Effects of nitrogen and sulfur deposition on CH₄ and N₂O fluxes in high-altitude peatland soil under different water tables in the Tibetan Plateau

Abstract

The effects of nitrogen (N) and sulfur (S) deposition on methane (CH₄) and nitrous oxide (N₂O) emissions under low (10 cm below soil surface) and high (at soil surface) water tables were investigated in the laboratory. Undisturbed soil columns from the alpine peatland of the Tibetan Plateau were analyzed. CH₄ emission was higher and N₂O emission was lower at the high water table than those at the low water table regardless of nutrient application. Addition of N (NH₄NO₃ (ammonium nitrate), 5 g N m⁻²) decreased CH₄ emission up to 57% and 50% at low and high water tables, respectively, but correspondingly increased N₂O emission by 2.5 and 10.4 times. Addition of S (Na₂SO₄ (sodium sulfate), 2.5 g S m⁻²) decreased CH₄ and N₂O emission by 64% and 79% at the low water table, respectively, but had a slightly positive effect at the high water table. These results indicated that the responses of CH₄ and N₂O emissions to the S deposition depend on the water table condition in the high-altitude peatland.

Key words:

• Climate change
• greenhouse gases
• wetland
• atmospheric deposition
• water level

INTRODUCTION

Peatlands naturally contain large amounts of carbon (C) and nitrogen (N) within their soils, and currently serve as sources for methane (CH₄) and nitrous oxide (N₂O) (Regina et al. 1999; Huttunen et al. 2003). CH₄ and N₂O, as two important greenhouse gases, are approximately 25 and 300 times more effective in global warming than carbon dioxide (CO₂), respectively (Hou et al. 2013). CH₄ is produced by methanogenesis, whereas N₂O is released mainly from nitrification and denitrification (Aerts and Ludwig 1997; Nykänen et al. 2002). These soil microbiological processes depend remarkably on the soil water conditions and nutrient status (Aerts and Ludwig 1997). Peatlands, similar to other ecosystems, undergo climate changes and nutrient [especially N and sulfur (S)] loading because of atmospheric deposition and other anthropogenic activities (Keller et al. 2006). These changes may alter the soil water table and nutrient status (i.e. N and S) and further affect the emission rates of CH₄ and N₂O from peatland soils. It is therefore urgent to assess the effect of water table conditions, as well as N and S inputs, on greenhouse gas flux in the peatland soils.

Several studies have recently examined the effects of water table on the CH₄ gas fluxes in peatland soils (Aerts and Ludwig 1997; Dinsmore et al. 2009; Chen et al. 2011b; Hou et al. 2013). These studies have found that lowering the water table decreases CH₄ emission. A few studies on the effect of water table depth on peatland soil N2O emissions have been conducted (Aerts and Ludwig 1997; Regina et al. 1999; Hou et al. 2013), in which the lowering of the water table resulted in a net increase in N₂O fluxes. However, the effects of N and S input, especially for S, on CH₄ and N₂O emissions of peatland soils are rare and limited to low-elevation peatlands of the boreal zone (Aerts and Ludwig 1997; Granberg et al. 2001; Nykänen et al. 2002; Melanie and Bridgham 2003). Uncertain conclusions have been raised on the CH₄ fluxes as affected by N and S amendment (Granberg et al. 2001). Studies on the effect of N addition on N₂O emission generally show that N stimulates N₂O emission from the boreal peatland soil (Nykänen et al. 2002; Zhang et al. 2007). Although S deposition may alter N₂O emission in soils (Sanderson et al. 2006), little is known about the impact of S deposition on N₂O emission in peatland. Until now the effects of water table on high-altitude peatlands have received less attention, and no information is available about the impact of N or S on the greenhouse gas emissions within these ecosystems.

The Tibetan Plateau is the highest [average 4000 m above sea level (a.s.l.)] plateau in the world, an area that contains the world’s largest extent of high-altitude peatlands. These alpine peatlands contain a large amount of soil organic C and play an important role in the greenhouse gas budget of peatlands in the world (Wang et al. 2002). The peatland areas of the Tibetan Plateau are currently receiving elevated rates of atmospheric N and S deposition (Wang et al. 2004; Liu et al. 2011). Although several studies have investigated the greenhouse gases in the alpine peatlands of the Tibetan Plateau, the majority of these studies have only analyzed the effects of temporal-spatial variations and vegetation on CH₄ and N₂O fluxes (Chen et al. 2011a, 2011b). Investigations on the impact of N and S applications and their interactions with different water tables in the alpine peatland are insufficient.

This study was designed to measure the role of N and S deposition on the CH₄ and N₂O emissions under various water table conditions in the alpine peatland soil on the Tibetan Plateau. We determined the effects of increased N and S addition in alpine peatland soil on CH₄ and N₂O emissions. We also evaluated whether the observed variations depend on the water table condition.

MATERIALS AND METHODS

Study sites

The study was conducted at Hongyuan County (33°54′N, 102°36′E, at 3500 m a.s.l.), located on the eastern Tibetan Plateau, Southwest China. This area is characterized by a harsh continental climate. The mean annual temperature is 1.1°C. The highest monthly mean temperature is 10.9°C in July and the lowest is –10.3°C in January. The mean annual precipitation is 753 mm, with approximately 86% received from May until September. The vegetation of the study site is dominated by Carex muliensis (Hand.-Mazz.), accompanied by Caltha scaposa (Hook. F. et Thoms), Chamaesium paradoxum (H. Wolff) and Sanguisorba filiformis ((Hook. f.) Hand.-Mazz.). The soil of the study site was classified as peat. The peat layer of the investigated site is more than 2 m deep. The soil characteristics of the study site are shown in Table 1.

Table 1 Soil characteristics in the study site

Depth (cm)	pH	OC (g kg^−1)	TN (g kg^−1)	C/N	TP (g kg^−1)
0–10	5.33 ± 0.07	252.51 ± 10.05	20.80 ± 1.28	12.16 ± 0.67	0.88 ± 0.06
10–20	5.17 ± 0.04	229.89 ± 16.91	19.62 ± 1.09	11.72 ± 0.75	0.80 ± 0.04
20–30	5.28 ± 0.06	218.54 ± 13.88	19.28 ± 0.74	11.29 ± 0.50	0.77 ± 0.03

OC, organic carbon; TN, total nitrogen; C/N, carbon/nitrogen; TP, total phosphorus. Values are means and standard errors for four replicate samples.

Experimental setup

Forty undisturbed soil columns (11 cm diameter, 30 cm depth) were collected by inserting polyvinyl chloride (PVC) tubes into the peatland in August 2012. Vegetation and litter layer were removed before inserting the tubes. All columns were cooled with freezer blocks and returned to the laboratory in insulated boxes. The soil columns were then randomly divided into two groups, and each group consisted of 20 columns. The water tables in the soil columns in one group were adjusted with distilled water to 10 cm below the soil surface and to the soil surface for the other group. The columns were then pre-incubated in a dark climate room at 15°C for 5 days before the nutrients treatments were applied. Incubation temperature was designated as the soil temperature during soil column sampling in the field. After the pre-incubation period, independent, as well as combined, N and S additions were tested and compared with untreated control columns within each group. The soil columns were supplied once with 10 mL of nutrient solution. The nutrient supply consisted of 1.7 mmol NH₄NO₃ (ammonium nitrate) or 0.7 mmol Na₂SO₄ (sodium sulfate), which is equivalent to 5.0 g N m⁻² and 2.5 g S m⁻², respectively. The amount of N treatment corresponds closely to the amounts supplied by local farmers to grasslands in the study area. The amount of S treatment is close to the atmospheric S deposition in this region (Liu et al. 2011). Control columns were supplied with 10 mL of distilled water. Then, the water tables were adjusted with distilled water to 10 cm below the soil surface and to the soil surface in soil columns for the two groups to further compensate for the evaporative water loss from the column during the pre-incubation period. The water tables were then maintained until the end of the experiment by adding distilled water every 2–3 d. To minimize the effects of the water fluctuation caused by water addition on gas emissions, the distilled water was added at least 1 d prior to gas sampling. Each treatment was replicated five times. All soil columns were incubated in a dark climate room at 15°C during the period of this investigation.

Measurements

The gases for CH₄ and N₂O were determined at day 2, 4, 8, 12, 16, 20, 24, 28, 32, 36 and 40 after nutrient application. For measurements, a PVC cap fitted with a septum sampling valve was securing to the top of each column for 1 h. A needle was inserted into the septum while fitting the cap to minimize pressure disturbance. Gas samples (10 mL) were taken from the columns after 20, 40 and 60 min using a polypropylene syringe. All gas samples were collected between 9:00 AM and 11:00 AM. The concentrations of gases within the samples were analyzed using a gas chromatograph (Hewlett Packard 5890N) within 24 h after the gas sampling. The gas chromatograph was equipped with a flame ionization detector for CH₄ analysis and an electron capture detector for N₂O analysis.

Calculation and statistical analysis

The fluxes of CH₄ and N₂O were calculated from the linear changes in the gas concentration of the headspace volume for each soil column (Chen et al. 2011). Cumulative CH₄ and N₂O emissions from each column were calculated by linear interpolation between sampling dates (Ni et al. 2012).

Statistical analyses were performed using SPSS 16.0 (SPSS Inc., Chicago). Data were tested for normality and log-transformed before analysis of variance (ANOVA) was conducted. One-way ANOVA was used to determine the effect of nutrients on the CH₄ and N₂O emission rates and on cumulative amounts. Differences between the two soil table conditions with the same nutrient addition were tested using Mann-Whitney U tests. Significance was accepted at P < 0.05.

RESULTS

The CH₄ emission rates from this alpine peatland soil at low water table were generally lower than those at the high water table during the investigation days (Fig. 1a and b). Addition of N had a significant negative effect on the CH₄ fluxes under the two water table conditions for 40 d of incubation. Addition of S decreased the CH₄ flux in the low water table, but tended to increase CH₄ flux in the high water table.

Figure 1 The methane (CH₄) fluxes in control (C), nitrogen (N), sulfur (S) and both nitrogen and sulfur (NS) additions under (a) low water table and (b) high water table during the investigation period. Values are means and standard errors for five replicate samples.

The N₂O emission rates of the control soil at low water table were higher than those at high water table over the entire incubation period (Fig. 2a and b). Addition of N had a significant positive impact on N₂O fluxes regardless of water levels and resulted in a large increase in the N₂O emission rate on the first sampling day (Fig. 2a and b). Addition of S significantly inhibited the N₂O fluxes at the low water table during the investigation period (Fig. 2a).

Figure 2 The nitrous oxide (N₂O) fluxes in control (C), nitrogen (N), sulfur (S) and both nitrogen and sulfur (NS) additions under (a) low water table and (b) high water table during the investigation period. The panel situated in the upper right-hand corner of the figure is to show N₂O fluxes from day 8 to day 40 more clearly. Values are means and standard errors for five replicate samples.

The cumulative emissions of CH₄ for 40 d in soils with N addition decreased by 57% and 50% in the low and high water tables, respectively, compared with those of the control soils (Fig. 3a and b). Addition of S reduced cumulative CH₄ emissions in the low water table by more than 60% compared with the control soil, but had a slightly positive impact in the high water table (Fig. 3a and b).

Figure 3 The cumulative methane (CH₄) and nitrous oxide (N₂O) emissions in control (C), nitrogen (N), sulfur (S) and both nitrogen and sulfur (NS) additions under (a, c) low water table and (b, d) high water table during the investigation period. Values are means and standard errors for five replicate samples. Different letters indicate significant differences at P < 0.05 using the least significant difference (LSD) test.

The N₂O cumulative emissions during the investigation period were 2.5 and 10.4 times higher in soils with N addition than those in the control at low and high water tables, respectively (Fig. 3c and d). Compared with the control, S addition decreased N₂O cumulative emissions up to 79% in the low water tables, but tended to increase N₂O cumulative emissions in the high water table (Fig. 3c and d).

DISCUSSION

CH₄ emission

The CH₄ emission from the peatland soil was found to be higher in the high water table than in the low water table in this study, which was similar to the conclusion from the previous studies (Aerts and Ludwig 1997; Dinsmore et al. 2009; Yamamoto et al. 2011). The amount of CH₄ released from peatland soil to the atmosphere is the difference between the production and oxidation of CH₄ in soil (Nykänen et al. 2002). Increased water table level may result in decreased oxygen availability in the upper soil layer, which would limit methane oxidation and lead to an increase in the soil CH₄ emission (Aerts and Ludwig 1997; Kotelnikova 2002).

N inputs can affect the activity of both CH₄-producing and CH₄-oxidizing microbes (Nykänen et al. 2002). Nitrate (NO₃^–) can decrease methanogenesis because of microbes that use nitrate as electron acceptors while the microbes are oxidizing organic substrates, and then methanogens cannot compete for their substrates with nitrate reducers (Chowdhury and Dick 2013). Ammonium (NH₄⁺) compounds generally reduce the activity of the CH₄-oxidizing bacteria and increase CH₄ release in soil (Nykänen et al. 2002). However, Rigler and Zechmeister-Boltenstern (1999) and Bodelier et al. (2000) reported that the both NO₃^– and NH₄⁺ can increase methane oxidation and decrease CH₄ release in N-limited conditions. Our study showed that N addition inhibits the CH₄ production at the two water tables. Considering that the amounts of NO₃^– and NH₄⁺ added into the soils are almost equal in our study, the negative effects of N on CH₄ production cannot be attributed completely to the decrease in methanogenesis by adding NO₃^–. Rigler and Zechmeister-Boltenstern (1999) and Bodelier et al. (2000) reported that NO₃^– and NH₄⁺ can increase methane oxidation and decrease CH₄ release in nitrogen-limited conditions. This result could explain the soil CH₄ responses to NH₄NO₃ addition in this alpine peatland where low N is available in the soils because low air temperature causes low rates of organic matter decomposition.

The sulfate ion (SO₄^2–) addition to the peatland soil with the low water table reduced CH₄ emission in this study. This result is comparable to those of others who found a decrease in CH₄ production caused by SO₄^2– in peat soil under aerobic conditions (Yavitt et al. 1987; Dise and Verry 2001; Granberg et al. 2001). The major effect of SO₄^2– on CH₄ production is exerted through its effects on the competition for substrate (i.e. electron donors, hydrogen and acetate) between sulfate-reducing bacteria and methanogens (Gauci et al. 2004; Eriksson et al. 2010). When SO₄^2– is applied, the sulfate-reducing bacteria in peat, stimulated by the increasing SO₄^2– availability, consumed more substrate and inhibited methanogens. However, the inhibition of CH₄ production by SO₄^2– addition in wetland soil could be reversed by increasing acetate or hydrogen (Winfrey and Zeikus 1977). Yavitt et al. (1987) also reported that SO₄^2– addition to the deep peat led to an increased production of these substrates by starting a cascade of anaerobic reactions. The increased production of substrates caused the small increase in CH₄ emission with SO₄^2– addition, which was measured in the high water table in this study. These results suggested that the water table plays an important role in regulating the effect of S deposition on CH₄ in the high-altitude peatland.

N₂O emission

N₂O is produced mainly during both nitrification and denitrification in soil (Aerts and Ludwig 1997; Nykänen et al. 2002). Nitrification requires aerobic environments, whereas denitrification requires anaerobic conditions (Aerts and Ludwig 1997). In the current study, higher N₂O emission was measured at a low water level, similar to previous studies that were performed in both field and laboratory experiments (Aerts and Ludwig 1997; Zhu et al. 2008; Hou et al. 2013). The high N₂O emission of the low water table is due to the following: (1) the low water table increased aerobic condition in the upper soil, which enhances nitrification of ammonium (Aerts and Ludwig 1997), and (2) the N₂O would be reduced to N₂ by denitrification at high water table because of low oxygen availability (Regina et al. 1999).

Addition of N generally results in the enhancement of N₂O fluxes in peatland soils because of the increase in the substrate during nitrification and denitrifcation after NH₄NO₃ application (Nykänen et al. 2002; Zhang et al. 2007). A similar result on the effects of N addition on N₂O flux was also observed in the present study.

The SO₄^2– addition inhibited N₂O production in the low water table, but stimulated N₂O emission in the high water table in the present study. To the best of our knowledge, no reports are available on the effects of SO₄^2– on N₂O emission in peatland soils. Several studies have investigated the effect of S on N₂O emission in aquatic sediments (Brunet and Garcia-Gil 1996), agricultural soils (Hasegawa et al. 2000) and subtropical forest soils (Cai et al. 2012). These studies showed that the effects of S addition on N₂O emission in soils varied depending on soil properties. N₂O production in SO₄^2–-treated soil is related to sulfur denitrification via sulfate reduction or heterotrophic denitrification (Yamashita et al. 2011). Under aerobic condition, sulfate-reducing bacteria simulated by adding SO₄^2– consume substrate (hydrogen and acetate) and inhibit sulfur denitrification (Winfrey and Zeikus 1977; Zhu et al. 2005). Under anaerobic conditions, the increased substrate simulated sulfur denitrification to reduce nitrate to nitrogen gas (Yavitt et al. 1987), indicating that the magnitude of the response of the N₂O production in the alpine peatland soil to the S addition is also dependant on the water table level.

Overall, this study demonstrated that N addition inhibits CH₄ production and increases N₂O emission regardless of water table conditions in the alpine peatland, but the effects of SO₄^2– on CH₄ and N₂O emissions vary depending on the water table levels. Given the importance of alpine peatland to the global emission of CH₄ and N₂O, a better understanding of the effects of the N and S inputs with different water tables on the these gases at the ecosystem level should be a strong priority for future studies.

ACKNOWLEDGMENTS

This research was supported by the National Basic Research Program (973) of China (No. 2012CB417101), and the National Science Foundation (No. 40801089 & 41271276). We gratefully acknowledge the assistance of Changbing Zhang and Ping Li during the field work. We greatly appreciate the constructive comments of the editors and two anonymous reviewers on an earlier version of the manuscript.