Methane emissions reduce the radiative cooling effect of a subtropical estuarine mangrove wetland by half

Abstract

The role of coastal mangrove wetlands in sequestering atmospheric carbon dioxide (CO 2) and mitigating climate change has received increasing attention in recent years. While recent studies have shown that methane (CH 4) emissions can potentially offset the carbon burial rates in low-salinity coastal wetlands, there is hitherto a paucity of direct and year-round measurements of ecosystem-scale CH 4 flux (F CH4) from mangrove ecosystems. In this study, we examined the temporal variations and biophysical drivers of ecosystem-scale F CH4 in a subtropical estuarine mangrove wetland based on 3 years of eddy covariance measurements. Our results showed that daily mangrove F CH4 reached a peak of over 0.1 g CH 4-C m −2 day −1 during the summertime owing to a combination of high temperature and low salinity, while the wintertime F CH4 was negligible. In this mangrove, the mean annual CH 4 emission was 11.7 ± 0.4 g CH 4-C m-2 year −1 while the annual net ecosystem CO 2 exchange ranged between −891 and −690 g CO 2-C m −2 year −1 , indicating a net cooling effect on climate over decadal to centurial timescales. Meanwhile, we showed that mangrove F CH4 could offset the negative radiative forcing caused by CO 2 uptake by 52% and 24% over a time horizon of 20 and 100 years, respectively, based on the corresponding sustained-flux global warming potentials. Moreover, we found that 87% and 69% of the total variance of daily F CH4 could be explained by the random forest machine learning algorithm and traditional linear regression model, respectively, with soil temperature and salinity being the most dominant controls. This study was the first of its kind to characterize ecosystem-scale F CH4 in a mangrove wetland with longterm eddy covariance measurements. Our findings implied that future environmental changes such as climate warming and increasing river discharge might increase CH 4 emissions and hence reduce the net radiative cooling effect of estuarine mangrove forests.

Drought‐Induced Salinity Enhancement Weakens Mangrove Greenhouse Gas Cycling

“…The seasonal variation of CH 4 emissions from this mangrove, showing higher emissions in the wet season than the dry season, is consistent with the findings in other subtropical/tropical wetlands with similar seasonal variations in temperature and rainfall (Dalmagro et al, 2019;Philipp et al, 2017;Tang et al, 2018). In comparison with a recent EC study on mangrove NME (Liu et al, 2020), daily NME of this mangrove shows less noticeable seasonal variation pattern. The difference in the seasonality may ZHU ET AL.…”

“…Second, in comparison with Rosentreter et al (2018b) assuming a 50% daily inundation duration, this mangrove has a much shorter daily inundation duration (∼5% of time), which could lead to lower CH 4 emissions and its offset potential. Third, in relation to Liu et al (2020), the obviously lower CH 4 offset potential of this mangrove results from a stronger CO 2 sink (−1,075.8 vs. −782.3 g CO 2 -C m −2 year −1 ) and a weaker CH 4 source (3.1 vs. 11.7 g CH 4 -C m −2 year −1 ), which are mostly likely attributed to the difference in annual rainfall (∼1,000 vs. ∼2,000 mm) since more rainfall usually accompanies with less PAR (weaker CO 2 sink) and lower salinity (stronger CH 4 source).…”

“…These EC studies provide long-term continuous ecosystem fluxes with improved spatial/temporal representations, but few of them simultaneously measure CO 2 and CH 4 fluxes in mangroves (Knox et al, 2019), which strongly limits the ability to accurately quantify the magnitudes of GHG fluxes and their contributions to net GHG balance. Based on 3-year continuous EC measurements in a subtropical estuarine mangrove, Liu et al (2020) found that ecosystem CH 4 emissions could offset half of the radiative cooling effect by CO 2 uptakes over a 20-year time horizon, highlighting the critical role of CH 4 emissions in regulating net GHG balance. However, more EC-based GHG flux measurements are highly needed to assess whether this strong CH 4 offset potential is applicable to other estuarine mangroves.…”

Methane emissions respond to soil temperature in convergent patterns but divergent sensitivities across wetlands along altitude

“…In addition, exponential fitting between methane emissions and soil temperature in a boreal fen has indicated a rapid increase of methane emissions at a baseline temperature of 5°C–10°C (Rinne et al, 2018). In order to have an insight into the relationship between methane emissions and soil temperature in the altitude perspectives, we compared the available data from eight wetlands from various altitudes with our results, that is, Rocky Mountain wetland of USA, Bayinbuluk wetland of the Tianshan Mountain, Luanhaizi wetland, and Zoige wetland on the Tibetan Plateau as the four high altitude ones, Lake Wohlen in Switzerland as the mid‐altitude one and Mai Po wetland in Hong Kong SAR of China as the low altitude one (Table 5; Delsontro et al, 2010; He et al, 2014; Liu et al, 2020; Peng et al, 2019; Song et al, 2015; Wickland et al, 2001). The relationships can be characterized as exponential based on data from the respective literature, and their turning points were calculated accordingly (Table 5, the depth of soil temperature depends on the site specific conditions in each study).…”

“…Data were run on point emissions and properties, except those between methane emissions and DOC as they are averaged values of each sampling campaigns. To compare our results with the global data on methane emission and associated soil temperatures across wetlands, we use the data from Wickland et al (2001), Delsontro et al (2010), He et al (2014), Song et al (2015), Peng et al (2019), and Liu et al (2020) that were extracted by GetData Graph Digitizer 2.2.5. p < 0.05 indicates significant differences. Statistical analyses were conducted using SPSS 17.0.…”