"C-wetland" : A new paradigm to enhance conservation of carbon-rich wetlands

Wetlands have the highest ecosystem service value among all terrestrial ecosystems. Among their ecosystem services, carbon sequestration and its potential ability to mitigate climate change have been extensively discussed recently, not only in the academic community but in media during COP27 of UNFCCC and COP14 of Ramsar. In particular, peatlands—peat-accumulating wetlands—accumulate 600 Gt C globally. However, peatlands were historically exploited for their peat and reclaimed to other land-use types. In fact, the definition of peatlands is based on their resource value lying in the organic matter content in peat. Therefore, under goals of conservation and carbon neutrality, “peatlands” may not be a precise concept for estimating and conserving the substantial carbon stock of this type of wetland. For highlighting the carbon stock of wetlands rather than considering their resource characteristics, a new systematic description of carbon-rich wetlands is needed for accurately understanding the distribution of the wetlands and for better conserving their carbon stock. Therefore, we propose a new term, “C-wetland,” for carbon-rich wetlands and map the global C-wetland distribution (Figure 1). Since peatland refers to lands with enough peat deposits, we should first know that “peat” is defined as dead and partially decomposed plant material accumulated in situ, which often contains more than 30% organic matter (OM) by dry mass.2United Nations Environment ProgrammeGlobal Peatlands Assessment – the State of the World’s Peatlands: Evidence for Action toward the Conservation, Restoration, and Sustainable Management of Peatlands. United Nations Environment Programme, Nairobi, 2022Google Scholar Different countries have different minimum standards for organic matter content (20%–60%), probably based on their peatland extent, with countries having more peatland distribution making higher standards.3Chen H. Wu N. Wang Y. et al.A historical overview about basic issues and studies of mires (in Chinese).Sci. Sin. -Terrae. 2020; 51: 15-26Google Scholar Besides the organic matter content of peats, the minimum thickness is another important indicator to define peatland, which is often no less than 30 cm, again with variable standards (0–60 cm) in different countries.3Chen H. Wu N. Wang Y. et al.A historical overview about basic issues and studies of mires (in Chinese).Sci. Sin. -Terrae. 2020; 51: 15-26Google Scholar Therefore, “peatland” is a term for land with an in-situ-accumulated peat layer meeting the defining standards of organic matter content and peat thickness. Peatlands usually include both intact and degraded ones. Intact peatlands are actively forming peat, while degraded peatlands no longer form peat but may lose it.2United Nations Environment ProgrammeGlobal Peatlands Assessment – the State of the World’s Peatlands: Evidence for Action toward the Conservation, Restoration, and Sustainable Management of Peatlands. United Nations Environment Programme, Nairobi, 2022Google Scholar,3Chen H. Wu N. Wang Y. et al.A historical overview about basic issues and studies of mires (in Chinese).Sci. Sin. -Terrae. 2020; 51: 15-26Google Scholar Generally, “wetland” is a term for all wet habitats permanently or periodically saturated. Due to such saturated or flooded conditions, wetlands usually also have another two attributes: hydric soils and hydrophytes growing in water or saturated soils (https://water.usgs.gov/nwsum/WSP2425/definitions.html). By such a definition, intact peatlands, with all three important attributes of wetlands (saturated conditions, hydric soils, and hydrophytes), should be regarded as a typical wetland. Degraded peatlands, some with enough water, soil, and vegetation attributes, can also be regarded as wetlands. From the point of view of conservation, intact peatlands, as well as some degraded peatlands, should be included for conservation as wetlands.3Chen H. Wu N. Wang Y. et al.A historical overview about basic issues and studies of mires (in Chinese).Sci. Sin. -Terrae. 2020; 51: 15-26Google Scholar As the most valuable ecosystem on Earth, wetlands including most peatlands provide substantial ecosystem services to human beings. Therefore, as an important part of wetlands, peatlands should not be evaluated by their organic matter content and peat thickness but rather by their important ecological function, especially for their substantial long-term accumulation of soil carbon. Due to the water-logged conditions of wetlands, decomposition of their soil organic carbon (SOC) is limited, making wetlands a carbon sink with or without peat accumulating. Among them, intact peatlands are generally a peat-accumulating wetland with strong carbon sequestration ability. For example, peatlands hold more than 30% of all soil carbon despite their area of about only 3%–4% of the global terrestrial area.2United Nations Environment ProgrammeGlobal Peatlands Assessment – the State of the World’s Peatlands: Evidence for Action toward the Conservation, Restoration, and Sustainable Management of Peatlands. United Nations Environment Programme, Nairobi, 2022Google Scholar,4Yu Z. Loisel J. Brosseau D.P. et al.Global peatland dynamics since the last glacial maximum.Geophys. Res. Lett. 2010; 37: L13402Crossref Scopus (800) Google Scholar However, the soil carbon stock estimate is controversial for the world’s peatlands since the global extent of peatlands remains uncertain due to the varied definition standards for peatlands among countries. Moreover, still, many wetlands with peat deposits or rich organic matter accumulation serve as important carbon stocks and carbon sinks but may not be regarded as peatlands due to their lower organic matter content or shallower peats than the arbitrary definitions of peatlands. Their being excluded leads to underestimation of the soil carbon stock of carbon-rich wetlands. Therefore, for sustaining soil carbon stocks, leading scientists in climate change urge the prioritizing of the conservation of peatland in order to ensure more carbon in soil.5Rumpel C. Amiraslani F. Koutika L.S. et al.Put more carbon in soils to meet Paris climate pledges.Nature. 2018; 564: 32-34Crossref PubMed Scopus (91) Google Scholar In summary, the varied defining standards of peatlands have not only raised great uncertainty in estimates for their soil carbon stock by ignoring that of non-peatland carbon-rich wetlands but also partly resulted in soil carbon loss by peat exploitation and reclamation. A new paradigm for all carbon-rich wetlands including intact peatlands, some degraded peatlands, and non-peatland wetlands is urgently needed to mitigate climate change. By replacing resource-oriented “peat” by conservation-oriented “organic soil material”, we conceive a new framework for “carbon-rich wetlands”, with an emphasis on conserving organic carbon rather than consuming organic matter from wetlands in this commentary (Figure 1A). In soil science, the boundary between organic soil material and mineral soil material is defined at 12% of organic carbon (by weight), which has also been adopted by the recent Global Peatlands Assessment.2United Nations Environment ProgrammeGlobal Peatlands Assessment – the State of the World’s Peatlands: Evidence for Action toward the Conservation, Restoration, and Sustainable Management of Peatlands. United Nations Environment Programme, Nairobi, 2022Google Scholar From the point of view of conserving organic soil material, defining carbon-rich wetlands should not involve the accumulating thickness. According to the above-mentioned rationales, we coined a new term, “C-wetland”, for carbon-rich wetlands defined as a typical wetland with accumulated organic soil materials containing (by weight) no less than 12% organic carbon. Similar to the degradation of peatlands, some C-wetlands degraded during the past decades. Due to rich organic carbon content in soils, although degraded C-wetlands no longer accumulate organic soil materials as carbon sinks, they still hold substantial soil carbon stocks and should be restored to resume their organic soil material accumulating. In a word, the new framework of C-wetlands can not only lead to a comparatively precise estimate of their distribution and carbon store but can also lead to an ultimately better conserved global carbon-rich wetland. Based on a detailed wetland database,1Lehner B. Döll P. Development and validation of a global database of lakes, reservoirs and wetlands.J. Hydrol. X. 2004; 296: 1-22Crossref Scopus (1511) Google Scholar modified by the Global Soil Organic Carbon Map (Food and Agriculture Organization, http://54.229.242.119/GSOCmap/) through excluding pixels with soil organic carbon less than 12% in the top 30 cm soil, we derived a preliminary map of global intact C-wetlands (Figure 1B). In total, the estimated global intact C-wetland area is around 5.70 to 7.39 million km2, contributing to more than 70% of the world’s wetland area (excluding the area of inland waters, ranging from 7.86 to 9.76 million km2),1Lehner B. Döll P. Development and validation of a global database of lakes, reservoirs and wetlands.J. Hydrol. X. 2004; 296: 1-22Crossref Scopus (1511) Google Scholar indicating that carbon-rich wetlands are the major component of wetlands. Furthermore, besides intact C-wetlands, there are still many degraded C-wetlands. Therefore, if the proportion of degraded C-wetlands was similar to that of degraded peatlands of the global peatlands (12%),2United Nations Environment ProgrammeGlobal Peatlands Assessment – the State of the World’s Peatlands: Evidence for Action toward the Conservation, Restoration, and Sustainable Management of Peatlands. United Nations Environment Programme, Nairobi, 2022Google Scholar the estimated global C-wetland area would be 6.47 to 8.36 million km2. The large estimated area of C-wetlands further highlights the importance of wetlands as long-term carbon sinks and substantial soil carbon stocks, suggesting that protecting soil carbon stocks should be the priority for wetland conservation. Therefore, considering non-peatland C-wetlands not included in peatlands area, the current peatland C stock is greatly underestimated. We propose this new term of C-wetland and calculate its substantial distribution not only to highlight the function of carbon-rich wetlands as long-term carbon sinks and their substantial carbon stock in climate mitigation but also to appeal for the prioritizing of the conservation of global wetland carbon stock, especially under the framework of global carbon neutrality. This study was supported by the Second Tibetan Plateau Scientific Expedition (2019QZKK0304) and the Strategic Priority Research Program of Chinese Academy of Sciences (XDA2005010404). We give special thanks to W. Xiong for her careful editing of the draft. The authors declare no competing interests.