Abstract
International initiatives set ambitious targets for ecological restoration, which is considered a promising greenhouse gas mitigation strategy. Here, we conduct a meta-analysis to quantify the impacts of ecological restoration on greenhouse gas emissions using a dataset compiled from 253 articles. Our findings reveal that forest and grassland restoration increase CH4 uptake by 90.0% and 30.8%, respectively, mainly due to changes in soil properties. Conversely, wetland restoration increases CH4 emissions by 544.4%, primarily attributable to elevated water table depth. Forest and grassland restoration have no significant effect on N2O emissions, while wetland restoration reduces N2O emissions by 68.6%. Wetland restoration enhances net CO2 uptake, and the transition from net CO2 sources to net sinks takes approximately 4 years following restoration. The net ecosystem CO2 exchange of the restored forests decreases with restoration age, and the transition from net CO2 sources to net sinks takes about 3-5 years for afforestation and reforestation sites, and 6-13 years for clear-cutting and post-fire sites. Overall, forest, grassland and wetland restoration decrease the global warming potentials by 327.7%, 157.7% and 62.0% compared with their paired control ecosystems, respectively. Our findings suggest that afforestation, reforestation, rewetting drained wetlands, and restoring degraded grasslands through grazing exclusion, reducing grazing intensity, or converting croplands to grasslands can effectively mitigate greenhouse gas emissions.
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Introduction
Global temperature is approaching a threshold that will have irreversible consequences for the future of our Earth, mainly due to the increasing concentrations of atmospheric greenhouse gases (GHG) such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O)1. Over the last 220 years, global CO2 concentrations increased from 283 to 419 parts per million (ppm), CH4 increased from 750 to 1925 parts per billion (ppb), and N2O increased from 273 to 336 ppb2. Land-use change and ecosystem degradation have caused massive anthropogenic emissions of GHG and altered natural ecological ecosystems from net sinks to net sources3,4. Restoring the degraded ecosystems and converting lands back to healthy ecosystems has been proposed as a vital strategy for stabilizing the Earth’s climate5. To limit global warming below the 2 °C threshold, there is an urgent need to reduce atmospheric GHG concentrations by restoring degraded ecosystems such as forests, grasslands and wetlands6. Ecological restoration is the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed (Society for Ecological Restoration and Policy Working Group 2002). The United Nations (UN) has declared 2021-2030 as the ‘UN Decade on Ecosystem Restoration’ and calls on countries to meet commitments to restore one billion hectares of land. The Bonn Challenge and the New York Declaration on Forests have established ambitious targets to restore 350 million hectares of forests worldwide by 20305. Thus, systematically understanding the impacts of ecological restoration on GHG emissions is imperative for making better restoration policies and improving the Intergovernmental Panel on Climate Change (IPCC) guidance of GHG inventories.
Forests occupy approximately 30% of the global land surface and play a crucial role in regulating the global carbon (C) cycle and reducing global warming7,8,9. A recent estimation reported that global forests maintained a net C sink of −7.6 Gt CO2e yr−1, reflecting a balance between gross C removals (−15.6 Gt CO2e yr−1) and gross emissions (8.1 Gt CO2e yr−1) from deforestation and other disturbances, e.g., clear-cut, fire, windthrows, insects, etc10. Afforestation and reforestation could change biomass accumulation and alter soil biogeochemical, physical and hydrological properties, thereby affecting the GHG fluxes9,11,12,13. Previous work found that converting croplands to forests increased CH4 uptake due to the decreased soil bulk density, and afforestation decreased N2O emissions due to the reduced nitrogen (N) substrate availability11. The conversion of grasslands to forests might decrease CH4 emissions but increase N2O emissions12. Although many studies showed that afforestation could enhance the CO2 sink function of ecosystems10,14, some studies observed that forest lands continued to act as a CO2 source even after several years of afforestation15. These diverse results suggest that the magnitude and direction of GHG dynamics driven by forest restoration are highly uncertain and could be affected by multiple factors, including ecosystem types, restoration ways, and restoration age12,16,17. It is undoubtedly necessary to explore the general patterns and the major controlling factors of GHG emissions in the restored forests.
Grassland ecosystems constitute approximately 40% of the terrestrial biosphere18, and natural grasslands are usually identified as efficient sinks of atmospheric CH4 and CO219, but sources of N2O20. Grassland degradation leads to changes in soil nutrient content, soil moisture, and plant composition, which influences the pattern of GHG emissions21,22. It has been found that grassland degradation might decrease CH4 uptake by 40%23. However, whether grassland restoration can reduce GHG emissions is still inconclusive19,24. Previous work reported that grassland restoration increased C accumulation and enhanced CH4 uptake24, but some studies found that grassland restoration might stimulate N2O and CO2 emissions and shift grassland from a C sink to a C source19. Furthermore, the effects of grassland types, restoration measures, and restoration age on GHG emissions in the restored grasslands at a global scale are still unclear.
Wetlands are considered to be one of the most efficient ecosystems for sequestrating CO2 from the atmosphere25, mainly because inundation creates anaerobic conditions that prevent the decomposition of dead plant material and restore sequestered C in soil26,27. Despite covering only 5–8% of the Earth’s landscape, global wetlands store 20–30% of soil C on the Earth and thereby play an important role in the global C cycle28. In general, wetland drainage and degradation decrease CH4 emissions but enhance CO2 and N2O emissions to the atmosphere, converting the wetlands from C sinks into sources29,30. However, the impacts of restoration on wetland GHG and the driving factors remain controversial31,32,33. Previous work reported that wetland restoration could shift the ecosystems into net GHG sources32,34,35 or net sinks36. The inconsistent results are probably attributed to the wetland restoration types, restoration age, climate, water table depth, and soil properties32,37. Since wetland restoration generally decreases CO2 emissions but increases CH4 emissions32,34,36, the overall effects of wetland restoration on the global warming potentials (GWP) considering three major GHGs (i.e., CO2, CH4, and N2O) are not well understood.
Despite numerous studies investigating the effects of ecological restoration on the emission of individual or a few GHGs at the plot or regional level38, the general pattern of the impacts of ecological restoration on the three major GHGs at a global scale has not yet been analyzed. Furthermore, there is currently a lack of comprehensive global assessments for the three major ecosystems (i.e., forests, grasslands, and wetlands) which are crucial for the global GHG budget and the ‘UN Decade on Ecosystem Restoration’6,7,8,9. In addition, detailed data on the responses of GHG to ecological restoration are lacking in the IPCC reports, the IPCC Guidelines for National Greenhouse Gas Inventories, and the Good Practice Guidance for Land Use, Land-Use Change and Forestry. To fill these knowledge gaps, we compiled a global dataset from 253 peer-reviewed articles and conducted a meta-analysis to assess the effects of ecological restoration on GHG emissions (Fig. 1). Our specific objectives were to (1) quantify the impacts of ecological restoration on CH4 and N2O emissions and net ecosystem CO2 exchange (NEE) in forest, grassland and wetland ecosystems, (2) explore the patterns of GHG emissions with restoration age, and (3) determine the key factors influencing the response of GHG emissions to ecological restoration.
In this work, we show that forest and grassland restoration increases CH4 uptake, mainly due to the changes in soil properties. Conversely, wetland restoration increases CH4 emissions, primarily attributed to elevated water table depth. Forest and grassland restoration has no significant effect on N2O emissions, while wetland restoration reduces N2O emissions. Overall, forest, grassland and wetland restoration enhances C sink, reduces the global warming potentials, and can serve as strategies for mitigating GHG.
Results
Effects of ecological restoration on CH4 emissions
Overall, forest and grassland restoration significantly decreased CH4 emissions, and the weighted response ratios (RRd) of CH4 emissions were ‒2.3 (95% CI: ‒2.9 to ‒1.6) and ‒1.6 (95% CI: ‒2.4 to ‒0.8) under forest and grassland restoration, respectively (Fig. 2a). Compared with the paired control ecosystems, forest and grassland restoration averagely increased CH4 uptake from 1.0 to 1.9 kg C ha‒1 year‒1 (by 90.0%) and 2.6 to 3.4 kg C ha‒1 year‒1 (by 30.8%), respectively (Fig. 3b, c). Among the types of grassland restoration, temperate steppe & meadow and desert steppe increased CH4 uptake from 2.6 to 3.8 kg C ha‒1 year‒1 (by 46.2%) and 7.7 to 11.4 kg C ha‒1 year‒1 (by 48.4%), respectively (Fig. 3c). Wetland restoration significantly increased CH4 emissions by 544.4% (RRd: 2.9; 95% CI: 2.4‒3.4; P < 0.05) (Fig. 2a). The average CH4 emissions increased from 23.4 kg C ha‒1 year‒1 to 150.8 kg C ha‒1 year‒1 after wetland restoration (Fig. 3a). Among the types of wetland restoration, the conversion of grasslands to wetlands showed the largest increase in CH4 emissions, with an average increase from 61.2 kg C ha‒1 year‒1 (in paired control) to 284.8 kg C ha‒1 (in restored wetlands) (Fig. 3a). In contrast, there was no significant change in CH4 emissions when aquaculture ponds were converted to wetlands (RRd: ‒1.7; 95% CI: ‒3.8 to 0.5) and mangroves were restored (RRd: 1.1; 95% CI: ‒0.4 to 2.5) (Fig. 2a).
Effects of ecological restoration on N2O emissions
Overall, forest (RRd: ‒0.4; 95% CI: ‒1.3 to 0.4) restoration did not affect N2O emissions, while grassland and wetland restoration reduced N2O emissions by 21.7% (RRd: ‒0.7; 95% CI: ‒1.4 to ‒0.1) and 68.6% (RRd: ‒2.9; 95% CI: ‒3.9 to ‒1.9; P < 0.05) (Fig. 2b), respectively. When considering the types of forest restoration, the conversion of croplands to forests averagely decreased N2O emissions from 3.7 kg N ha‒1 year‒1(in paired control) to 1.4 kg N ha‒1 year‒1 (in restored forests) (RRd: ‒3.3; 95% CI: ‒4.7 to ‒1.9; P < 0.05) (Fig. 3e). Among the types of wetland restoration (Fig.2b), the conversion of grasslands to wetlands averagely reduced N2O emissions from 5.2 kg N ha‒1 year‒1 (in paired control) to 2.6 kg N ha‒1 year‒1 (in restored wetlands), and the conversion of croplands to wetlands averagely decreased N2O emissions from 17.0 kg N ha‒1 year‒1 to 2.3 kg N ha‒1 year‒1. Compared with the paired control ecosystems, peatland restoration averagely reduced N2O emissions from 2.2 kg N ha‒1 year‒1 to 0.5 kg N ha‒1 year‒1. However, floodplains restoration did not significantly affect N2O emissions (Fig. 2b). Among the measures of grassland restoration, the conversion of croplands to grasslands averagely decreased N2O emissions from 2.3 kg N ha‒1 year‒1 to 0.7 kg N ha‒1 year‒1 (Supplementary Fig. S1b). Similarly, prairie restoration reduced N2O emissions from 4.8 kg N ha‒1 year‒1 to 0.1 kg N ha‒1 year‒1 (RRd: ‒10.9; 95% CI: ‒14.7 to ‒7.1; P < 0.05 (Fig. 3f).
Effects of ecological restoration on CO2 fluxes and GWP
Overall, wetland restoration significantly reduced NEE by 138.8% (RRd: ‒3.2; 95% CI: ‒3.8 to ‒2.5; P < 0.05) (Figs. 2c and 4a, Table 1). Compared with the paired control ecosystems, the conversion of grasslands to wetlands averagely reduced NEE from 231.9 g C m‒2 year‒1 to ‒219.5 g C m‒2 year‒1, and the conversion of aquaculture to wetlands averagely reduced NEE from ‒41.9 g C m‒2 year‒1 to ‒151.5 g C m‒2 year‒1(Supplementary Table S1). Bogs restoration averagely reduced NEE from 159.2 to ‒35.8 g C m‒2 year‒1 (Fig. 4a). The conversion of grasslands to wetlands decreased gross primary productivity (GPP) and ecosystem respiration (ER), while bogs restoration increased GPP and ER (Supplementary Fig. S2). The floodplains and mangrove restoration showed no significant effect on GPP and ER (Supplementary Fig. S2).
Overall, grassland restoration decreased NEE by 146.9% (RRd: ‒4.7; 95% CI: ‒5.8 to ‒3.5; P < 0.05) (Figs. 2c, 4h and Supplementary Fig. S1c). Compared with the paired control ecosystems, grassland restoration by grazing exclusion averagely decreased NEE from ‒245.3 g C m‒2 year‒1 to ‒703.0 g C m‒2 year‒1, grassland restoration by reducing grazing density averagely reduced NEE from ‒587.9 g C m‒2 year‒1 to ‒1460.1 g C m‒2 year‒1, and the conversion of cropland to grassland averagely reduced NEE from 10.3 g C m‒2 year‒1 to ‒75.8 g C m‒2 year‒1 (Supplementary Fig. S1c, Table S2). Grassland restoration increased GPP and ER (Supplementary Fig. S2).
Due to the small sample size for the paired restored-control measurements for the NEE, GPP, and ER in forests, the effects of forest restoration on CO2 fluxes were not tested by the RRd and t-test (Figs. 2c, 4d–f). Based on the restoration chronosequence sub-dataset, the NEE in restored forests decreased first and then tended to be stable and showed a negative exponential relationship with afforestation age, while the GPP and ER showed a positive exponential relationship with afforestation age (Fig. 5e; P < 0.001). Similarly, the NEE was negatively and exponentially correlated with reforestation age and time since restoration after disturbance (Fig. 5f, g; P < 0.001).
On average, the C budget (CO2 and CH4) was −295.5, −506.5 and −53.4 g C m−2 year−1 for forest, grassland and wetland restoration, respectively, indicating the capacity of enhanced C sink in the restored ecosystems (Table 1). On average, forest, grassland and wetland restoration decreased the GWP by 327.7%, 157.7% and 62.0% compared with their paired control ecosystems, respectively (Table 1).
Changes of CH4 and N2O emissions and NEE with restoration age
Given the critical impact of restoration age on GHG emissions in restored ecosystems, the patterns of CH4 and N2O emissions and NEE with restoration age were first explored. Overall, the restoration age had a significant effect on CH4 emissions (Fig. 5c and Supplementary Fig. S3b). The RRd of CH4 emissions in the restored forests was negatively correlated with restoration age (i.e., years since restoration) (Fig. 5c). The soil CH4 uptake showed no response to the afforestation age within 10 years, while soil CH4 uptake increased with afforestation age for longer time intervals (Supplementary Fig. S3b). The RRd of CH4 emissions in the restored wetlands was exponentially and positively correlated with restoration age and achieved a relatively stable value in about 10 years since restoration (Fig. 5a). The RRd of N2O emissions in the wetland was negatively correlated with restoration age (Fig. 5b).
Restoration age was an important factor influencing CO2 fluxes (Fig. 5). The GPP/ER showed low values (<1 or ≈1) in the early years following afforestation and restoration from fire and clear-cutting (Fig. 5g, h). The GPP/RE became greater than 1 (i.e., NEE < 0) by 4 years, 6 years, 13 years, and 8 years after restoration for the afforestation sites, clear-cutting sites, post-fire sites, and all disturbances sites, respectively (Fig. 5). The GPP/ER ratio varied with afforestation age and time since restoration after disturbance, and had an asymptote of 1.19 and 1.09, respectively (Fig. 5g, h; P < 0.001). The NEE in the wetlands was exponentially and negatively correlated with restoration age and the switchover time from net CO2 sources to net CO2 sinks was estimated to be approximately 4 years (Fig. 5d).
Factors influencing the response of CH4, N2O, and NEE to ecological restoration
Forest restoration significantly increased concentrations of soil organic C (SOC), NH4+-N and dissolved organic C (DOC), but reduced soil temperature, soil water-filled pore space (WFPS), soil moisture, and pH (P < 0.05; Fig. 6b). Grassland restoration remarkably increased soil WFPS, soil moisture, vegetation coverage, and grassland aboveground and belowground biomass, but reduced soil bulk density (BD) and soil NO3‒-N concentrations (P < 0.05; Fig. 6c). Wetland restoration significantly increased water table depth, soil SOC and total N (TN), but decreased soil BD, soil redox potential (Eh), pH and NH4+-N concentrations (P < 0.05; Fig. 6a).
The RRd of CH4 emissions in the restored forests was positively correlated with the RRd of soil WFPS (P < 0.01; Fig. 7b). The RRd of CH4 emissions in the restored forests and grasslands was negatively correlated with the RRd of BD (Fig. 7a). Afforestation decreased CH4 emissions regardless of the tree types (i.e., coniferous and deciduous forest) (P < 0.01; Supplementary Fig. S3c). The RRd of N2O emissions in the forests and grasslands were positively correlated with the RRd of soil NH4+-N and NO3‒-N concentrations (Fig. 7d, e), and the RRd of N2O emissions in the forests was negatively correlated with the RRd of soil pH (P < 0.01; Fig. 7f).
The RRd of CH4 emissions in the restored wetlands was exponentially and positively correlated with water table depth (P < 0.01; Fig. 5a and Fig. 7c). The RRd of N2O emissions in the wetland was positively correlated with the RRd of soil NH4+-N concentrations (Fig. 7g). The N2O emissions and NEE of the restored wetlands were negatively related to water table depth (P < 0.001; Supplementary Fig. S4c).
Across all restoration groups, GPP and ER were positively correlated with the temperature and precipitation (P < 0.01; Supplementary Fig. S5). The RRd of CH4 emissions in all systems was negatively correlated with the aridity index (P < 0.01; Supplementary Fig. S6c). When the precipitation was larger than 900 mm, the RRd of N2O emissions in all ecosystems was positively correlated with precipitation (P < 0.05; Supplementary Fig. S6e). The RRd of NEE in all ecosystems was negatively correlated with the aridity index when the aridity index was greater than 0.9 (P < 0.05; Supplementary Fig. S6i).
Discussion
Forest and grassland restoration increased CH4 uptake while wetland restoration enhanced CH4 emissions
We found that forest and grassland restoration significantly increased CH4 uptake (Table 1, Fig. 2), suggesting the great potential of forest and grassland ecosystem restoration in enhancing sink function for CH4. The conversion of croplands and grasslands to forests increased CH4 uptake by 84.8% and 106.8% (Fig. 3), respectively, indicating high CH4 removal efficiency. The RRd of CH4 emissions linearly increased with the RRd of WFPS (Fig. 7b), indicating that lower soil moisture and WFPS enhanced CH4 uptake and inhibited CH4 emissions from soil. Afforestation significantly decreased WFPS in the forest ecosystems (Fig. 6b), mainly because trees had deeper roots and higher water demands than crops and grasses, and afforestation enhanced evapotranspiration and canopy interception of precipitation39,40. The decrease in soil WFPS caused by afforestation can enhance the diffusion of atmospheric O2 and CH4 into the soils, thereby increasing CH4 oxidation and uptake in the afforested soils39. Soil compaction by machinery in the agricultural lands and trampling by livestock in the grasslands may increase soil bulk density and reduce soil porosity41,42,43. Our results showed that grassland restoration significantly reduced soil bulk density (Fig. 6c), and the RRd of CH4 emissions showed a negative relationship with the RRd of soil bulk density (Fig. 7a), implying that the lower soil bulk density in the restored grasslands increased CH4 diffusion from atmosphere into soils and thus increased CH4 uptake44. Grassland restoration by reducing grazing intensity or grazing exclusion increased belowground biomass (Fig. 6c), which may form “root holes” and improve soil aerobic conditions for diffusion of atmospheric CH4 into the soil profiles and the growth of methanotrophs45, thereby enhancing CH4 uptake in the restored grassland. In addition, the increase in SOC in the afforested lands (Fig. 6b) could enhance soil macropores and the number of coarse pores46, and thus create favorable environments for methanotrophs growth and CH4 oxidation47. Taken together, the increased CH4 uptake in the restored forests and grasslands could be attributed to the changes in soil properties.
Forest restoration significantly increased CH4 uptake with the increase of afforestation age (Fig. 5c and Supplementary Fig. S3b), which could be mainly attributed to the increased SOC and decreased soil moisture and WFPS following afforestation (Figs. 6b and 7b)48,49. Bárcena et al. reported that soil SOC concentrations increased with stand age and therefore increased the abundance and activity of methane-oxidizing bacteria growth by supplying abundant substrates50, consequently resulting in an enhanced CH4 oxidation rate with afforestation age. Gatica et al. found that, with the increase of afforestation age, soil moisture was decreased by the combined effects of increasing rainfall interception51 and tree transpiration in the older forest stands39, and thus enhanced CH4 consumption with time. Therefore, these individual observations support our results and inferences48,49,50,51.
We found that wetland restoration significantly increased annual CH4 emissions by 5.4 times compared with the paired control ecosystems, indicating that wetland restoration enhanced the CH4 source strength. Among the types of wetland restoration, the conversion of grasslands to wetlands showed the largest increase in CH4 emissions, followed by the conversion of croplands to wetlands (Fig. 3a). These results indicated that greater attention should be paid to the increased CH4 emissions from the restored wetlands in global GHG accounting. Previous work showed that CH4 emissions in the wetlands were mainly controlled by water table level26,38, nutrient status44, plant species52, and microbial activity53. Unlike the CH4 emissions in forest and grassland ecosystems which were influenced by soil properties (Fig. 7a, b), the RRd of wetland CH4 emissions was positively correlated with water table depth, suggesting that the wetland restoration facilitated the production of CH4 by creating an anaerobic environment through elevated water table levels (Figs. 6a, 7c). Long-time waterlogging during the restoration period reduced O2 penetration into the sediments and thus induced a reduction in the redox potential of 1.23-fold compared with the paired control ecosystems (Fig. 6a), which in turn stimulated methanogen growth and activity, thereby enhancing CH4 emissions54. Bog restoration by rewetting may be beneficial to the proliferation of aerenchymatous vascular plants, and thus allow CH4 to bypass the oxidized surface soil, consequently enhancing CH4 emission into the atmosphere via the plant-mediated transport32. In addition, restoration of wetlands by rewetting created an anaerobic environment which may inhibit microbial activity and reduce SOC decomposition55, thus leading to a higher SOC concentration in the restored wetlands (Fig. 6a). The higher SOC concentrations in the restored wetlands could provide more substrates for methanogen growth than in the paired control ecosystems44,56. Thus, elevating the water table level and increasing the substrate supply for methanogen is likely to result in higher CH4 fluxes in the restored wetlands (Figs. 6a, 7c, and Supplementary Fig. S4a).
The RRd of wetland CH4 showed exponentially relationship with restoration age and achieved a relatively stable value in about 10 years since restoration (Fig. 5a). Similarly, Mitsch et al. reported that wetland restoration initially stimulated CH4 emissions but decreased over time and reached CH4 emissions comparable to the natural wetland after 13-15 years28. The rapid response of CH4 emissions to wetland restoration at the initial stage was mainly due to the restoration of the natural hydrology and the inundation of easily decomposable plant litters, which created an anaerobic environment and sufficient substrate for the growth of methanogens and CH4 production53,57,58,59.
Diverse responses of N2O emissions to ecological restoration
Our findings revealed that the conversion of agricultural lands to forests significantly decreased N2O emissions and the conversion of grasslands and wetlands to forests stimulated N2O emissions, indicating the response patterns of N2O emissions to forest restoration depends on the prior land-use type. The RRd of N2O emissions in forests was positively related to the RRd of NH4+ and NO3‒ (Fig. 7), indicating that the cessation of fertilization in the afforested croplands may lead to a reduction in soil N2O emissions compared with the fertilized croplands11. The main reasons for the increased N2O emissions in the forests converted from grasslands could be attributed to that afforestation in grasslands increased the concentrations of soil SOC and NH4+ and decreased soil pH (Fig. 7, Supplementary Fig. S7). The increased SOC and NH4+ could increase soil C and N availability and soil nitrification for N2O production12,60. Our results demonstrated that the RRd of N2O emissions showed a negative relationship with the RRd of soil pH (Figs. 6b, 7f). The reduction of soil pH may inhibit the activity of the N2O reductase enzyme and in turn increase N2O/N2 ratios in the denitrification, consequently increasing N2O emissions from denitrification in the afforested soils4,61. Grassland restoration by conversion of cropland to grassland sharply decreased N2O emissions (Supplementary Fig. S1b), mainly due to the decreased concentrations of soil NO3‒ by stopping fertilization (Figs. 6c, 7). In contrast, artificial assisted restoration in the degraded grasslands by applying organic or mineral fertilizer may increase N2O emissions by increasing the availability of N for N2O production62. Thus, our results indicated that different grassland restoration measures showed distinct impacts on N2O emissions (Supplementary Fig. S1b).
Wetland restoration significantly decreased N2O emissions (by 68.6%) and soil NH4+ concentration, and the RRd of N2O emissions in wetlands was positively related to the RRd of NH4+(Figs. 2b, 3d, 6a, 7g), indicating that the reduction in soil NH4+ concentration, driven by wetland restoration, contributes to the decreased N2O emissions in the restored wetlands (Fig. 6a). Previous work showed that the conversion of agricultural lands and grasslands to wetlands significantly decreased N fertilizer and animal waste inputs, thus reducing the substrates of inorganic N for nitrifying and denitrifying microorganisms63. Raising the water table in rewetted peatlands can potentially increase the diffusional barrier for deep soil N2O emissions into the atmosphere and enhance the microbial complete reduction of N2O to N2 by denitrifying bacteria30,64,65, thereby reducing N2O emissions in the restored peatlands30. In addition, Brummell et al. attributed the reduction of N2O emissions in the restored peatlands to the rapid reestablishment of vascular plants66, which effectively take up soil N by plant roots and thereby decrease N availability for nitrification and denitrification. Therefore, reducing soil N availability and raising water levels could create unfavorable environments for N2O emissions.
Forest, grassland, and wetland restoration enhances C sink and reduces the GWP
Our results demonstrated that the NEE decreased with afforestation and reforestation age, and the estimated time required for the transition from CO2 sources to net sinks was approximately 3-5 years (Fig. 5), indicating that restored forests have the capacity to rapidly become CO2 sinks. At the early stage of restoration, forests may act as weak CO2 sources (Fig. 5), primarily due to the low foliar biomass and the rapid decomposition of residuals in the ground and soils17. As trees grow, the increases in GPP surpassed the rise in ER. The enhanced ability of C assimilation and the subsequent increase in annual woody biomass production are the key factors driving restored forests to function as CO2 sinks67,68,69. The estimated switchover time of restored forests from CO2 source to net sink after the disturbance was approximately 6 years for the clear-cutting sites and 13 years for the post-fire sites (Fig. 5), indicating a relatively slow recovery of the C sink function in the burned sites. Forest restoration progressively increased the GPP/ER ratio with restoration age, eventually reaching a stable value of 1.1-1.2 after approximately 20 years. Interestingly, this value aligns with the average GPP/ER ratio of 1.2 observed in mature forests worldwide70.
Grassland restoration markedly reduced the NEE (Fig. 2c), suggesting that grassland restoration effectively increased C sink capacity. The RRd of NEE in grassland was positively related to the RRd of soil moisture (Fig. 7h), indicating that higher soil moisture increased grassland CO2 sinks. Grassland restoration by grazing exclusion or reducing grazing density is conducive to the recovery of grassland and the increase of vegetation coverage (Fig. 6c), thereby reducing evaporative water loss from the soil71. The increase in soil moisture in the restored grassland decreased NEE by increasing GPP relatively more than ER72. The results are consistent with the general observations that higher soil water content increased vegetation leaf area index and GPP, and thereby resulted in a great C sink capacity in the grassland ecosystems73. In addition, the significant relationship between grassland NEE and aboveground biomass suggested that grassland restoration by grazing exclusion favored the regrowth of grasses and increased above- and belowground biomass (Figs. 6c and 7h), and thus increased gross ecosystem photosynthesis and eventually resulted in a significant decrease in NEE74. Grassland restoration by converting cropland to grassland remarkably decreased NEE (Supplementary Fig. S1c), indicating an increased C sink. The large residual of dead roots in the soil with cropland harvest would decompose and release large CO2 emissions, while the grassland had a large live root and thereby decreased ecosystem respiration and increased net CO2 uptake in the grassland systems75. Therefore, the enhanced CO2 uptake in the restored grassland was mainly due to the increase in soil moisture and vegetation biomass (Figs. 6c, 7h).
Overall, wetland restoration significantly reduced the NEE and shifted the ecosystems into CO2 sinks (Fig. 4a), highlighting the effectiveness of wetland restoration in enhancing CO2 sequestration. The significant negative correlation of NEE with water table depth in the restored wetlands indicated that the rise of the water table plays a crucial role in promoting the CO2 sink (Supplementary Fig. S4c). This pattern is expected since individual observations have confirmed that restoring the drained bog by raising the water table could create suitable conditions for vegetation growth, and the recolonization of bryophytes and vascular plants in restored wetlands contributed to the net CO2 uptake26,27,29. Returning aquaculture to wetland significantly enhanced CO2 sinks (Fig. 2c), which may be mainly due to the increase of the aquatic vegetation biomass and the shift of dominant species from submerged vegetation to flotation vegetation76. In addition, we found that the NEE of wetlands showed a negative exponential relationship with restoration age, and the transition time from net CO2 sources to net CO2 sinks was estimated to be around 4 years (Fig. 5d). Similarly, Waddington et al. reported that the restored peatlands could serve as net C sinks (20 g C m‒2) after two years restoration, mainly due to the rapid increase in moss cover and biomass after rewetting77. Lee et al. reported that the eighth year of the restored peatlands exhibited a net C sink (163 g C m−2 yr−1)78, falling within the range of C sink values reported for pristine peatlands (50-267 g C m−2 yr−1)26,79,80,81. Over longer time frames, we can anticipate a gradual stabilization of the CO2 sink as the biomass pool in the restored wetland approaches a steady C sequestration rate.
Regarding the C balance (excluding DOC fluxes), wetland restoration increased CH4 emissions and CO2 uptake, resulting in net sinks with a mean of 53 g C m−2 yr−1(Table 1). The balance between CO2 sinks and the strong warming potentials of CH4 and N2O emissions strongly determined the net climatic impact of the restored wetlands78. We found that wetland restoration resulted in a significant reduction in the GWP (Table 1), mainly due to the substantial reduction in CO2 emissions and the accompanying decrease of N2O emissions compared with the paired control ecosystems. Generally, wetland restoration could reduce GWP by 43-90% (Table S1). Among the wetland restoration measures, rewetting, moss layer transfer, and replanting & rewetting could significantly decrease GWP (Table S2). Returning croplands to wetlands is the most effective way to reduce GWP (Table S1). Our results were in accordance with the previous findings that wetland restoration remarkedly decreased GWP to a neutral range (3.3 to 6.7 t CO2-eq ha−1 yr−1) at the restored wetlands in comparison with the drained wetlands, mainly due to the increased CO2 uptake by precluding oxidation of the residual in the restored wetlands29,33,78. Previous work found that the restoration of bogs through rewetting could yield exceptionally high CO2 sink, which could effectively offset CH4 emissions, thus resulting in a substantial reduction in the GWP29. Afforestation increased CH4 and CO2 uptake (Table 1), thus leading to a significant reduction in GWP with a mean of −10.3 t CO2-eq ha−1 yr−1. Afforestation provided an effective strategy for GWP mitigation largely due to the increase in woody biomass C82. Grassland restoration resulted in a decrease in GWP across all the grassland types and restoration measures (Table 1, Table S2 and S3), mainly by increasing the net CO2 sink. Similarly, Rong et al. observed that grassland restoration brought about a greater increase in GPP than ER in a heavily grazed grassland, therefore decreasing NEE in the grassland72. Taking together, our study suggested that, forest, grassland, and wetland restoration could serve as an effective strategy for mitigating GHG emissions and reducing GWP.
Implications for the IPCC reports, Guidelines for National Greenhouse Gas Inventories, and future research
The present study provides a comprehensive understanding of the impacts of ecosystem restoration on GHG emissions at a global scale. Based on the compiled dataset, our results demonstrate that ecological restoration has vast potential to mitigate GHG at a global scale and provide insights and data for the IPCC reports and Guidelines for National Greenhouse Gas Inventories, particularly in relation to ecological restoration and land use change. In the context of “Agriculture, Forestry and Other Land Use (AFOLU)”, wetlands are recognized for their high level of uncertainty in the national greenhouse gas inventory reports submitted by States Parties to the United Nations Framework Convention on Climate Change (UNFCCC). Over the years, the IPCC has issued a series of methodological documents, such as “IPCC 2006 Guidelines for National Greenhouse Gas Inventories”, “2013 Supplement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories”, and “2019 Refinement to the 2006 IPCC Guidelines”. However, there is very little data regarding ecological restoration and its impacts on GHG emissions. The emission factors provided by the IPCC mainly focus on the dynamics of GHG in drained and rewetted organic soils83,84,85. Our study has expanded the available datasets for the restored wetlands and their paired control lands including different wetland types (bogs, mangroves, and floodplains wetland) and various land-use changes (i.e., conversion of cropland to wetland, conversion of grassland to wetland, conversion of forest to wetland, and returning aquaculture to wetland) (Figs. 3, 4). In addition, our study provides an updated N2O emission factor for rewetted wetlands, estimated at 2.1 kg N ha‒1 yr‒1 (95% CI: 1.4 to 2.8) (Fig. 3d), which is considerably higher than the assumed default value of 0 kg N ha‒1 yr‒1 for rewetted organic soils in the IPCC guidelines due to the limited data79. For the forests, the IPCC Guidelines only considered non-CO2 gases from biomass burning and assumed that the conversion of croplands, grasslands and other lands to forest lands tended not to alter the sources and removals of non-CO2 gases85. However, this assumption may not always hold true mainly due to the changes in soil properties resulting from land conversion (Fig. 6 and Supplementary Fig. S7)12,21. In our study, we conducted a detailed analysis for each land conversion type individually (i.e., conversion of croplands to forests and conversion of grasslands to forests) and developed a meta-data for non-CO2 gases (i.e., CH4 and N2O emissions) (Figs. 2, 3), thereby providing valuable data for refining the non-CO2 gases inventory methodology for the conversion of other lands into forests.
Our results confirmed that afforestation and reforestation, as well as rewetting the drained wetlands, should become critical for future ecological restoration to mitigate GHG (Tables S1 and S2, Fig. 5). In addition, previous work reported that the aquaculture systems are an important source of CH486, and our results verified that returning aquaculture to wetlands was an effective measure to enhance C sinks (Table S1). Wetland restoration by converting drained forests, grasslands, and croplands into wetlands increased CO2 sinks by decreasing ecosystem respiration, effectively transforming the ecosystems from CO2 sources to sinks29. Bog restoration by rewetting can be an active restoration strategy to recover vegetation and convert the extracted bog from GHG sources to GHG sinks26 (Table S2). For the grassland, restoring the degraded grassland either by grazing exclusion, reducing grazing intensity, or converting croplands to grassland is an effective strategy for mitigating GHG (Tables S2 and S3). These findings offer valuable insights for policymakers to select effective ecological restoration measures.
Our study highlights the significance of restoration age in regulating GHG emissions in restored ecosystems, underscoring the importance of considering the time in assessing or modeling the effects of restoration or land-use change on GHG emissions (Fig. 5 and Supplementary Fig. S3). Although restoration measures can be implemented and completed quickly, the re-establishment of plant coverage and microbial communities is a gradual process50,77,78. The process of biomass accumulation changes over time15,77, and soil physical, biogeochemical, and hydrological properties change with restoration time39,41,50. Consequently, restoration age plays a significant role in regulating GHG budgets. Although biogeochemical processes in restored ecosystems have been studied in recent years12,16,30,39, there remains considerable uncertainty regarding the duration required for a restored ecosystem to transition into a net CO2 sink. By compiling data from peer-reviewed literature, we identified the temporal patterns of NEE for forest and wetland restorations and determined the switchover time needed for the restored ecosystem to become a CO2 sink. These temporal patterns of NEE highlighted the need for policymakers and planners to prioritize measures that facilitate the long-term recovery of the degraded systems in order to maximize climatic benefit and better achieve the goals of the UN Decade on Ecosystem Restoration (2021-2030). Moreover, the empirical equations and insights gained from our study regarding temporal patterns of NEE following restorations can provide important information for ecological modeling efforts. In addition, the soil CH4 and N2O emissions are mainly governed by methanogens, methanotrophs, nitrifying and denitrifying microbes. However, studies about the effects of ecological restoration on these microbial communities are still insufficient57,87, which limits the explanation and prediction of the patterns of CH4 and N2O emissions under ecological restoration. Therefore, future investigations should prioritize examining the microbial mechanisms underlying changes in soil CH4 and N2O emissions during the ecological restoration process.
Methods
Data source
We compiled a global dataset on GHG emissions associated with ecological restoration from published literature (Fig. 1). We systematically searched the peer-reviewed literature from Google Scholar, Web of Science, and the China National Knowledge Infrastructure using the following keywords: TS = (restoration * OR rehabilitation * OR revegetation * OR recovery * OR reconstruction * OR reclamation * OR restored * OR restoring * OR recovering *) AND TS = (methane * OR CH4 * or nitrous oxide * OR N2O * or carbon dioxide * OR CO2 * or greenhouse gas *) AND TS = (wetland * or forest * or grassland *). The search results were filtered to include articles published between December 1999 and June 2023. Peer-reviewed studies were selected by the following criteria: (1) the selected experiments were conducted in the field from restored sites with paired control sites, or chronosequence sites; (2) each treatment was required to have at least three replicates; (3) the measurement covered an entire year or at least one growing season88; (4) the selected studies reported at least one type of GHG. Finally, the dataset used in this study included a paired restored-control samples sub-dataset and a chronosequence sub-dataset, which were compiled from 253 peer-reviewed articles (Supplementary Fig. S8). The paired sub-dataset included 679 paired measured cases, and the chronosequence sub-dataset included 1289 data points with restoration age (i.e., years since restoration) (Supplementary Data 1–5).
The dataset included: (1) GHG fluxes, including CH4, N2O, GPP, ER and NEE; (2) environmental factors, including longitude, latitude, mean annual air temperature (MAT), and mean annual precipitation (MAP); (3) restoration age, i.e., the years since restoration; (4) soil properties obtained from individual studies, including soil water table depth (WT), soil temperature (ST), and WFPS, Eh, BD, soil pH, SOC, TN, soil NH4+ and soil NO3−. The fluxes of CH4, N2O and CO2 were usually measured with the static chamber technique and eddy covariance method. The NEE is calculated as the differences of GPP and RE. If the GPP is lower than RE, then the NEE is positive, indicating net CO2 sources to the atmosphere. In contrast, negative NEE indicates net CO2 uptake from the atmosphere. The means, standard deviation (SD), and sample sizes for all variables in both restored and control ecosystems were extracted. If some studies (mainly eddy fluxes data) did not include SD values, SD was calculated as 1/10 of the mean88. Data in graphical figures and plots were extracted using Web Plot Digitizer (version 4.2).
In this study, we used the definition of “ecological restoration” proposed by the Society for Ecological Restoration. Wetlands have been disturbed by human activities, including the draining of natural wetlands for croplands, grasslands and forests, the conversation of wetlands to aquaculture ponds, peatland extraction, floodplain drainage, mangroves deforestation, etc3,76. Wetland restoration is defined as the process of rebuilding the pre-disturbance ecosystem, including the biogeochemical and hydrological processes typical of water-saturated soils, as well as the recovery of vegetation to the former natural ecosystem29. According to the collected data in this study, the main types of wetland restoration included the following categories: (1) conversion of drained grasslands to wetlands, (2) conversion of drained croplands to wetlands, (3) conversion of drained forests to wetlands, (4) returning the aquaculture ponds to wetlands, (5) floodplain restoration by rewetting, (6) bog restoration by rewetting and moss layer transfer technique, and (7) mangroves restoration by planting. Based on the collected forest data that meet the selection criteria, forest restoration in this study included the conversion of grasslands to forests and the conversion of croplands to forests43. In addition, forest restoration from the disturbances (i.e., clear-cutting, fire and windstorm) was included in the chronosequence sub-dataset (Fig. 5). Grassland degradation was mainly due to overgrazing, land abandonment, or land conversion to croplands89. Thus, restoration measures included recovering degraded grassland via grazing exclusion, reducing grazing intensity, artificial assisted restoration, and conversion of croplands to grasslands. The types of grasslands were classified as prairies, temperate steppe & meadow (TGM), alpine steppe & meadow (AGM), and desert steppe (DS)90.
Data analysis
We used meta-analysis to examine the response of GHG and environmental factors to ecological restoration. Wetlands, forest and grassland ecosystems can both release CH4 or CO2 into the atmosphere (positive values) and uptake CH4 or CO2 into the atmosphere (negative values). Thus, the natural logarithm-transformed response ratio (RR) is not suitable for meta-analysis in our study. Here, Hedges’ d was used to evaluate the weighted response ratios (RRd) as it ranges from −∞ to +∞43,44. The Hedges’ d was calculated as follows (Eq. 1, Eq. 2):
The variance (vd) was estimated with the following equation (Eq. 3):
where Xt and Xc are the means of the concerned variable in the restored and paired control groups, respectively. S is the pooled standard deviation. Nt and Nc refer to the sample size of variables of the restored and paired control groups, and St and Sc are the corresponding standard deviation.
MetaWin 3 software was used to calculate the overall effect size with a categorical random effects model43. Confidence intervals (95%; CIs) were calculated by bootstrapping (9999 iterations). The responses of variables to ecological restoration can be considered significant if the 95% confidence interval of the weighted effect size does not overlap zero (α = 0.05). If the 95% CI overlapped with zero, it assumed that there was no significant difference between the restored and paired control ecosystems. The effects of ecological restoration on the soil properties also were calculated using the above equations.
To estimate the greenhouse effect of GHG emissions in ecosystems, GWP (t CO2-eq ha−1 year−1) from CH4, N2O and CO2 fluxes (CH4 and CO2: g C m−2 year−1; N2O: g N m−2 year−1) was calculated as follows (Eq. 4)1,4,29:
where the fractions 16/12, 44/28 and 44/12 were used to transform the mass of C for CH4 and CO2 and nitrogen for the N2O to CO2 equivalent, respectively44. The 27.2 and 273 are the GWP values for CH4 and N2O, respectively, to CO2 over a 100-year time horizon1.
One-way analysis of variance (ANOVA) was used to test the differences in GHG fluxes (CH4, N2O, GPP, NEE and ER) and soil variables between the restored and paired control ecosystems using IBM SPSS Statistical Tool (Version 23.0; SPSS Inc.). To explore the relationships between GHG fluxes and environmental factors, the mixed meta-regression was adopted after the calculation of the random effect model. In all statistical tests, the level of significance was set at P < 0.05.
Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.
Data availability
All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Information files. Source data are provided with this paper.
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Acknowledgements
This research was supported by the National Natural Science Foundation of China (Nos. 32130069 to X.L.C., 31922060 to K.R.Z., 42007044 to T.H.H., U23A2017 to H.J.X., 32030069 to Q.F.Z. and U1906220 to W.X.D., equally) and Youth Innovation Promotion Association CAS (No. Y2022091 to K.R.Z.).
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K.Z. designed the research, conducted analysis, and wrote the manuscript. T.H., W.D., X.C., Y.Z., Y.C., H.X., X.W., J.Z., K.Z., and Q.Z. collected samples, conducted analysis, and wrote the manuscript. All authors assisted in the discussion of the results and preparation of the manuscript.
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He, T., Ding, W., Cheng, X. et al. Meta-analysis shows the impacts of ecological restoration on greenhouse gas emissions. Nat Commun 15, 2668 (2024). https://doi.org/10.1038/s41467-024-46991-5
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DOI: https://doi.org/10.1038/s41467-024-46991-5
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