Variations in CO2 and CH4 Exchange in Response to Multiple Biophysical Factors from a Mangrove Wetland Park in Southeastern China

Abstract

Mangrove ecosystems can be both significant sources and sinks of greenhouse gases. The restoration of mangrove forests is increasingly used as a natural climate solution tool to mitigate climate change. However, the estimates of carbon exchanges remain unclear, especially from restored mangroves. In this study, we observed the temporal variations in carbon dioxide (CO2) and methane (CH4) fluxes and their biophysical controls for 4 years, based on a closed-path eddy covariance (EC) system. The measurements were conducted in a mangrove wetland park with 14-year-old restored mangroves surrounded by open waters in Guangdong Province, China. The EC measurements showed that the mangrove ecosystem acted as a CO2 source with a net CO2 ecosystem exchange (NEE) of 305 g C m−2 from January 2019 to May 2020 by the 5-m tower measurement. After the tower was adjusted to 10 m, the mangrove showed a CO2 sink with an NEE of −345 g C m−2 from June 2020 to December 2022. The change in tower height influenced the interpretation of interannual trends on NEE. There were no significant interannual trends in the gross primary productivity (GPP) and the ecosystem respiration (Re) values. The change from CO2 source to sink may be attributed to the decrease in land surface proportion by the tower replacement, which reduces the proportion of the mangrove canopy respiration and, therefore, captures lower CO2 fluxes from open waters. The restored mangroves indicated strong CH4 sources of 23.2–26.3 g C m−2 a−1. According to the random forest analysis, the land surface proportion, radiation, and relative humidity were the three most important predictors of NEE, while the CH4 flux was most sensitive to air temperature. Compared to the natural and long-term restored mangroves, this 14-year-old restored mangrove had not yet achieved a maximum carbon sequestration capability. Our study highlights the need for the careful design of long-term observations from restored mangroves and proposes future needs in the context of carbon neutrality.

Global Warming Potentials of CO2 Uptake, CH4 Emissions, and Albedo Changes in a Restored Mangrove Ecosystem

“…Mangrove restoration efforts need 40 years to achieve biomass recovery (Sasmito et al., 2019). A 12‐year restored mangrove ecosystem acted as a C source during 2019–2020, probably because of the young age of the mangroves and the large area of water surface (C. Wang et al., 2023; Zhao et al., 2022). Annual GPP of our restored mangroves in 2020, 2021, and 2022 were 1,543.93, 1,416.46, and 1,688.91 g C m −2 yr −1 , respectively, similar to the mangroves restored for 12 years with a GPP value of 1,466 g C m −2 yr −1 (Zhao et al., 2022).…”

“…The NEP of natural mangrove ecosystems showed significant CO 2 sinks, with NEP ranging from 73.8–1,170 g C m −2 yr −1 and canopy heights of 3–20 m (Alvarado‐Barrientos et al., 2021; Barr et al., 2010; H. Chen et al., 2014; Gou et al., 2023; Leopold et al., 2016; X. Zhu, Qin, & Song, 2021). In mangrove restoration, the NEP varied significantly, ranging from −305 to 1,105 g C m −2 yr −1 for mangroves restored for 6–20 years (Cui et al., 2018; C. Wang et al., 2023; Zhao et al., 2022). The NEP of our restored mangroves fell within these ranges and indicated the role of a CO 2 sink from 2020 to 2022.…”

“…Their results showed notable discrepancies, with NEE ranging from 1,105 to 305 g C m 2 yr 1 (Cameron et al, 2019;Cui et al, 2018;J. G. Liu & Lai, 2019;C. Wang et al, 2023;Zhao et al, 2022).…”

“…The Leizhou mangroves restored more than 20 years have a similar CO 2 uptake potential to the Gaoqiao natural mangroves (Cui et al, 2018). However, a 12-year restored mangrove ecosystem appeared to function as a CO 2 source, contrary to the conventional understanding of mangroves as sinks (C. Wang et al, 2023;Zhao et al, 2022). The assessment of climate benefits in the early stages of restored mangroves remained unclear due to the limited number of studies compared to those conducted on natural mangroves.…”

“…Previous studies have observed the net ecosystem CO 2 exchange (NEE) of restoration and assessed their climatic effects utilizing the eddy covariance (EC) system. Their results showed notable discrepancies, with NEE ranging from −1,105 to 305 g C m −2 yr −1 (Cameron et al., 2019; Cui et al., 2018; J. G. Liu & Lai, 2019; C. Wang et al., 2023; Zhao et al., 2022). The Leizhou mangroves restored more than 20 years have a similar CO 2 uptake potential to the Gaoqiao natural mangroves (Cui et al., 2018).…”