Peatland is a kind of wetland ecosystem formed by the gradual accumulation of peat layer due to the inhibition of organic matter decomposition under long-term flooded anaerobic environment. It is one of the most valuable ecosystem types on the earth and plays a vital role in biodiversity protection, water purification and water cycle regulation, carbon sequestration and climate change mitigation. Peatland only accounts for 2.84% of the global land surface area (about 4 million square kilometers), but it is a vital soil carbon pool in the global terrestrial ecosystem. Its soil carbon storage reaches 500-700 billion tons of carbon, accounting for 21-47% of the global land soil carbon storage (150-2400 billion tons of carbon), close to the global total atmospheric carbon pool (860 billion tons of carbon) and the total forest ecosystem carbon pool (79.1-927 billion tons of carbon). Therefore, peatland is considered as a long-term and important terrestrial carbon sink, which plays an important role in the global realization of the 'carbon neutrality' solution based on nature and the sustainable development goals of the UN Climate Action.
However, over the past century, the global peatland has been extensively affected by man-made drainage activities and climate drying. About 50% of European peatland has been severely affected by man-made drainage activities; 50-75% of Southeast Asian (subtropical) peatland has experienced man-made drainage; 43.5% of the Zoige plateau alpine peatland (one of the most extensive plateau peat swamp complexes in China and the world, and the largest plateau peat swamp distribution area in China) is affected by human drainage activities. According to statistics, about 11-13% of the world's peatland is disturbed by human drainage activities. After drainage, peatland is mainly used for crop planting, livestock grazing, forage production, forestry management or peat mining. Drainage activities have caused a sharp drop in the water level of peatland, which has led to the rapid oxidation and decomposition of the peat soil that had been accumulated for tens of thousands of years in an anaerobic environment and was directly exposed to the atmosphere, releasing many greenhouse gases such as carbon dioxide (CO2) and nitrous oxide (N2O), causing global warming, and triggering a large-scale collapse of peatland. It has significantly changed the surface morphology, destroyed soil structure, and brought great difficulty to its ecological recovery. However, until now, there is no clear understanding about whether the large amount of soil carbon released from peatland after artificial drainage (i.e., soil respiration) comes from the oxidative decomposition of soil organic carbon (i.e., soil heterotrophic respiration) or the underground root respiration of plants in peatland (i.e., soil autotrophic respiration) or both. Moreover, there is a lack of building a robust quantitative relationship between water level decline, soil carbon emissions and surface collapse rate of peatland under human drainage activities, leading to the lack of fine quantification of global soil carbon emissions (soil respiration and its components, soil heterotrophic respiration and autotrophic respiration) and surface collapse rate of peatland caused by human drainage activities, which seriously restricts human understanding of the importance of drainage peatland in global carbon emissions.
In view of the above key scientific issues, Ma Lei, a young researcher of the Earth atmosphere Interaction and global change Team of the College of Atmospheric Sciences of Lanzhou University, and professor Zuo Hongchao, used meta-analysis, bootstrap sampling, global upscaling and other methods to clarify for the first time that water level decline caused by artificial drainage mainly promotes soil respiration by promoting soil heterotrophic respiration rather than autotrophic respiration (Figure 1), thus causing large-scale collapse of peatland surface, and this cascade effect widely exists in different land use types in different climatic zones around the world (different land use modes such as farmland and grassland after artificial drainage) (Figure 2).On this basis, the research team constructed a formula to calculate the annual soil heterotrophic respiration emissions of different climate zones and different land use types of peatland in the world based on the surface collapse rate, and accurately estimated the spatial distribution characteristics of annual soil heterotrophic respiration emissions caused by human drainage activities and the global total emissions (Figure 3 and Table 1). It is estimated that the total annual emissions of soil respiration in peatland caused by human drainage activities are 2.36 (1.47 – 3.76) billion tons of CO2 (Table 1). The results show that the global peatland affected by human drainage activities only accounts for~0.3% of the global land surface, but contributes~14% (8.7 – 22%) of the total annual CO2 emissions of global land use change (16.87 billion tons of CO2), and~4.6% (3.0 – 7.3%) of the total annual CO2 emissions of global fossil fuels and land use change (51.33 billion tons of CO2). These results highlight the important role of protecting natural peatland and restoring degraded peatland in mitigating climate change.
Figure 1 Relative changes in soil respiration (SR) and its components, heterotrophic respiration (HR) and autotrophic respiration (AR), due to water table (WT) decline in global pristine peatlands. a Distribution of the in situ observations. b Frequency distributions of the relative changes in SR and HR due to WT decline (AR was not shown due to the scarcity of data points (n = 35)). c Relative changes in SR, HR and AR due to WT decline. d Pearson correlation of the growing season HR and SR rates in pristine and drained boreal peatlands. e Pearson correlation of the annual HR and SR rates in pristine and drained tropical peatlands. No data were observed in pristine and drained temperate peatlands. Each data point for WT decline impacts on in situ SR and its components was classified on the basis of the driver of WT decline and duration of WT decline in (a). The grey–blue regions in (a) indicate the global distributions of peatlands derived from PEATMAT5. The larger solid circles and horizontal bars in (c) denote the weighted means of relative changes and their 95% confidence intervals. The smaller circles and numbers in (c) indicate individual relative changes due to WT decline and the number of data points, respectively. The open circles and bars in (d, e) show the means and standard deviations for individual data points. The asterisks *, **, *** indicate significance at the levels of p < 0.05, 0.01 and 0.001, respectively, and n.s. indicates no significance (for details, see Meta-analysis in Methods).
Figure 2 Drainage duration controls peat subsidence rate (Rps) and the proportion of peat oxidation to Rps (Po). a, b Relationships of Rps with drainage duration (i.e., years since drainage) for drained peatlands across different climate zones and land uses. c, d Proportion of subsidence attributed to peat oxidation (Po) with drainage duration for drained peatlands across different climate zones and land uses. The fitted regression curves and their 95% confidence intervals are shown. The same Po was used for drained boreal and temperate peatlands under different land uses due to a lack of sufficient observations. Similarly, the same Po was used for drained tropical peatlands under different land uses.
Figure 3 Peat subsidence rate (Rps) (cm yr–1) due to oxidation and associated soil heterotrophic CO2 emissions. Spatial variations in the (a) estimated Rps due to oxidation of drained peatlands, (b) soil organic carbon (SOC) concentration (g kg–1) of pristine peatlands, (c) soil bulk density (BD, g cm–3) of pristine peatlands, and (d) estimated soil heterotrophic CO2 emissions due to peat oxidation (kg C ha–1 yr–1) among global drained peatlands. For the calculation processes of Rps and HR, see the Methods. The grey?blue regions indicate the global distributions of peatlands derived from PEATMAT5.
Figure 4 Contributions of soil heterotrophic respiration (HR) to soil respiration (SR). a Contributions for pristine and drained peatlands. b Contributions for drained boreal and tropical peatlands. The same lowercase letter within the plot indicates no significances of median or mean between pristine and drained peatlands or drained boreal and tropical peatlands at the level of p < 0.05 using Mann-Whitney U-Test. The notes in (a) are also applied to (b).
The above research results were published in the new Nature journal Communications Earth & Environment (Figure 5) on October 29 under the title of A globally robust relationship between water table decline, subsidence rate and carbon release from peatlands, and were selected as the cover paper by the editorial department.After the article was published online, at the invitation of Nature Portfolio Communities Senior Manager Dr. Eve Satkevic, the research team shared the story behind the research work at Nature Portfolio Earth and Environment Community under the title Critical importance of conserving (new) pristine peatlands and restoring drained peatlands for climate change mitigation. They highlighted the importance of protecting global natural peatlands and restoring degraded peatlands for climate change mitigation.(Fihure6)
Figure 5 Published paper in Communications Earth & Environment
Human drainage of peatland and associated land use changes such as grazing can strongly affect the net exchange of CO2 between peatland and the atmosphere. Based on this, the research team commented on the article 'Plant take of CO2outspaces losses from permafresh and plant respiration on the Titanium Plate' published in PNAS in 2021. The research team affirmed the view that the alpine ecosystem of the Qinghai Tibet Plateau as a whole is a carbon sink, and pointed out that the study did not consider the impact of diverse human activities such as alpine peatland drainage and alpine grassland grazing on the carbon balance of the ecosystem, while ignoring these diverse human activities may be one of the important reasons why the carbon sink of the Qinghai Tibet Plateau estimated by the authors is four times that of previous estimates, that is because a large amount of alpine peatland on the Qinghai Tibet Plateau is artificially drained and used for grazing, and alpine grassland (the most widely distributed alpine ecosystem on the Qinghai Tibet Plateau) is mainly used as grazing grassland (Figure 7). Based on this, the research team reanalyzed the data submitted by the authors and supplemented information on human activities in the corresponding observation plots. The research team found that the annual net ecosystem productivity (NEP) of natural alpine peatland (mean ± 95% confidence interval: 1784 ± 454 kg C ha – 1 yr – 1) was significantly higher (P<0.001) than that of drainage peatland (1784 ± 454 kg C ha – 1 yr – 1) (Figure 8A), while there was no significant difference in annual NEP between natural grassland, shrub and grazing grassland, and shrub (Figure 8B). In addition, livestock feeding will largely remove the net primary productivity of grazing peatland and grassland, while the carbon returned through livestock excreta is less. Therefore, it can be inferred that various human activities will further reduce the net carbon sink capacity of alpine peatland and alpine grassland. Ignoring these important carbon loss processes may collectively explain why the authors found that the net carbon sequestration in the Qinghai Tibet Plateau was approximately three times higher than previous model predictions or soil resampling measurements. From this perspective, in future research, the impact of various human activities should be considered when evaluating the net carbon balance of the alpine ecosystem in the Qinghai Tibet Plateau. The inclusion of information on human activities also helps to find solutions for reducing large amounts of carbon emissions from hot emission sources (such as drainage alpine peatland) (Figure 8A). The review article was titled Quantifying net carbon fixation by Titanium aluminum ecosystems should consider multiple anthropogenic activities and was published in Letter format on PNAS on February 1st (Figure 9).
Figure 7 Landscapes of peatland draining (A) and grassland grazing (B) on the Tibetan Plateau (photo by Lei Ma).
Figure 8 Net ecosystem production of alpine ecosystems under different disturbances. The *** and n.s. indicate P< 0.001 and no significance, respectively. The datasets are directly extracted from Wei et al. (2).
Figure 9 Published Letter in PNAS
In addition, peatland is also an important nitrogen pool, and its nitrogen reserves account for~10% of the total reserves of global terrestrial ecosystems. As human drainage activities lead to the direct exposure of the surface peat soil to the air, which significantly promotes the nitrification and (or) denitrification of the surface peat soil, resulting in a large amount of production and emission of greenhouse gas N2O, peatland is an important source of atmospheric N2O. However, there is still a lack of detailed understanding of N2O emission magnitude, spatial distribution characteristics and driving mechanism under different conditions of peatland (natural vs. drainage vs. restoration), different climatic zones (tropical vs. temperate vs. frigid), and different nutrient types (Minerotrophic vs. Ombrotrophic), This has seriously restricted the accurate estimation of total N2O emissions from peatland at the regional to global scale and the fine assessment of its emission reduction potential. However, there is still a lack of detailed understanding of N2O emission magnitude, spatial distribution characteristics and driving mechanism under different conditions of peatland (natural vs. drainage vs. restoration), different climatic zones (tropical vs. temperate vs. frigid), and different nutrient types (Minerotrophic vs. Ombrotrophic). It has seriously restricted the accurate estimation of total N2O emissions from peatland at the regional to global scale and the fine assessment of its emission reduction potential.
Based on this, the research team used comprehensive analysis, bootstrap sampling, global upscaling and other methods, and used the rich annual N2O observation data in European peatland (492 annual N2O observation data samples, and other continents lack of observation data) to find that the annual N2O flux in peatland presents huge spatial variability. The variation range is between -1.08 to 33.40 kg N2O-N ha-1 yr-1 (Figure 10); This spatial variability is affected by the state of peatland, climate conditions and nutrient supply types. Drainage activities significantly promote N2O emissions from minerotrophic peatland, but do not affect N2O emissions from ombrotrophic peatland, which is true in different climatic zones. Similarly, recovery can significantly reduce N2O emissions from minerotrophic peatland, but cannot effectively reduce N2O emissions from ombrotrophic peatland (Figure 10). These results indicate that N2O emissions from peatland are jointly driven by water level and soil nitrogen availability (Figure 11). The study also found that European peatland is a huge source of N2O emissions (the base year emissions in 2020 were 90.42 (64.49~122.57) Gg N2O-N yr-1, accounting for 17.38% (15.23 – 20.15%) of European agricultural N2O emissions), of which drainage peatland accounted for 87.27% of all peatland N2O emissions, while drainage land used for forestry accounted for 46.67% of all drainage peatland N2O emissions, grassland accounted for 29.82%, farmland accounted for 22.74%, and peat mining was the least, Only 0.77% (Figure 12). These results emphasize the importance of N2O emission reduction in drainage peatland in Europe. Therefore, this study developed an empirical model (Figure 13) that dynamically considered the changes in water level and soil carbon nitrogen ratio (C/N) during the restoration of drainage peatland, set up eight peatland restoration scenarios, and assessed the N2O emission reduction potential (Figure 14). The results showed that the earlier the drainage peatland recovered, the more conducive to reducing N2O emissions. On the premise of not harming food security, restoring the water level of all drainage peatland for forestry and peat mining to the surface, and restoring the water level of drainage peatland for agricultural and grassland production to 15 cm below the ground (reducing the inhibitory effect on the growth of gramineous plant roots) can reduce N2O emissions from peatland in Europe to the greatest extent, without causing crop and forage yield reduction.
Figure 10 (a) Distribution of synthesized annual flux datapoints across European peatlands. (b) Frequency distributions of annual nitrous oxide (N2O, kg N2O-N ha-1 yr-1) fluxes for natural, drained and rewetted peatlands. (c) Boxplot for multiple comparisons of annual mean N2O fluxes from European peatlands under different peatland statuses (N = natural wetland, D = drained wetland, and R = rewetted wetland), climatic regimes (T = temperate, B = boreal, and S = subarctic) and nutrient supply types (M = minerotrophic and O = ombrotrophic), as well as three simultaneously considered factors; the numbers in parentheses indicate the datapoints within each subgroup. The boxplot shows the median (central thick lines), mean (rhomboidal points), 25% and 75% quartiles, and 1% and 99% ranges (i.e., the upper and lower whiskers) around the median. Datapoints above (below) the upper (lower) whisker indicates outliers. The same lowercase letter indicates a lack of mutually significant differences at the 95% confidence level. The distribution of European peatlands was derived from PEATMAP (http://peatdatahub.net/).
Figure 11 Binary relationship between annual mean N2Ofluxes and annual mean water table depth (WTD) and soil (0–30 cm) carbon to nitrogen ratio (C/N) across different peatlands in Europe. The annual N2O fluxes, WTD and soil C/N ratios were log-transformed. For the data analysis method, see the statistical analysis section in the body text.
Figure 12 Total annual N2O emissions from European peatlands (a) and drained peatlands (b) in 2020 (baseline emissions). The vertical error bars indicate the 95% confidence intervals which were obtained on the basis of a bootstrapping method. For the emission estimation method details, see the statistical analysis section in the body text.
Figure 13 Regression analysis of the annual change rate of peatland water table depth (WTD) or soil (0–30 cm) carbon to nitrogen ratios (C/N) versus years since drainage (a and c), and rewetting (b and d). For subfigure d, we were unable to fit regression curves for different land-use histories before rewetting due to the lack of enough observations.
Figure 14 Estimated 2020–2100 annual mean (a) and cumulative (b) N2O emissions across European peatlands under different scenarios. The shaded areas of the annual and cumulative mean N2O emissions represent the 95% confidence intervals (CIs), which were obtained on the basis of a bootstrapping method. For details of the estimation method, see the statistical analysis section in the body text.
The above research results were published on May 15th in the important journal Environmental Pollution in the field of ecology and environmental science, titled Comprehensive assessment of nitrous oxide emissions and mitigational potentials across European lands (Figure 15).
Figure 15 Published paperin Environmental Pollution
These studies were jointly supported by the Second Comprehensive Scientific Expedition and Research Program on the Qinghai Tibet Plateau (STEP) (2019QZK0103), the 'Double First Class' Construction Fund of Lanzhou University (561120206), the Sino German Postdoctoral Scholarship Program (CSC – DAAD, 57460082), the Natural Science Foundation of Shanxi Province – Youth Science Foundation (201901D211420) and the Key Laboratory of Inland River Basin Ecology and Hydrology of the Chinese Academy of Sciences (KLEIRB-ZS-20-05). Ma Lei, a young researcher from the College of Atmospheric Sciences of Lanzhou University, and Zuo Hongchao, a professor from the College of Resources and Environment of Lanzhou University, led the research work. The collaborators include Professor Zhu Gaofeng, a senior engineer from the College of Atmospheric Sciences of Lanzhou University, Chen Bolong, a postdoctoral fellow from the Department of Mathematics and the College of Biological Sciences of Hong Kong University, Niu Shuli, a researcher from the Institute of Geographic Science and Resources of the Chinese Academy of Sciences, and Wang Jinsong, an associate researcher Professor Phillipe Ciais from the French National Laboratory of Climate and Environmental Sciences, and Dr. Lin Fei and Dr. Ma Xiaohong from Taiyuan Normal University.
Relevant paper information (* corresponding author):
（1）Lei Ma, Gaofeng Zhu, Bolong Chen, Kun Zhang, Shuli Niu, Jinsong Wang, Phillipe Ciais, Hongchao Zuo*. 2022. A globally robust relationship between water table decline, subsidence rate, and carbon release from peatlands. Communications Earth & Environment. 3, 254. https://doi.org/10.1038/s43247-022-00590-8
（2）Lei Ma* & Hongchao Zuo. 2022. Quantifying net carbon fixation by Tibetan alpine ecosystems should consider multiple anthropogenic activities. Proceedings of the National Academy of Sciences of the United States of America, 119, 1– 2.https://doi.org/10.1073/pnas.2115676119
（3）Fei Lin, Hongchao Zuo, Xiaohong Ma, Lei Ma*. 2022. Comprehensive assessment of nitrous oxide emissions and mitigation potentials across European peatlands. Environmental Pollution, 301, 119041.https://doi.org/10.1016/j.envpol.2022.119041
