitrous oxide flux from water to air (g m-2h-1); Cw is the nitrous oxide

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concentration in surface water (mol L-1); Ceq is the nitrous oxide concentration in surface water

140

that is in equilibrium with the atmospheric concentration (mol L-1) according to Henry’s law

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using temperature dependent solubility (Weiss and Price 1980); K is the gas transfer velocity (m

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h-1); M is the molar mass of nitrous oxide (44 g mol-1).

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Nitrous oxide concentration in surface water was measured using the headspace equilibration

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method (Whitfield et al. 2011, Wang et al., 2009). Briefly, a 20-ml water sample was collected

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from 5 cm below the surface using a 60-ml polypropylene syringe equipped with three-way

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stopcocks. 20 ml ambient air was added to the syringe to create a headspace. Then, the sample

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syringe was shaken vigorously for 2 min. The equilibrated headspace gas was injected into a

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pre-evacuated Exetainer® vial (839 W, Labco, UK) for storage until analysis using a gas

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chromatograph (7890B, Agilent Technologies, USA). According to the Quality Index of the

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product, the gas pressure in the evacuated vials can reach 100 Pa, indicating 99.9% of gas was

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remove. Prior to usage, the vials were randomly chosen to test residual nitrous oxide in them. We

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filled 12 ml pure helium into the selected 12-ml vials, and the residual in all tests were below

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detection limit (the detect limit is < 10 ppb). Ambient air at each sampling site was also

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analyzed. The concentration of dissolved nitrous oxide (Cw, µg L-1) in surface water was

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calculated using the following equation (3): =

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! × "!

× "! # $ × ! × "% "%

(3)

157

where C0 is nitrous oxide concentration in the headspace before shaking, which is the nitrous

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oxide concentration in ambient air, µg L-1; Ch is nitrous oxide concentration in the headspace

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after shaking, µg L-1; Vh is headspace volume, ml; Vw is water volume, which is 20 ml in this

160

study; α is Bunsen coefficient (Weiss and Price 1980).

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The gas transfer velocity was calculated using the following equation (4) from (Liss and Merlivat 1986): = 2.07 + 0.215 ∙ +

8

.,

.

∙ -0 / 1

2

(4)

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where U10 is the wind speed at a height of 10 m above water surface (m s-1); Sc is the Schmidt

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number at the water surface temperature according to (Wanninkhof 2014); n is -2/3 for U10 ≤ 3.7

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m s-1 and -1/2 for U10 > 3.7 m s-1 (Encinas Fernández et al., 2014).

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2.5. DNA extraction and qPCR

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Denitrifiers in sediments were quantified using the quantitative polymerase chain reaction

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(qPCR) method. There are many functioning genes in the denitrification process (Shrewsbury

171

et al., 2016), of which nirS for encoding nitric oxide reductase and nosZ for encoding nitrous

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oxide reductase are the most common gene markers for denitrifiers (Guo et al., 2013,

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Throbäck et al., 2004, Morales et al., 2010, Hou et al., 2014), and were used in the qPCR in

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this study. DNA was extracted from sediments using a FastDNA Power-Max Soil DNA

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Isolation Kit (MP Biomedical, USA) according to the manufacturer’s instructions. This DNA

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subsequently served as a template for qPCR amplification. The qPCR assay was performed using

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the primer cd3aF/R3 cd targeting nirS gene and the primer nosZ2F/nosZ2R targeting nosZ gene,

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respectively (Hou et al. 2014). Gene copies were amplified and quantified in a Bio-Rad cycler

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equipped with the iQ5 real-time fluorescence detection system and software (version 2.0,

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Bio-Rad, USA). All reactions were completed in a total volume of 20 µL containing 10 µL

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SYBR® Premix Ex TaqTM (Toyobo, Japan), 0.5 mM of each primer, 0.8 µL of bovine serum

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albumin (3 mg mL-1, Sigma, USA), double distilled H2O, and template DNA. The qPCR

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program for nirS was as follows: 95°C for 60 s, followed by 40 cycles of 95°C for 30 s, 57°C for

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45 s, and 72°C for 60 s. The qPCR program for nosZ commenced with 95°C for 60 s, followed

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by 40 cycles of 95°C for 30 s and 60°C for 45 s and 72°C for 60 s. A standard curve was

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established by serial dilution (10-2–10-8) of known concentration plasmid DNA with the target

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fragment. All PCRs were run in triplicate on 96-well plates (Bio-Rad, USA) sealed with

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optical-quality sealing tape (Bio-Rad, USA). Three negative controls without the DNA template

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were included for each PCR run.

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2.6. Statistical analysis

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One-way analysis of variance (ANOVA) was employed to test the statistical significance of

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differences between sampling sites. Post-hoc multiple comparisons of different sampling sites

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were performed using Tukey’s least significant difference procedure. Prior to analysis of

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variance, the normality test using Kolmogorov-Smirnov and variance homogeneity test were

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conducted. Logarithmic transformation was conducted to ensure the data to be satisfied for

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ANOVA assumptions. Correlation analysis between nitrous oxide emissions and water

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temperature was conducted using linear regression. All statistical analyses were carried out using

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SPSS (v22.0, SPSS Inc., North Chicago, IL, USA). The level of significance was P < 0.05 for all

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tests.

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3. Results

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3.1. Sediment total nitrogen, organic carbon and grain size

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The contents of total nitrogen and organic carbon in sediments were about 0.26 and 1.91 mg

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g-1 in the upstream channel, respectively, which were significantly lower than the downstream

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reservoirs (P < 0.05). The total nitrogen and organic carbon in cascade reservoirs reached

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0.87−1.60 and 6.52−14.83 mg g-1, respectively (Fig. 2A). In contrast, sediment grain size (d50) in

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the upstream channel was higher than the downstream reservoirs, which was 25.4 and 6.2−18.3

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µm, respectively (Fig. 2B). Inside the reservoir, both total nitrogen and organic carbon in

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sediments increased initially and then deceased from the tail of the reservoir to the front of the

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dam, while sediment grain size (d50) showed a gradual decrease trend. In the JH Reservoir,

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sediment total nitrogen and organic carbon increased from 0.10 and 1.80 mg g-1 at the tail of the

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reservoir to the maximum of 1.57 and 18.0 mg g-1, and finally decreased to 1.22 and 5.76 mg g-1

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at the front of the dam, respectively (Fig. 2C); sediment grain size gradually decreased from 12.6

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µm at the tail of the reservoir to 6.8 µm at the front of the dam (Fig. 2D).

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3.2. Nitrous oxide fluxes at air-water interfaces

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Compared with the upstream channel, nitrous oxide emissions from downstream cascade

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reservoirs were enhanced, and showed a gradual increase along the flow direction. Nitrous oxide

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fluxes gradually increased from 0.47 µg m-2h-1 in the upstream channel to 1.08 µg m-2h-1 in the

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JH Reservoir (Fig. 3A). In contrast, nitrous oxide emissions showed no significant differences

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between sites on the Nujiang River (P > 0.05), which were 0.72−1.02 µg m-2h-1 (Fig. 3B). Inside

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the reservoir, nitrous oxide emissions at the tail of the reservoir were less than the downstream

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area, where there were no significant differences between sites (P > 0.05). In the JH Reservoir,

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nitrous oxide fluxes were 0.59−0.80 µg m-2h-1 at the tail of the reservoir, and they increased to

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1.00−1.05 µg m-2h-1 at the downstream area (Fig. 3C).

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3.3. Sediment denitrification potentials

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Denitrification potential in sediments was low at 0.38 µg kg-1h-1 in the upstream channel, and

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exhibited a general increase trend in the downstream reservoirs along the flow direction,

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reaching 15.80 µg kg-1h-1 in the JH Reservoir (Fig. 4A). Inside the reservoir, there was a peak of

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sediment denitrification potential at the transition zone. In the JH Reservoir, the denitrification

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potential increased from 2.15 µg kg-1h-1 at the tail to the maximum of 21.59 µg kg-1h-1 at the

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transition zone, and finally decreased to 6.92 µg kg-1h-1 at the front of the dam (Fig. 4B).

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3.4. Denitrifier abundance

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Sediment denitrifiers were distributed non-uniformly along the upper Mekong River, which

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were abundant in the downstream cascade reservoirs but scarce in the upstream channel. The

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denitrifier reached 0.18 × 106−0.74 × 106 gene copies per gram of sediment in the downstream

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reservoirs, but only 0.01 × 106 gene copies per gram of sediment were detected in the upstream

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channel (Fig. 5A). Inside the reservoir, the distribution of sediment denitrifiers exhibited a

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similar pattern to dentrification potentials (Fig. 5B, Fig. 4B). In the JH Reservoir, the denitrifier

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abundance increased from 0.80 × 106 gene copies per gram of sediment at the tail of the reservoir

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to the maximum of 1.10 × 106 at the transition zone, and then decreased to 0.69 × 106 gene

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copies per gram of sediment at the front of the dam (Fig. 5B).

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4. Discussion

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4.1. Hydrological alteration and nutrient accumulation by dams

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Rivers continuously transport terrestrial substances to oceans, which is essential to driving

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biogeochemical processes in river-coastal systems (Beaulieu et al. 2014, Borges et al., 2018).

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Dams convert rivers into lentic reservoirs with subsequent decrease in flow velocity, increase in

249

hydraulic retention time and suspended particle settlement (Maeck et al. 2013). The flux of

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terrestrial sediment to global coastal ocean was estimated to be reduced by 26% due to dam

251

constructions (Syvitski et al., 2005). Particulate organic matters and soluble organic matters

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absorbed by particles will settle in reservoirs. In this study, the contents of total nitrogen and

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organic carbon in the reservoir sediment were higher than the upstream channel (Fig. 2A). The

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upper Mekong River has a small rocky catchment (Fig. 1) with limited impacts of tributaries on

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the mainstream; meanwhile, the flow was jointly regulated by cascade reservoirs without the

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occurrences of frequent drawdown events. As the flow velocity decreases, coarse particles easily

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settle down, while fine particles can travel a long way downstream (Tang et al., 2018, Yi et al.,

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2017), leading to a general decrease in grain size along the flow direction in both the inter- and

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inner-reservoirs (Fig. 2B, 2D). Compared with coarse particles, fine particles often have stronger

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sorption ability because of the larger surface area in a unit mass (Xia et al., 2009, Pan et al., 2013,

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Manohar et al., 2002). As a result, the contents of total nitrogen and organic carbon in sediments

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generally increased from the upstream channel to the downstream reservoirs (Fig. 2A), and from

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the tail of the reservoir to the area adjacent to the front of the dam (Fig. 2C). The data deviation

264

at the MW (Fig. 2A) was possibly attributed to the disturbances induced by severe sand mining

265

activities in the reservoir (Fig. S2 in the supporting information). The decreases of sediment total

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nitrogen and organic carbon at the front of the dam (Fig. 2C) might be caused by the discharge

267

via hydropower stations. The retention time of the upper Mekong River was estimated to be less

268

than 0.07 years before cascade dam constructions, while it was prolonged to 5.72 years after

269

cascade dam constructions (Table 1). Hence, dam constructions increased river hydraulic

270

retention time and trapped nitrogen and organic carbon within riverine sediments, which

271

potentially alter nitrogen transformations and nitrous oxide emissions in rivers.

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4.2. Regulation of sediment denitrification by cascade dams

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Denitrification is a microbe-mediated process where nitrate is reduced and ultimately produces

274

nitrogen gas through a series of intermediate gaseous nitrogen oxide products, which can leak

275

nitrous oxide (Beaulieu et al. 2011). The majority of denitrifiers are facultative aerobic

276

heterotrophs, which need organic carbon to satisfy energy to maintain life (Gómez-Alday et al.,

277

2014, Wang et al., 2014). Hence, the availability of nitrogen substrates and organic carbon often

278

act as two main limiting factors of denitrification in aquatic systems (Kim et al., 2016, Zarnetske

13

279

et al., 2011, Trauth et al., 2018). For example, Zarnetske et al. (2011) demonstrated that the rate

280

of denitrification was more than double that under elevated dissolved organic carbon conditions

281

in riverine riparian zones.

282

In unregulated river systems with natural flow regimes, rivers mainly act as pipes for nitrogen

283

transport from terrestrial systems to lakes or coastal systems, with limited nitrogen

284

transformations because of the short retention time of nitrogen substrates and organic matters. As

285

a result, weak denitrification (Fig. 4A) and poor denitrifiers (Fig. 5A) in sediments were detected

286

in the upstream channel. Prolonged hydraulic retention time, associated with dams, enhanced the

287

accumulation of total nitrogen and organic carbon within riverine sediments. This provided

288

suitable substrates and environmental conditions to enhance denitrification in the sediments. The

289

sediment denitrification rate (Fig. 4A) and denitrifier abundances (Fig. 5A) in cascade reservoirs

290

were relatively higher than the upstream channel. This was further supported by the results in the

291

JH Reservoir. Under flow regulations by the dam, nitrogen and organic carbon accumulated and

292

created the hotspot of denitrification at the transition zone in the JH Reservoir. The

293

denitrification potential rate (Fig. 4B) and denitrifier abundance (Fig. 5B) exhibited similar

294

spatial patterns to the contents of sediment total nitrogen and organic carbon (Fig. 2C) from the

295

tail of the reservoir to the front of the dam. In addition, the availability of oxygen might be

296

another contributor to the spatial heterogeneity of denitrification inside the reservoir. As oxygen

297

was consumed with the flow, the anoxic-oxic transition may occur at the transition zone, which

298

is favourable to denitrification. This needs further studies in future to analyse oxygen profiles at

299

the sediment-water interface by collecting intact sediment cores (Shang et al., 2013).

300

4.3. Spatial heterogeneity of nitrous oxide emissions in cascade reservoirs

14

301

In the dammed upper Mekong River, the enhanced sediment denitrification could increase

302

nitrous oxide production and emissions. We have detected higher fluxes of nitrous oxide from

303

the water to the atmosphere in cascade reservoirs than the upstream river channel (Fig. 3A). As

304

the river system moves downstream, the increased water temperature (Fig. S3) enhanced

305

microbe-mediated denitrification for nitrous oxide production. As a result, nitrous oxide

306

emissions exhibited a gradual increase trend along the flow direction in the cascade reservoirs

307

(Fig. 3A). Although there is also a temperature gradient along the natural river continuum

308

without reservoirs, suspended particles rarely settle in the running river to supply nitrogen

309

substrates and organic carbon for nitrous oxide production. Thus, we did not detected the

310

increase trend of nitrous oxide emissions along the undammed Nujiang River, but found no

311

significant differences between sites along the river (P > 0.05, Fig. 3B). Hence, we consider it is

312

the general rule for nitrous oxide emissions in cascade reservoirs since sediment retention and

313

temperature gradient are the features of cascade reservoirs, which is also supported by nitrous

314

oxide emissions from cascade reservoirs on the mainstream of the Wujiang River, China (Liang

315

et al., 2019). Unlike nitrous oxide, the increase trend of methane emissions was not detected

316

according to our previous study in the cascade reservoirs (Shi et al. 2017). It is possibly

317

attributed to the higher sensitivity of denitrifiers to temperature than methanogens. This needs

318

further studies in future using other molecular biology techniques, such as DNA/RNA-based

319

stable isotope probing (Dumont and Murrell, 2005). In cascade reservoirs, the huge overlying

320

water may also contribute to nitrous oxide emissions from the surface water. However, we did

321

not observe the correlation between nitrous oxide fluxes with the storage capacity in these

322

cascade reservoirs. Future studies are needed to quantify the exact contribution of nitrous oxide

323

production in the overlying water to net nitrous oxide fluxes from the reservoir. Inside the

15

324

reservoir, nitrous oxide emissions increased from the tail of the reservoir and reached the

325

maximum at the hotspot of denitrification at the transition zone, but maintained constant in the

326

downstream area where the sediment denitrification decreased (Fig. 3C). This is because the

327

released nitrous oxide was homogenized with the flow during the diffusion.

328

Compared with reservoirs in other river systems (19.60 µg m-2h-1) (Deemer et al. 2016), the

329

cascade hydropower reservoirs in the upper Mekong River emit much less nitrous oxide to the

330

atmosphere (0.47–1.08 µg m-2h-1). Different landscapes store different amounts of organic

331

matters in soils and vegetation, and the potential for gas production and loss varies from site to

332

site. Reservoirs that submerge peat lands often emit more greenhouse gases than those built in

333

canyons, which have thin soil layers and no peat deposits. The upper Mekong River has a small

334

rocky catchment with little soil (Fig. 1). The relatively low amounts of submerged organic

335

matters potentially caused the lower nitrous oxide emissions.

336 337

5. Conclusions

338

In this study, we investigated nitrous oxide emissions from cascade reservoirs in the upper

339

Mekong River, which has been heavily dammed for hydropower production. The main findings

340

are as follows:

341

(1) Cascade hydropower dams increased nitrous oxide emissions from rivers.

342

(2) Nitrous oxide emissions exhibited an increase trend along the flow direction in cascade

343 344 345

reservoirs. (3) Sediment accumulation by dams supplied sufficient nitrogen substrates and organic carbon, creating hotspots of denitrification for nitrous oxide production in reservoirs.

16

346 347

(4) Nitrous oxide emissions in the upper Mekong River were much lower than other riverine systems due to the small rocky catchment with little soil.

348 349 350 351

Acknowledgements

352

This work was supported by the National Natural Science Foundation of China [No. 91547206,

353

51425902, 51709181 and 51709182], and the Fundamental Research Funds for the Central

354

Universities [Y918018].

17

355

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24

Table 1. The main features of cascade hydropower reservoirs in the upper Mekong River. GGQ

XW

MW

DCS

NZD

JH

25°36'39"N

24°42'53"N

24°37'43"N

24°01'40"N

22°38'24"N

22°03'03"N

99°19'44"E

100°5'37"N

100°26'59"E

100°22'06"E

100°25'40"E

100°45'39"E

Construction time

2012

2009

1993

2003

2014

2009

Dam height (m)

105

292

132

115

261.5

108

Normal water level (m)

1307

1240

994

899

812

602

Storage capacity a (108 m3)

3.5

149.1

5.0

9.4

237.0

11.4

318.5

381.6

388.0

419.0

545.6

574.0

Hydraulic residence time (yr)

0.01

2.36

0.78

0.30

1.87

0.40

Installed capacity (106 kW)

0.90

4.20

1.50

1.35

5.85

1.75

Location

Annual discharge volume (108 m3)

a

It is the storage at the normal water level.

Fig. 1. Location of cascade dams in the upper Mekong River and sampling sites in this study. The light blue and red shadings show river catchments.

Fig. 2. The spatial patterns of TN, OC and grain size in sediments. (A) Sediment TN and OC in cascade reservoirs; (B) Sediment grain size in cascade reservoirs; (C) Sediment TN and OC in JH Reservoir; (D) Sediment grain size in JH Reservoir. TN = total nitrogen, OC = organic carbon. S1-S8 in Fig. 2C, 2D are the sampling sites from the tail of the JH Reservoir to the front of the JH dam. Error bars indicate standard deviations.

Fig. 3. Nitrous oxide fluxes across air-water interfaces. (A) Cascade reservoirs on the upper Mekong River; (B) Nujiang River; (C) JH Reservoir. N1-N9 in Fig. 3B are the sampling sites along the flow direction on the Nujiang River (Fig. 1). S1-S8 in Fig. 3C are the sampling sites from the tail of the JH Reservoir to the front of the JH dam. Error bars indicate standard deviations.

Fig. 4. The spatial patterns of sediment denitrification potentials in the upper Mekong River. (A) Cascade reservoirs on the upper Mekong River; (B) JH Reservoir. S1-S8 in Fig. 4B are the sampling sites from the tail of the JH Reservoir to the front of the JH dam. Error bars indicate standard deviations.

Fig. 5. The distribution of sediment denitrifiers in the upper Mekong River. (A) Cascade in the upper Mekong River; (B) JH Reservoir. The abundance of denitrifiers was the sum of nirS and nosZ genes. S1-S8 in Fig. 5B are the sampling sites from the tail of the JH Reservoir to the front of the JH dam.

• N2O emissions in the upper Mekong River were increased by cascade dams. • N2O emissions exhibited an increasing trend along the flow direction. • Sediment retention by dams supplied substrates and carbon for N2O production. • Denitrification hotspot was created at the transition zone in the reservoir.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: