Effects of Lotus (Nelumbo nucifera Gaertn.) on the Methane Emission in Littoral Zones of a Subtropical Lake, China

Abstract

Freshwater lakes represent a potential source of methane (CH₄) emission into the atmosphere. However, the CH₄ emission contribution to the total emission in the littoral zones of lakes, especially emergent macrophytes (e.g., lotus), is poorly known. Lotus has been cultivated in almost all provinces in China; it is not only an aquatic plant, but also a kind of vegetable. In this study, two sampling zones (lotus plant and open water) were established in the lake of the middle reaches of the Yangtze River. The CH₄ emission was measured using a floating opaque chamber and gas chromatography between April and December in the years 2021 and 2022. The results indicated that the flux of CH₄ emissions ranged from 0.10 to 59.75 mg m⁻² h⁻¹, with an average value of 5.61 mg m⁻² h⁻¹, in the open water, while ranging from 0.19 to 57.32 mg m⁻² h⁻¹, with an average value of 17.14 mg m⁻² h⁻¹, in the lotus plant zone. The maximal CH₄ emissions occurred in July and August for the open water, which was highly related to the air and water temperature, whereas it happened in September for the lotus plant zone, possibly due to the high vegetation biomass, indirectly enhancing the high soil organic carbon content, plant-mediated CH₄ emission, as well as the lower dissolved oxygen concentration, thus strengthening the production and emissions of CH₄. Considering the carbon emissions (both CH₄ and CO₂) and plant productivity, although greater CH₄ emission occurred in the lotus plant zone, it could still represent a potential carbon sink (213 g m⁻² yr⁻¹) compared to the open water.

1. Introduction

It is estimated that there are up to 117 million global lakes greater than 0.002 km² in size, covering a total area of 5.0 million km² and corresponding to 3.7% of Earth’s non-glaciated land surface [1].

Although the methane (CH₄) emissions of freshwater (lakes and rivers) are usually less substantial than carbon dioxide (CO₂) fluxes [2], the warming potential of CH₄ is 28 times greater than CO₂ over 100 yr time frames [3]. Globally, the CH₄ emissions of freshwater lakes and rivers are about 122–159 Tg yr⁻¹ and account for 20% of CH₄ emission to the atmosphere [3,4], and their contribution is expected to increase in the future climate change scenarios [5]. Because of the spatio–temporal variability of CH₄ emission fluxes from lakes, recent CH₄ emissions from global lakes were estimated to range from 8 to 48 Tg yr⁻¹ [6,7]. Therefore, it is necessary to further understand the dynamics of the CH₄ source in lakes under different environmental factors.

Deemer et al. [8] indicated that a variety of biological, morphometric, and physical properties have been used as important predictors of CH₄ emission from freshwater to the atmosphere. These factors could be temperature [9,10], water depth [11,12], ecosystem productivity [9,12,13], and surface area [14,15], among others. However, the relationship between productivity and CH₄ emission has been shown empirically in various freshwaters [12,16,17]. Hence, in the littoral zone of lakes and rivers, most aquatic plants—such as Typha, Nymphaea, and Nelumbo—not only have high productivity (wet weight of 10 kg m⁻²) [18], but also morphological features adapt to waterlogged habitat [19]. Even if the littoral area of freshwater is relatively small, it can contribute to high CH₄ emission [20,21,22]. Moreover, the contribution of plant-dependent CH₄ emission to the total emission could increase to 80–90% in freshwater with emergent macrophytes [23,24,25]. Therefore, two studies conducted in the boreal lakes revealed that about 66–81% of CH₄ was released from the littoral zones, demonstrating that it is needed to reinforce the estimates of lakes’ CH₄ release in the vegetated littoral zone [21,22].

Lotus (Nelumbo nucifera Gaertn.), as an important aquatic plant and a type of vegetable, has been cultivated in China for more than 2000 years, especially around the Yangtze River [26]. Hubei, located in the middle reaches Yangtze River, is one of the largest provinces in lotus cultivation [26]. Hong Lake, covering an area of around 350 km², is the largest freshwater lake in Hubei province [27,28]. The lake was originally connected with the Yangtze River in the 1950s when the area of the lake was 760 km²; now, it is semi-connected to the Yangtze River because of the construction of sluices around the lake between the years 1955 and 1975 [27]. Although the water surface area of the lake has greatly decreased over these decades [27], there are large lotus aquatic plants living in the littoral zone of the lake, which has become a holy place for human recreation. Lotus aquatic plants could affect the CH₄ release of the freshwater, which should be further investigated in Hong Lake.

The purpose of this study was to evaluate the effect of lotus aquatic plant on CH₄ emissions in the freshwater lake in subtropical China. Two observation sites (lotus plant and open water) were set up in Hong Lake. CH₄ emission was measured by using the floating opaque chamber and a gas chromatography method. In addition, the driving factor of CH₄ emission, the vegetation biomass, soil organic carbon concentration, temperature, and dissolved oxygen concentration were measured as well, in order to investigate the relationships between environmental factors and CH₄ emissions.

2. Materials and Methods

2.1. Study Sites

The research area lay in the Hong Lake Natural Reserve in the middle reach of the Yangtze River, located between 29°40′ N and 29°58′ N, and between 113°12′ E and 113°26′ E [28]. Hong Lake is the seventh largest freshwater lake in China and the largest lake in Hubei Province [27]. The lake has a surface area of 344 km², with an open water area of 308 km², a littoral area of 36 km², and a mean water depth of 1.34 m [28,29]. This region is in the northern fringe of a humid subtropical monsoon climate, with an average annual temperature of 15.9–16.6 °C and average annual precipitation of 1060–1331 mm, as well as around 74% of precipitation, which occurs from April to October [27,28,29]. Paddy and fluvo-quic soil are the typical soils. Additionally, Hong Lake became one of the international important wetlands of the Ramsar convention in 2008 and one of the Chinese Wetland Ecosystem Research Stations in 2014 [28].

In the present study, two sites in the Hong Lake were selected to monitor CH₄ emission flux in the open water site (KKs) and the emergent vegetation of the lotus (N. nucifera Gaertn.) site (NNs) (Figure 1). No vegetation was grown in the site KKs. On the other hand, the NNs site was covered by the lotus, sparse areas containing Zizania latifolia Griseb Stapf., and invasive plant Eichhornia crassipes Mart Solme.

2.2. CH₄ Measurements

The floating opaque chambers were used to measure CH₄ emission from the open water and N. nucifera Gaertn. containers. CH₄ gases were sampled from April to December in 2021, and from April to October in 2022, and gas samples were collected once per month on a clear day between 8:30 and 11:30 a.m. (Beijing standard time, GMT + 8 h), with sampling carried out twice in September 2021 and June 2022, and three times in September 2022. The chamber was composed of acrylic organic glass (40 cm height × 35 cm diameter) [28]. Air inside the chambers was circulated with battery-driven fans during the measurement to ensure the gas samples were well mixed. In addition, the top of the chamber was set up with a thermometer sensor, while the open end of the chamber was fitted with a cystosepiment and tyre as floating equipment (Figure 2a). During the measurement, three chambers at each site were placed upside down at a distance of 50 to 100 cm apart on the water surface, and the open end of the chamber remained approximately 10 cm below the water surface. Generally, gas samples of the chamber were collected 0, 5, 10, and 15 min after enclosure using a 60 mL polypropylene syringe attached to a three-way stopcock. Subsequently, all gas samples were immediately injected into evacuated bags composed of polymer film and aluminum foil (Hoonpo Inc., Ningbo, Zhejiang province, China), then taken to the laboratory for determination within three days.

The hanging opaque chambers were used to measured plant-dependent CH₄ emission in the NNs site (Figure 2b). The chamber was composed of acrylic organic glass (30 cm height × 15 cm radius). The stem and leaf of the lotus above the water surface were enclosed in the chamber, and the end of the chamber was sealed by two half rounds of acrylic glass plates. In addition, the center of the acrylic glass plate had a small round hole (~3 mm) for passing through the stem of the plant, and the edge of the hole was sealed by plasticine when the plant was sealed in the chamber. Meanwhile, the bottom edge of the chamber was closed with transparent tape to guard against gas leakage. The inner part of the chamber was also set up with the thermometer sensor and fans, its function being similar to the floating opaque chambers. Before sampling, the stem and leaf of the plant were closed in the hanging opaque chamber; subsequently, three chambers were hung on the six bamboo poles (Figure 2b). Gas samples were immediately collected 0, 10, 20, and 30 min after chamber closure using a 60 mL polypropylene syringe attached to a three-way stopcock. Similarly, the gas samples were also injected into evacuated bags and transferred to the lab, where they were tested with the CH₄ concentration. The gas samples for the plant-dependent CH₄ emission in the NNs site were collected twice, namely, on 25 August and 21 September in 2022.

The CH₄ concentrations of all gas samples were determined by gas chromatography (Agilent, 7890A, GC system, Agilent Co., Wilmington, DE, USA) equipped with a flame ionization detector. In the study, about 20 mL of each gas sample was extracted from the gas bags to test the CH₄ concentration. We then assessed the flux value: 0.70 and above of the absolute coefficient R² values were 83%, 58% of which was 0.90 and above, and these dates were considered valid. The gas flux was calculated using the following equation [28]:

$$F=\left(\frac{dc}{dt}\right)\times \left(\frac{M}{{{\displaystyle V}}_0}\right)\times \left(\frac{273.15}{T}\right)\times \left(\frac{V}{A}\right)$$

(1)

where F (mg m⁻² h⁻¹) is the CH₄ emission flux at the time of chamber closure; dc/dt (ppm h⁻¹) is the linear slope of the CH₄ concentration change in the chamber along with time; M and V₀ are the molar mass and molar volume of CH₄ under standard conditions; T (K) is the absolute air temperature in the chamber; V (m³) represents the effective volume of the chamber; and A (m²) is the base area of the chamber.

2.3. Determination of the Vegetation Biomass and Environmental Factors

The temperatures of the air, water temperature, and chamber air, as well as the water depth near the sampling chamber, were measured at the same time as collecting the gas samples. The temperatures of the air and the inside of the chamber were measured using a digital thermometer (TM-902C, Factory of Lihuajin Instrument, Guangzhou, China). The water depth at two sites was measured using a ruler and bamboo. Water temperature, pH, and dissolved oxygen (DO) concentration at a water depth of 10 cm were measured using a portable multi-parameter water quality meter (Multi 3630 IDS, WTW Co., Munich, Germany).

The plant biomass, covering an area of 50 cm × 50 cm, was sampled at the NNs site in 2021. Three duplicate plant samples were randomly selected at the site; approximately 300 mg of each fresh plant duplicate sample was transported to a lab, where they were over-dried at 70 °C for 48 h for the dried biomass. The plant biomass was obtained using the following formula:

$$B_{plant}=4\times \left(\frac{W_d}{W_s}\times W_t\right)$$

(2)

where B_plant (g m⁻²) is the biomass of plant in the NNs; W_d (g) is the dried weight for each plant sample; W_s (g) is the fresh weight for each plant sample; and W_t (g) is the total fresh weight for each plant area (50 cm × 50 cm).

Three soil samples were also collected at a depth of 10 cm; subsequently, the soil samples were transferred to the laboratory and dried at 70 °C for 48 h. All soil samples were milled and passed through a 0.149 mm sieve, and soil samples of 100–500 mg were weighed to determine the soil organic carbon (SOC) content using the potassium dichromate oxidation external heating method. A soil sample of 10 g was weighed for pH measurements; the water-to-soil ratio was 2.5 to 1, and the pH value of the soil was measured using the potentiometric method [30]. Soil samples of 1000 mg were weighed to measure the total nitrogen (TN) content by using the Kjeldahl method with H₂SO₄ digestion, and soil samples of 200 mg were weighed to determine the total phosphorus (TP) content of the soil samples using the colorimetry by alkali fusion with NaOH [28].

2.4. Data Analysis

SPSS v 18.0 software was used for data processing and statistical analyses [30]. The independent-samples t-test was used to determine the statistical significance of differences in mean CH₄ emission, SOC content, TN, TP, plant biomass, and soil pH. Meanwhile, the paired t-test was used to assess the statistical significance of differences in the mean of the temperature, pH, DO, and water depth. Pearson’s rank correlation coefficient was used to investigate the correlations between CH₄ emission and temperatures, water depth, DO, and pH. The statistical significance of the differences was considered at p < 0.05 levels.

3. Results

3.1. Environmental Factors

Table 1. The characteristics of the soil and plant in Hong Lake.

Sites	Vegetation		Soil
	Vegetation Types	Biomass/g m−2	pH	SOC/g kg−1	TN/g kg−1	C/N	TP/g kg−1
KKs	No the vegetation	-	8.12 ± 0.05 a	16.63 ± 1.54 a	1.33 ± 0.14 a	12.54 ± 0.21 a	0.64 ± 0.01 a
NNs	N. nucifera	798.68 ± 12.34	7.27 ± 0.08 a	35.57 ± 1.67 b	3.07 ± 0.18 b	11.63 ± 0.40 a	0.62 ± 0.02 a

Note: different lowercase letters indicate a significant difference exists between two sites.

shows that the average vegetation biomass at the NNs site was 798.68 g m⁻². The soil pH at NNs was lower than that at KKs, whereas the SOC content and TN at NNs were two times that at KKs (p < 0.05). There was no significant difference (p > 0.05) in the carbon-to-nitrogen ratio (C/N) and TP between the NNs and at KKs (Table 1).

The mean air temperature was 26.1 and 26.4 °C, and the mean water temperature was 24.4 and 24.1 °C at KKs and NNs for 2021–2022, respectively. The mean water depth at NNs (108 cm) for 2021–2022 was significantly (p < 0.05) lower than that of the KKs (156 cm). The mean pH at a 10 cm water depth in the KKs and NNs was 8.4 and 7.8 for 2021–2022, respectively. In addition, the mean DO concentrations at a 10 cm water depth for 2021–2022 at NNs (4.5 mg/L) were significantly (p < 0.05) lower than that at KKs (7.9 mg/L) (Figure 3).

3.2. CH₄ Emission Flux

The temporal variations in CH₄ emission fluxes for 2021–2022 were recorded at the NNs and KKs, and the peak values of the KKs occurred in July and August, whereas this was in September at the NNs (Figure 4). The CH₄ emission fluxes ranged from 0.10 to 59.75 mg m⁻² h⁻¹ at the KKs for 2021–2022 and from 0.19 to 57.32 mg m⁻² h⁻¹ at the NNs for 2021–2022, respectively (Figure 4).

Plant-mediated CH₄ emission at the NNs was 9.58 and 15.27 mg m⁻² h⁻¹ on 25 August and 21 September 2022, accounting for 30% and 85% of total emissions, respectively (Figure 5a). Mean CH₄ emissions from the KKs and NNs were 9.68 and 18.18 mg m⁻² h⁻¹ for 2021 and 1.55 and 16.09 mg m⁻² h⁻¹ for 2022, respectively (Figure 5b). Meanwhile, the mean CH₄ emission flux (17.14 ± 1.98 mg m⁻² h⁻¹) at the NNs was three times than that at the KKs for 2021–2022 (5.61 ± 1.86 mg m⁻² h⁻¹) (Figure 5b).

3.3. Relationship between CH₄ Emission and Environmental Factors

All CH₄ emission fluxes were significantly (p < 0.05) and positively correlated with air and water temperatures (Figure 6a,b). Luo et al. [31] proposed that for the computational formula of temperature quotient (Q₁₀ = e^10b) in Figure 6b, the coefficient b is 0.137, the Q₁₀ value is 3.94, and the increase rate of CH₄ is 0.06 mol m⁻² h⁻¹ when the water temperature increases by 10 °C. When two values of the CH₄ emission flux (57.72 and 59.75 mg m⁻² h⁻¹) were removed, the CH₄ emission fluxes were significantly (p < 0.05) and negatively correlated with the DO concentration in the water (Figure 7).

4. Discussion

In recent years, studies have reported that freshwater lakes are important natural sources of CH₄ emission into the atmosphere [2,32]. In this study, the CH₄ emission flux shows very large temporal variations at Hong Lake, with a range from 0.10 to 59.75 mg m⁻² h⁻¹ during 2021–2022 (Figure 4). The CH₄ emissions in Hong Lake were high and comparable to Dong Lake of the largest Chinese urban lake (0.06–5.53 mg m⁻² h⁻¹) [33] and the largest shallow eutrophic lake Taihu in the Chinese subtropical region (0.006–0.37 mg m⁻² h⁻¹ for the diffusion emission) [34]. This could be attributed to the three pathways of CH₄ release, namely: diffusion, ebullition, and plant transport. For example, Xing et al [33] highlighted that there was no grown vegetation in their observation site, while Xiao et al. [34] indicated that the CH₄ flux only included diffusion. Generally, ebullition is a major pathway for CH₄ release from lake sediments to the atmosphere [35]. However, our results did not measure the ebullition of CH₄, which could limit the analysis in the three pathways of CH₄ emission. Wang et al. [36] indicated that the macrophyte-covered littoral zones were the “hotspots” of CH₄ emission in Lake Taihu, ranging from −1.7 to 131 mg m⁻² h⁻¹ from August 2003 to August 2004, emphasizing the importance of vegetation for the CH₄ emission. In addition, Gondwe et al. [37] also reported that the CH₄ emission in the swamps in the Okavango Delta, Botswana, varied between 0.24 and 293 mg m⁻² h⁻¹, and indicated that high CH₄ emission could probably be regulated by high temperature in the tropical wetlands. Hence, the CH₄ emission fluxes in the present study were significantly higher than the results of Xing et al. [33] and Xiao et al. [34], but significantly lower than the results of Wang et al. [36] and Gondwe et al. [37]. The CH₄ emission fluxes measured in Hong Lake were within the range reported for other subtropical/tropical wetlands. The results showed an obvious difference in the CH₄ emission in the lake in different regions.

In our study, the maximal CH₄ emission occurred in the summer (July and August) and autumn (September), which is consistent with previous studies [36,38,39]. However, there was a significant difference in the temporal variation of CH₄ emission between the KKs and NNs, e.g., maximal CH₄ emissions in July and August for the KKs, while in September for the NNs (Figure 4). The significant difference of CH₄ emission was ascribed to ecological determinants, e.g., climate (temperature), water depth, and vegetation types. During two growing seasons, we found that CH₄ emission fluxes increased exponentially with the air and water temperatures (Figure 6a,b). The results are in agreement with the observations in previous studies, which revealed that temperature could obviously affect the seasonal CH₄ emission in lakes and peatlands [33,38,40,41]. It is well-established that methanogenic microbial communities of lake sediments increase exponentially with temperature between 2 °C and 30 °C [42,43,44]. This could explain the maximum CH₄ emission in July and August in the KKs (Figure 4); namely, temperature was a key factor on the CH₄ emission for the open water. In addition, we analyzed the sensitivity of CH₄ emission fluxes to water temperature. The Q₁₀ was 3.94, suggesting that the CH₄ emission from Hong Lake will continue to strengthen under the scenarios of future climate change, thus contributing to climate warming.

However, compared with the KKs, the maximum values of CH₄ emission in the NNs occurred in September, which may be linked to the emergent macrophytes, N. nucifera Gaertn. On the one hand, numerous studies have indicated that vegetation is the key factor of CH₄ release in wetlands and the high emission is possibly attributed to primary production, which could supply organic matter incorporated into the sediment and induce the production of CH₄ by methanogenesis [16,17,21,43]. Table 1 shows that the average vegetation biomass (798.68 g m⁻²) and SOC content (35.57 g kg⁻¹) in the NNs were significantly higher than that of the KKs. The high vegetation biomass indirectly affects the carbon accumulation in sediments, thus contributing to the high SOC content, and should stimulate the production and emission of CH₄, which can lead to maximal CH₄ emission. However, Kim et al. [41] pointed out that the peak in CH₄ emission flux lagged the peak in biomass production by 2–3 weeks. Burke et al. [45] found the rates of highest CH₄ emission from Florida everglades with emergent aquatic vegetation, which is likely attributed to the organic matter incorporated into sediments. On the other hand, numerous examples in the literature have provided evidence that the contribution of plants transported through the aerenchyma to total CH₄ emission in the freshwater wetlands is 80–90% [23,24,25]. Most of the CH₄ released from the shallow sediment escaped oxidation and reached the atmosphere, and it was estimated that about 60–80% of the CH₄ emission was from the littoral zones in boreal lakes [21,22]. In our studies, the contribution of plant-dependent CH₄ emission to the total emission in the NNs was 30–85% (Figure 5a). Hence, the lotus plant zone in the Hong Lake has been shown to emit more CH₄ than that of open water (Figure 5b), which is probably attributed to the supply of fresh organic matter from the lotus plant in the littoral zone and the greater transport of CH₄ through the plant. This implies that the littoral zone vegetation of the lake can be the key factor in regulating the CH₄ emission in the NNs.

In general, water depth in wetlands is a major factor affecting the spatial and temporal variation of CH₄ emission flux [21,46]. Although we did not find a significant linear relationship between the CH₄ emission and water depth, there was a significantly negative correlation between the CH₄ emission fluxes and DO concentrations, but the R² value was much lower (Figure 7). One study reported that lower DO concentrations in water overlying the sediment in the lakes led to a higher CH₄ production [47], and thus increased the release of the CH₄ in a eutrophic lake [17,48]. In our study, the average DO concentration (4.5 mg/L) in the NNs was significantly lower than that of the KKs (7.9 mg/L, Figure 3g,h). Some studies have shown that small and shallow lakes emit more CH₄ than larger and deeper lakes, which could be attributed not only to the rich substrate supply from the littoral zone production of organic matter, but also to less time for CH₄ oxidation during passage through water column [14,40,49,50]. This could further explain the higher CH₄ emission fluxes in the NNs, compared to the KKs.

Based on our results, the need to consider the CH₄ emission in the littoral region of freshwater was further emphasized, which could influence the sink of carbon (C) in lakes. Therefore, the average CH₄ emissions (12.86 mg C-CH₄ m⁻² h⁻¹ in the NNs) reported in the present study were higher than the average CO₂ emission (3.78 mg C-CO₂ m⁻² h⁻¹ with no vegetation growth) in Chinese subtropical lake Donghu, as reported by Xing et al. [33] in the same climate region. We made a preliminary estimate that the annual C emission in the NNs was 113 g C m⁻² yr⁻¹ of CH₄ (12.86 mg C-CH₄ m⁻² h⁻¹ × 0.001 × 24 h × 365 d for the annual estimates of CH₄ emission), and was 33 g C m⁻² yr⁻¹ of CO₂ (3.78 mg C-CO₂ m⁻² h⁻¹ × 0.001 × 24 × 365 d for the annual estimates of CO₂ emission), respectively. In addition, the net primary productivity (dried weight) of the emergent vegetation, N. nucifera Gaertn., was 798.68 g m⁻² yr⁻¹ (Table 1). Subsequently, the transformation coefficient of C was usually 0.45, meaning that the C fixed by plants was 359.41 g C m⁻² yr⁻¹, and thus the C sink for the NNs was 213.41 g m⁻² yr⁻¹ (359.41 g C m⁻² yr⁻¹ minus 113 g C m⁻² yr⁻¹ minus 33 g C m⁻² yr⁻¹ for the budgets of C sink). Although there was greater CH₄ emission in emergent vegetation in the N. nucifera Gaertn. zone, it can still represent a potential sink of carbon. The results indicated that the lotus, as an important aquatic plant and a type of vegetable in China, was an important factor in the sink of carbon; moreover, it also implied that the restoration of aquatic vegetation in degraded lakes will contribute to an increase in carbon sink and to the mitigation of global warming.

5. Conclusions

In the present study, Hong Lake was found to be a potential source of CH₄ emission into the atmosphere. The massive plant biomass, more effective plant-mediated CH₄ emission, and higher SOC content could significantly enhance the release of CH₄ in the emergent vegetation of N. nucifera Gaertn. Moreover, the lower DO concentration in water overlying the sediment further stimulated the production of CH₄ in the emergent vegetation zone. Considering the carbon emission (CH₄ and CO₂) and the net primary productivity, the CH₄ emission in the emergent vegetation, N. nucifera Gaertn. zone was larger than that of the open water, but it can still represent a potential sink of carbon.