Objective Maximizing carbon dioxide (CO₂) uptake in wetlands is part of the global effort to combat climate warming. Yet, the simultaneously emitted potent greenhouse gas methane (CH₄) from wetlands may counterbalance their CO₂ uptake potentially. Studying the offset effect of annual CH₄ emissions to annual CO₂ uptake across different time scales and across different types of wetlands can avoid the misjudgment of “single carbon sink”, is conducive to fully understanding the carbon sink function of wetlands.

Method In this study, using the meta-analysis method, 141 years of data on net ecosystem CO₂ exchange (NEE) and 41 years of data on CH₄ fluxes were collated from different types of natural wetlands in China, including inland peatlands, marshes, coastal marshes, and mangroves.

Result We find that, in terms of wetland types, all wetlands function as net sinks for atmospheric CO₂ (negative flux represents sink and a smaller negative value indicates a stronger sink), and annual NEE increased following the order of coastal mangroves (–29 672 (–32 228 ~ –27 391) kg/ha), coastal marshes (–12 029 (–15 078 ~ –9 173) kg/ha), inland marshes (–8 819 (–10 786 ~ –6941) kg/ha), fens (–5 006 (–6 520 ~ –3 480) kg/ha), and peatland bogs (–4 745 (–7 136 ~ –2 452) kg/ha), while all wetlands function as net sources for atmospheric CH₄, and annual CH₄ emission rates were significant greater from peatland fens (437 (385 ~ 483) kg/ha) and inland marshes (319 (228 ~ 404) kg/ha) than those from coastal marshes (120 (75 ~ 165) kg/ha), peatland bogs (91 (74 ~ 113) kg/ha) and coastal mangroves (87 (53 ~ 126) kg/ha) (data is indicated as mean with 95% confidence interval, the same below). For the wetland location, annual NEE of coastal wetlands (–19 719 (–22 420 ~ –16 940) kg/ha) is significantly lower about two times than inland wetlands (–6 215 (–7 384 ~ –5 052) kg/ha) while annual CH₄ emission rate from coastal wetlands (96 (63 ~ 130) kg/ha) is approximately one-third of inland wetlands (282 (222 ~ 345) kg/ha). With regard to the climate zone, subtropical wetlands (–24 992 (–27 590 ~ –22 428) kg/ha) showed significant lower annual NEE than temperate wetlands (–6 237 (–7 372 ~ –5 147) kg/ha), while annual CH₄ emissions are significant greater for temperate wetlands (390 (323 ~ 449) kg/ha) than subtropical wetlands (109 (86 ~ 133) kg/ha). Considering the plant functional group, wetlands dominated by the forest (–29 672 (–32 198 ~ –27 434) kg/ha) or graminoid species (–11 646 (–14 052 ~ –9 396) kg/ha) showed significant lower annual NEE than wetlands dominated by moss (–7 036 (–10 564 ~ –3 350) kg/ha) or sedge species (–4 954 (–6 103 ~ –3 800) kg/ha), while annual CH₄ emissions is greatest in wetlands dominated by sedge species (410 (337 ~ 471) kg/ha), followed by graminoid species (259 (175 ~ 347) kg/ha) and lowest in wetlands dominated by moss (90 (72 ~ 115) kg/ha) and forest species (87 (53 ~ 127) kg/ha). Moreover, annual CO₂ uptake rates were significantly decreased and CH₄ emission rates were significantly increased with the increasing latitudes from 20°N to 55°N while exhibited parabolic trend with longitudes across natural wetlands in China. There exist distinct differences in the impacts of environmental factors on annual CO₂ uptake rates and CH₄ emission rates. Annual CO₂ uptake rates exhibits a linear increase with the increase of seven indicators, including mean annual temperature (MAT) and mean annual precipitation (MAP). In contrast, annual CH₄ emissions decrease linearly with MAT, MAP, and salinity; display a parabolic variation with water table depth (WTD), peaking within the range of 20 cm below the soil surface to 0 cm of soil surface; and show a logarithmic increasing trend with the increase of soil organic carbon (SOC) content. Across time scales spanning from 20 to 500 years, the net radiation balance of CO₂ uptake rates and CH₄ emission rates first decreases and then increases with longitude, while both linearly decline with the increase of MAT, MAP, soil temperature (T_s), and WTD, accompanied by a corresponding weakening of the warming effect.

Conclusion In conclusion, from the perspective of temporal variations of net radiative forcing, the positive radiative forcing of annual CH₄ emissions can completely and partially offset the negative radiative forcing of annual CO₂ uptake from inland and coastal wetlands, respectively, over 20- to 100-yr timeframes, while partially and marginally offset CO₂ uptake from inland and coastal wetlands, respectively, over 500-yr timeframe. Therefore, inland and coastal wetlands function as persistently C-based net greenhouse gas sinks from the long-term perspective and the latter is more effective natural climate solutions than the former on a per-unit-area basis.