J. Geogr. Sci. 2011, 21(5): 820-832 DOI: 10.1007/s11442-011-0882-1 © 2011
Science Press
Springer-Verlag
Seasonal and diurnal variations in N2O concentrations and fluxes from three eutrophic rivers in Southeast China YANG Libiao1,2, *YAN Weijin1, MA Pei1, WANG Jianing1 1. Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing 100101, China; 2. Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: This study was performed at three eutrophic rivers in Southeast China aiming to determine the magnitude and patterns of dissolved N2O concentrations and fluxes over a seasonal (in 2009) and diurnal (24 h) temporal scale. The results showed that N2O concentrations varied from 0.28 to 0.38 (mean 0.32±0.04), 0.29 to 0.46 (mean 0.37±0.07), and 2.07 to 3.47 (mean 2.84±0.63) μg N-N2O L−1 in the Fengle, Hangbu and Nanfei rivers, respectively, in the diurnal study performed during the summer of 2008. The study found that mean N2O concentration and estimated N2O flux (67.89 ± 6.71 µg N-N2O m−2 h−1) measured from the Nanfei River with serious urban wastewater pollution was significantly higher than those from the Fengle and the Hangbu Rivers with agricultural runoff. In addition, the seasonal study during June and December of 2009 also showed that the mean N2O concentration (10.59±14.67 μg N-N2O L−1) and flux (236.87±449.74 µg N-N2O m−2 h−1) observed from the Nanfei River were significantly higher than those from the other two rivers. Our study demonstrated both N2O concentrations and fluxes exhibited seasonal and diurnal fluctuations. Over three consecutive days during the summer of 2008, N2O accumulation rates varied within the range of 3.91–7.21, 2.76–15.71, and 3.23–30.03 µg N-N2O m−2 h−1 for the Fengle, Hangbu and Nanfei Rivers, respectively, and exponentially decreased with time. Keywords: nitrous oxide; concentration and flux; eutrophic river; seasonal and diurnal variations
1
Introduction
Nitrous oxide (N2O) is an important greenhouse gas that depletes stratospheric ozone and attributes to global warming. The anthropogenic greenhouse effect of N2O is about 300 times greater than CO2 (Ravishankara et al., 2009). The atmospheric N2O concentration has increased rapidly with intensive human activity. On a global scale, atmospheric N2O concentrations were about 270 ppbv in the pre-industrial era. As of 2005, the recorded value for N2O had risen to 319 ppbv (IPCC, 2007). Researchers are increasingly focused on N2O Received: 2010-11-22 Accepted: 2011-02-28 Foundation: National Natural Science Foundation of China, No.20777073 Author: Yang Libiao (1981–), Ph.D, specialized in water environment and ecology. E-mail:
[email protected] * Corresponding author: Yan Weijin, Professor, E-mail:
[email protected]
www.geogsci.com
springerlink.com/content/1009-637X
YANG Libiao et al.: Seasonal and diurnal variations in N2O concentrations and fluxes from three eutrophic rivers
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emissions from terrestrial and aquatic ecosystems as a key promoter of climate change. Riverine ecosystems receive substantial nitrogen loading from agricultural runoff, urban wastewater and human activity. These sources also serve as significant contributors of N2O to the atmosphere (Silvennoinen et al., 2008; McMahon et al., 1999). N2O is produced when nitrogen stored in water columns and sediments is decomposed by microbial processes. Part of the gas is released into the atmosphere across the water-air interface (Groffman et al., 2009). Previous studies reported that N2O emission has spatial and temporal variations due to changes in river chemistry (Beaulieu et al. 2008; Ferrón et al., 2007; Stow et al., 2005). It is suggested that N2O fluxes are not significantly changed during a short time scale, except in the case of significant events such as a sudden in precipitation or a large input of pollutants into the river channel (Amoroux et al., 2002). To date, only a few studies have observed the diurnal fluctuation in N2O flux on riverine ecosystems, though it is very common in terrestrial ecosystems (Clough et al., 2007). Additionally, observation of N2O emissions over a longer time period, for example seasonal variations, is meaningful for evaluating the contribution of the river ecosystem to the atmospheric N2O budget. In recent decades, many rivers in China have suffered from serious nitrogen pollution (Wang et al., 2007).Very few of the studies directly measured N2O fluxes from those rivers (Yan et al., 2004; Xiong et al., 2002). Thus, the magnitude and pattern of N2O emissions are still unclear. The purposes of this study were: 1) to investigate the seasonal and diurnal variations in dissolved N2O concentrations and N2O fluxes; 2) to determine if high nitrogen loading rivers support high dissolved N2O concentrations and N2O fluxes; 3) to investigate the dynamics of N2O accumulation rates over three consecutive days. Through this research, we expect to have a better understanding of N2O emissions from riverine ecosystems in Southeast China.
2 2.1
Materials and methods Study area
Lake Chaohu (116º24'30''–118º00'00''E, 30º58'00''–32º58'00''N) is located in central Anhui province of eastern China (Figure 1). The annual average temperature and precipitation
Figure 1
Map of sampling locations. Individual study sites are indicated with black circles
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of this watershed are 15.5℃ and 940 mm, respectively. There are 35 tributaries discharging into the lake. The Fengle River (FLR), the Hangbu River (HBR) and the Nanfei River (NFR) are the three major tributaries. The FLR and the HBR watersheds are dominated by fertilized agricultural land (Table 1). In recent years, because the two rivers receive substantial nitrogen via agricultural runoff, water quality has depreciated dramatically. The NFR flows through the Hefei city and has suffered from serious pollution due to urban wastewater inputs. Table 1
Properties of the studied rivers and the experimental time
River River length (km) Watershed area (km2) Main land use (%) Water quality* III
Experimental time
FLR
117
2152
Agriculture (78%)
20 to 25 Jul 2008, Jun to Dec 2009
HBR
139
2070
Agriculture (80%)
III
1 to 6 Aug 2008, Jun to Dec 2009
NFR
70
1446
Urban (>90%)
IV
14 to 20 Aug 2008, Jun to Dec 2009
*The levels of river water quality classified referring to the Environmental quality standards for surface water enacted by the Chinese government (GB3838-2002). Levels “III” and “IV” indicate river suffered from moderate and heavy pollution, respectively.
2.2
Experimental design
The seasonal variations in N2O concentrations and fluxes from the three studied rivers were investigated during June and December of 2009. The diurnal fluctuations in dissolved N2O concentrations and fluxes over a daily cycle and the dynamics of N2O accumulation rates over three consecutive days were examined consecutively in the three rivers during the summer of 2008 (from 20 July to 20 August) (Table 1). In the seasonal variation study, one sampling was conducted each month, and N2O flux was observed using the diffusion model methodology (Borges et al., 2004). At each sampling occasion, surface water (20 cm depth) samples (for N2O concentration measurements), and data concerning wind speed at a height of 10 m, and water temperature in situ were collected simultaneously. In this study, wind speeds were obtained from the meteorological station of Hefei City near our sampling locations. In the diurnal variation study, surface water (20 cm depth) samples were collected at 7:00, 11:00, 15:00, 19:00, 23:00, and 3:00 at a 4-h interval during a 24-h period, respectively, to measure the dissolved N2O concentration. Water temperature and wind speeds at a height of 10 m were measured simultaneously. Water surface N2O flux was then estimated using the two-layer diffusion model based on the gas concentration difference and transfer velocity. The diurnal variation of N2O flux was also synchronously measured using chamber. At each sampling occasion, the chamber was enclosed for 30 min (N2O emission time). The dynamics of N2O accumulation rates over three consecutive days were investigated using chamber. Chamber headspace was sampled corresponding to N2O emission times after 4, 8, 12, 24, 48, and 72 hours. The chamber was designed as a cube with a volume of 0.125 m3 (0.5 m × 0.5 m × 0.5 m), and was wrapped with silver paper in order to control the temperature inside. 2.3
Sample collection
After five chambers were deployed at the experimental site along the river reach, the cham-
YANG Libiao et al.: Seasonal and diurnal variations in N2O concentrations and fluxes from three eutrophic rivers
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ber headspace was sampled using a 100 ml airtight syringe equipped with stopcocks. Gas samples were stored in previously evacuated plastic gas-sampling bags (500 ml, Dalian Plaite Gas Packing Co., Ltd, China) that are impervious to N2O leakage. This is tested using a pump in the lab before the field experiment is performed. Surface water samples were collected using serum bottles (60 ml) avoiding bubbling and then sealed with butyl rubber stoppers (processed underwater to prevent headspace formation) after a crystal of KOH was added to the bottles (to stop biological activity). Water samples were stored at 4℃ and analyzed after transportation to the lab. 2.4
Laboratory analysis
N2O concentration of headspace gas samples was directly analyzed using the Gas Chromatograph (GC) equipped with an electron capture detector (ECD). Before analyzing the air sample, the Gas Chromatograph was calibrated using a standard air sample. Initial sample dissolved N2O concentration in water was measured according to the headspace-equilibrium method (Amoroux et al., 2002; Huttunen et al., 2002). Five milliliters highly purified N2 is injected into the sampling bottle using an airtight syringe and a 5ml water sample was displaced. Bottle headspace N2O concentration was analyzed after the bottle was vigorously shaken for 4 hours. Initial water sample N2O concentration (Cw) was calculated using methodology described by Johnson et al. (1990). The equilibrium concentration (Ce) of N2O in river water with atmosphere was calculated using Henry’s first law. The saturation of N2O was calculated by comparing Cw to Ce. 2.5
Flux calculation
N2O flux measured using chamber was calculated by the equation: F (μg N-N 2 O m −2 h −1 )= ΔC×V /(A×T)
(1)
−1
where ΔC (μg N-N2O L ) is the concentration difference between the gas sample in the chamber headspace and the background value in the atmosphere at 1m height above the water surface, V (L) and A (m2) are the volume and the area that the chamber enclosed, respectively, and T (h) is gas emission time. N2O flux estimated by the diffusion model was calculated according to the equation: F (μg N-N 2 O m −2 h −1 )= kN 2 O×ΔN 2 O
(2)
where ΔN2O (μg N-N2O L−1) is concentration difference between measured (Cw) and equilibrium concentration (Ce) with the atmosphere in river water. kN2O (cm h−1) is the gas transfer velocity. According to the work by Borges et al. (2004), an equation that accounts for both wind speed and water current speed was selected to estimate kN2O: kN 2 O =1.0+1.719 (w/h)0.5 + 2.58μ10
(3)
−1
where w is the water current speed (m s ), h is the depth of river water column (m), and μ10 (m s−1) is the wind speed at a 10-m height. 2.6
Statistical analysis
In this paper, statistical analyses were done using the SPSS 17.0 software package. One-Way ANOVA (LSD test) and the Independent-Samples T-test (2-tailed) were run to test the dif-
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ferences between groups mean values (α=0.05). Regression analyses were done to illustrate the relationships between dependent and independent variables.
3
Results
3.1
Seasonal variations of dissolved N2O and fluxes
Monthly sampling showed that dissolved N2O concentrations varied within the range of 0.49–3.79, 0.27–0.62, and 0.44–40.66 μg N-N2O L−1 for the FLR, HBR, and NFR, respectively. N2O concentrations observed in October from the FLR, in June, July and October from the NFR were significantly higher than those in other months. The overall mean N2O concentration in the NFR was significantly higher than that in the FLR and HBR (Table 2). Table 2
River
FLR
HBR
NFR
Seasonal variations of N2O concentrations and estimated N2O fluxes Wind speed
Water temperature
Concentration
Flux (Diffusion model)
Month
m s−1
℃
µg N-N2O L−1
µg N-N2O m−2 h−1
June
2.0
26.0
1.05
20.68
July
1.5
30.0
0.59
10.01
August
3.0
27.0
0.69
14.98
September
3.0
25.0
0.53
14.93
October
2.5
20.0
3.79*
88.77*
November
5.0
15.0
0.49
46.65
December
5.0
6.5
0.55
18.61
Mean
3.1±1.4
21.4±8.2
1.19 ±1.23 (713%)
28.71±28.25
June
2.0
27.0
0.45
8.89
July
1.5
31.0
0.62
9.15
August
3.0
25.0
0.27
7.56
September
3.0
26.0
0.43
11.99
October
2.5
22.0
0.55
13.01
November
5.0
15.0
0.59
18.32
December
5.0
7.0
0.37
13.56
Mean
3.1±1.4
21.9±8.2
0.44±0.13 (254%)
11.59±4.37
June
2.0
27.0
16.32*
317.65*
July
1.5
30.5
12.27*
183.76*
August
3.0
28.0
1.73
42.59
September
3.0
25.5
0.59
10.91
October
2.5
22.0
40.66*
947.69*
November
5.0
15.0
0.44
26.15
December
5.0
7.0
2.16
70.64
Mean
3.1±1.4
22.1±8.4
10.59±14.67 (5664%)
236.87±449.74
Data of wind speeds at a height of 10 m were obtained from the meteorological station of Hefei city near our sampling locations. Three rivers were simultaneously sampled at each sampling occasion, thus the wind speeds were the same for three studied rivers in each sampling month. Mean values presented as “mean ± SE, n = 7”. *indicating mean values between groups are significantly different at the 0.05 test level (LSD test). Data in the brackets indicate percentage saturation.
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825
Estimated N2O fluxes varied within the range of 10.01–88.77, 7.56–18.32, and 10.91–947.69 µg N-N2O m−2 h−1 for the FLR, HBR, and NFR, respectively. Like dissolved N2O concentration, N2O flux observed in October from the FLR, in June, July and October from the NFR were significantly higher than those in other months; the overall mean flux measured from the NFR was significantly higher than those from the FLR and HBR (Table 2). In addition, the data also presented a general pattern in dissolved N2O and fluxes for each river, high during the fall and low during the summer and winter. The higher N2O concentration observed in the NFR may have resulted from the high nitrogen concentration in this river. Mean observed concentrations of NO3− and NH4+ in the NFR were 1.42 and 12.26 mg N L−1, respectively, significantly higher than those in the FLR (NO3−: 0.69 mg N L−1, NH4+: 0.28 mg N L−1) and HBR (NO3−: 0.83 mg N L−1, NH4+: 0.27 N mg L−1) during the periods of 2005 to 2009 (Figures 2a and 2b). Therefore, more N2O could be produced due to high nitrogen loads, which can be found in other studies (Wang et al., 2009; Kenny et al., 2004). For example, in the incubation study of LaMontagne et al. (2003), a statistically significant positive correlation of dissolved N2O concentrations to nitrate concentrations was detected (r2 = 0.872, P
