Abstract
Increased CH4 emissions from rewetted organic soils can undermine the climate benefits of reduced CO2 release. This is especially problematic in low-lying areas that tend to remain waterlogged and act as potential CH4 hotspots. Here we test whether burning the soil surface before rewetting can reduce CH4 emissions. Using laboratory experiments with soil cores collected from degraded farmland in Denmark, we found that rewetting organic soils following burning reduced CH4 emissions by more than 95% over a 90-day period compared to rewetting alone. The reduction was likely associated with changed soil chemistry such as increased soil carbon stability and the decrease in methanogen abundance and activity. Our results suggest that targeted burning could help suppress short-term CH4 emissions after rewetting. However, long-term field studies are needed to understand whether this effect persists and to assess potential ecological risks such as pollution runoff, before any broader field-scale implementation is considered.
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Introduction
Organic soils store about 25% of global soil carbon on only 3% of the Earth’s land surface1. However, during the 20th century organic soils faced large-scale anthropogenic alteration, with 15% of organic soils worldwide being drained to repurpose the land mainly for agricultural use2. This degradation process has transformed organic soils from a carbon sink to a major source of CO2 emissions, contributing approximately 5% of all anthropogenic greenhouse gas (GHG) emissions3.
In recent decades, the importance of rewetting organic soils as a climate change mitigation strategy has become increasingly clear4. This realization has led to strong policy commitment and substantial public investment, driving the initiation of thousands of organic soil restoration projects over the past 50 years. An important policy development in this area is the Nature Restoration Law, which mandates that EU member states restore at least 30% of drained organic soils under agricultural use by 2030, with the target rising to 50% by 20505.
The ambition of this goal is commendable, especially since restoration through simple measures like blocking ditches and raising water tables can effectively convert these organic soils from carbon sources back into sinks. However, there is a critical issue that demands more attention: the quality and effectiveness of restoration efforts. Key concerns include whether the restored organic soils will meet the emission reduction targets set before the projects began and whether their full potential has been adequately explored. For example, the possibility of high CH4 emissions from rewetted organic soils could greatly offset the intended CO2 reduction benefits in the initial decades6, raising the question of whether such emissions can be effectively mitigated.
Given the complexities involved, including the lengthy and costly negotiations required to change land use practices with farmers or landowners, it is crucial to maximize the effectiveness of restoration efforts from the “obtained fruits”. Rather than simply expanding the restoration area, it is time for us to rethink the strategy and prioritize precision restoration actions ensuring that these efforts yield the greatest possible environmental benefits, particularly in terms of reducing GHG emissions.
Technically, while traditional rewetting strategies effectively reduce CO2 emissions on a landscape scale, they also create anoxic conditions that favor increased CH4 production, which can diminish the overall climate benefits of rewetting7. The increase in CH4 is not only evident in the initial years following rewetting8 but may also persist dominantly in the GHG balance even 18 years9 and 30 years10 after rewetting. Evans et al.11 also highlighted that while the reduction in CO2 emissions from rewetting generally outweighs the increase in CH4, the CH4 emissions still offset a substantial portion of climate benefits11. To address this biogeochemical trade-off, researchers are working on optimizing water table management during rewetting to maximize climate benefits. The prevailing approach recommends raising the water table around the soil surface, such as within 10 centimeters below the surface, to achieve the maximum net cooling effects11,12. However, the spatial heterogeneity of organic soils, further complicated by subsidence-induced low-lying topography from past drainage, creates a complex microtopography with varying elevations.
These localized low-lying microforms, though small in size (ranging from a few to several tens of square meters), are potential ponding areas in rewetted organic soils. These areas are often hotspots of anaerobic carbon turnover13 due to their prolonged water-saturated conditions14, high soil organic carbon content15, and increased nutrient availability16. The variability in CH4 emissions across different landscape gradients, from flooded local depressions to raised local elevations, clearly demonstrates the impact of these micro-scale variations17,18,19,20.
Although individual ponding areas may be small, their collective presence could account for 10-20% of the total rewetted area. Moreover, the intensified CH4 emissions in these hotspot areas can be 5-60 times higher than in other parts of the landscape and deserve high priority21,22,23,24. The critical question, therefore, is how we can effectively mitigate these intensified CH4 emissions in potential hotspot areas in a manner that is both operationally feasible and economically viable. This paper will explore whether controlled burning could serve as a pre-rewetting management strategy to reduce CH4 emissions after rewetting.
Regarding the feasibility, controlled burning has historically been a common land-use practice on drained bogs25 and is still used in some nature restoration management as well as in “near-natural” organic soils26. Importantly, controlled burning can be precisely applied to preselected, planned areas, making it especially suitable for patchy, low-lying sections of organic soils. Mechanistically, burning may reduce labile carbon pools27,28,29 and alter microbial community structure and activity30, potentially lowering methanogenic potential. However, these interactions are complex especially when combined with subsequent rewetting, which means that questions remain as to whether and to what extent controlled burning can achieve CH4 reductions, and, if so, what the real mechanisms are that drive these changes. In addition, inappropriate use of controlled burning on organic soils may raise concerns about ecological risks such as carbon loss, reduced biodiversity, nutrient release, and smouldering fires, but whether these problems are actually present when operating controlled burning in limited, low-lying areas of drained organic soils, and how such risks can be minimized or avoided needs detailed assessment and discussion. At the same time, as wildfires have become more common in drained organic soils31, understanding the interplay of burning, rewetting, GHG emissions, and ecological impacts is also important for evaluating how to rewet natural fire-affected organic soils.
To answer these questions, we established a series of microcosm experiments with soil cores collected from degraded organic soils (grazing land) to compare CH4 emission dynamics from rewetted soil cores with and without prior burning. To elucidate the mechanisms behind these emissions, we analyzed key soil physicochemical properties, the stability of soil carbon substrate, and microbial community diversity and abundance. We further quantified critical genes involved in CH4 cycling and utilized predictive metagenomic analysis to evaluate the functional potential of microbial communities. Through correlation analysis, we linked changes in soil properties and microbial dynamics to the observed CH4 emission patterns, providing a mechanistic understanding of how controlled burning prior to rewetting influences CH4 fluxes in organic soils. Finally, we discussed in detail the limitations of controlled burning as a practical pre-rewetting strategy. This work will inspire targeted strategies for reducing CH4 emissions in ponding-related hotspots of organic soils.
Results
GHG emissions
The cumulative emission data for different treatments during the 90-day incubation period are shown in Fig. 1a–d. At the end of the incubation period, cumulative CO2 emissions for drained organic soils were about 2700 ± 650 mmol CO2 m−2 at 10 °C and 3450 ± 660 mmol CO2 m−2 at 20 °C. Notably, a plateau in CO2 flux was observed around day 30 at both temperatures, likely due to gradual oxygen depletion in the sealed jars, which reduced aerobic respiration rates and subsequently limited CO2 production as anaerobic conditions set in, as also indicated by the decline in CO2 emission rates during the initial 30 days in Supplementary Fig. 1. Both rewetting strategies (i.e., direct rewetting and post-burn rewetting) significantly reduced cumulative CO2 emissions, with reductions of about 95% at 10 °C (Fig. 1a, p < 0.05) and 83% at 20 °C (Fig. 1b, p < 0.05). No significant difference was observed between rewetted organic soils without burning and rewetted organic soils with burning (Fig. 1a, b).
a Cumulative CO2 emissions of different treatments at 10 °C. b Cumulative CO2 emissions of different treatments at 20 °C. c Cumulative CH4 emissions of different treatments at 10 °C. d Cumulative CH4 emissions of different treatments at 20 °C. e CO2 equivalents of different treatments at 10 °C. f CO2 equivalents of different treatments at 20 °C. g Total global warming potential (GWP) of different treatments at 10 °C. h Total global warming potential (GWP) of different treatments at 20 °C. Treatments are distinguished by both color and symbol shape across panels. In panels a-d, treatments are represented by green triangles/circles (DOS), blue triangles/circles (ROS), and red triangles/circles (ROSB). In panels e-f, stacked bars show contributions from CO2 (purple) and CH4 (light green) to total CO2-equivalent emissions for each treatment. In panels g-h, total GWP is shown using blue bars (10 °C) and red bars (20 °C), with a dashed line linking the mean values across treatments to highlight treatment-level differences. Data are presented as means ± standard error (n = 3).
Regarding cumulative CH4 emissions, drained organic soils only emitted 32 mmol CH4 m−2 at 10 °C and 152 mmol CH4 m−2 at 20 °C, with large variability in these measurements. As expected, compared to drained organic soils, direct rewetting organic soils led to substantial increases in cumulative CH4 emissions, by about 6.0-fold at 10 °C and 3.0-fold at 20 °C (Fig. 1c, d). Differently, rewetting organic soils following burning effectively mitigated the increase in CH4 emissions associated with rewetting, ultimately reducing cumulative CH4 emissions by over 95% compared to direct rewetting organic soils at both temperatures and ultimately even lower than those from drained organic soils (Fig. 1c, d).
The calculated overall CO2 equivalents under various treatments are presented in Fig. 1e–1h. At both incubation temperatures, rewetted organic soils with burning had the lowest CO2 equivalents of emitted gases (Fig. 1e, f), resulting in the lowest total GWPs (Fig. 1g, h). For example, at 20°C, the GWP of rewetted organic soils with burning was decreased by 78% and 86% compared to drained and rewetted organic soils without burning, respectively (Fig. 1h, p < 0.05).
Soil physicochemical properties
The detailed soil basic physicochemical properties after the 90-day incubation are presented in Supplementary Table 1. Generally, these properties varied across different treatments. Rewetted organic soils, both with and without burning, showed a decrease in pH values compared to drained organic soils, with the lowest values observed in the rewetted organic soils without burning. Differently, when compared to drained organic soils, the electrical conductivity values significantly decreased in the rewetted organic soils without burning (p < 0.05) but increased in the rewetted organic soils with burning.
For total carbon, there was a slight decrease in rewetted soils with burning compared to drained soils and rewetted soils without burning. Similarly, the total organic carbon content in the rewetted soils with burning was lower than in soils that had not undergone burning (i.e., both drained and rewetted organic soils). A similar trend was observed for total nitrogen content, where the rewetted organic soils with burning exhibited lower values compared to both drained and rewetted soils without burning. Similarly, phosphorus levels were decreased in the rewetted soils after burning.
The Fourier transform infrared spectra of burnt and unburnt soil are presented in Fig. 2a, with shaded areas highlighting several relevant functional groups (Supplementary Table 2)29,32,33,34. The burning process resulted in increased maximum peak heights for refractory phenolics at ∼1420 cm−1 and aromatics at ∼1515 cm−1 and ∼1670 cm−1, while the peak for labile polysaccharides at ∼1050 cm−1 decreased. Consequently, compared to the unburnt soil, the humification degrees represented by SI1, SI2, and S13 in the burnt soil increased by 35%, 39%, and 17%, respectively (Fig. 2b). Moreover, the twin peaks associated with aliphatic groups were weakened in the burnt soil compared to the unburnt soil32,35, leading to a 21% increase in the aromaticity (SI4) (Fig. 2b).
a Fourier transform infrared spectra of burnt and unburnt topsoil (0 – 1 cm depth). Shaded areas highlight major absorption regions associated with key functional groups, including cellulose, aliphatics, aromatics, phenolics, and polysaccharides. b Stability indices of burnt and unburnt topsoil. Bars represent means ± standard error (n = 3). Green and purple lines or bars represent unburnt and burnt soil, respectively, across both panels.
Soil microorganisms
Figure 3 presents the Venn diagrams of bacterial and archaeal communities as well as the relative abundances of the top 10 families in both groups. Compared to rewetted soils without burning, rewetted soils with burning showed higher unique bacterial OTUs (Fig. 3a, b) but a lower number of unique archaeal OTUs (Fig. 3c, d) at both temperatures. Regarding the relative abundance of archaea (Fig. 3f), Methanobacteriaceae, the dominant methanogenic archaea in the community (22–32%), had the lowest relative abundance in the rewetted soils with burning compared to drained soils and rewetted soils without burning. This trend was consistent for Methanosarcinaceae and Methanomassiliicoccaceae, whose abundance was lower in the rewetted soils with burning compared to the rewetted soils without burning.
a Venn diagram of bacterial OTUs at 10 °C. b Venn diagram of bacterial OTUs at 20 °C. c Venn diagram of archaeal OTUs at 10 °C. d Venn diagram of archaeal OTUs at 20 °C. e Relative abundances of the top 10 bacterial families. f Relative abundances of the top 10 archaeal families. In panels a-d, different treatments are indicated by different colors: at 10 °C, treatments are represented by lavender (DOS10), sky blue (ROS10), and teal grey (ROSB10); at 20 °C, treatments are shown in olive green (DOS20), slate blue (ROS20), and dusty rose (ROSB20).
Figure 4 summarizes the most abundant KEGG pathways of organic metabolism predicted by PICRUSt using bacterial 16S rRNA sequences datasets. Generally, the enzymes associated with these pathways were more prevalent in the drained soils compared to the rewetted soils (both with and without burning). Among the rewetted treatments, soils subjected to burning exhibited the lowest enzyme abundances in most pathways, especially under 20 °C.
a Predicted metabolic pathway enrichment in different treatments at 10 °C. b Predicted metabolic pathway enrichment in different treatments at 20 °C. Functional predictions were generated using PICRUSt from 16S rRNA gene sequencing data and mapped to KEGG Level 3 metabolic categories. Z-scores represent the relative enrichment of various metabolic pathways across different treatments. Pathways are grouped by functional categories, such as amino acid metabolism, carbohydrate metabolism, other amino acid metabolism, lipid metabolism, and energy metabolism. Global and overview maps refer to broad, overarching pathways that provide a high-level summary of major biological processes, integrating various metabolic and cellular functions. Colored dots represent different treatments, with lighter colors used for 10 °C and darker shades for 20 °C. Specifically, green indicates DOS, blue indicates ROS, and red indicates ROSB. Dot position reflects the Z-score for each pathway under each treatment, illustrating shifts in microbial metabolic potential.
The two main methanogenesis pathways, acetoclastic (M00357 KEGG) and hydrogenotrophic (M00567 KEGG), and the enzymes involved are shown in Fig. 5a. PICRUSt functional analysis revealed that the abundance of key methanogenesis enzymes was highest in rewetted soils without burning and lowest in those with burning (Fig. 5c). This pattern was mirrored in the expression of the methanogenesis gene mcrA, which was significantly lower in rewetted soils with burning compared to both drained and rewetted soils without burning (Fig. 5d).
a KEGG methanogenesis pathways, including hydrogenotrophic methanogenesis and acetoclastic methanogenesis. b KEGG methanotrophy pathways, including anaerobic CH4 oxidation and aerobic CH4 oxidation. c Abundance of CH4 cycle-related enzymes. d Abundance of methanogenesis (mcrA) and main methanotrophy genes (pmoA and ANME-3). In panels a, b, enzyme names are color-coded according to functional groups and match the colors used in panel c. In panel d, data are presented as means ± standard error (n = 3). Abbreviations are explained in Supplementary Table 3.
The main pathways of CH4 oxidation and key enzymes involved are depicted in Fig. 5b. CH4 oxidation via reverse methanogenesis, also referred to as anaerobic CH4 oxidation, was identified as the dominant consumption pathway (solid lines in Fig. 5b), with the abundances of two key enzymes, tetrahydromethanopterin S-methyltransferase subunit (Mtr) and Formylmethanofuran dehydrogenase (Fwd), being significantly more abundant than those of other enzymes (Fig. 5c). Particularly, the highest abundances of these enzymes were in the rewetted soils without burning, while the lowest abundances were observed in the rewetted soils with burning (Fig. 5c). Regarding the expression of anaerobic CH4 oxidation-related genes, apart from ANME-3, other genes related to anaerobic CH4 oxidation were almost undetectable in the studied soils using qPCR quantification. The expression of ANME-3 genes was consistent with the trends predicted by functional analysis, showing the lowest levels in rewetted soils with burning (Fig. 5d).
Regarding aerobic CH4 oxidation (dotted lines in Fig. 5b), only genes related to particulate methane monooxygenase (pMMO) were mapped to the KEGG metabolic pathways, and their abundance was much lower than that of the main enzymes involved in anaerobic CH4 oxidation (Fig. 5c). qPCR revealed no significant difference in pmoA gene abundance between drained and rewetted soils without burning at both temperatures, but there was a significant reduction in rewetted soils with burning (Fig. 5d).
Discussions
Controlled burning prior to rewetting can significantly mitigate CH4 emissions
CH4 emissions are often viewed as an unavoidable trade-off with rewetting degraded organic soils8,9,10. While this is true over long timescales, focusing solely on this perspective overlooks more immediate challenges. If the goal of rewetting organic soils is to combat climate change in the near term, particularly within a few years or decades, then mitigating CH4 emissions becomes essential. Effective strategies to reduce these emissions would amplify the climate benefits of organic soil restoration, addressing both urgent climate action and long-term ecological restoration. Therefore, reducing CH4 emissions from rewetted organic soils should be a priority in both research and policy efforts.
To the best of our knowledge, this study is among the first to test the use of controlled burning as a pre-rewetting strategy specifically designed to reduce CH4 emissions from organic soils. While direct guidance from previous literature is limited, we were inspired by controlled burning practices commonly applied in the UK36, North America37, and Australia38 for soil and vegetation management, as well as studies on wildfire impacts on CH4 emissions from organic soils39,40,41 (as summarized in Supplementary Table 4), although without subsequent rewetting treatment. Some studies report reduced CH4 emissions post-burning, lasting from weeks to years40,42, with suppressed emissions persisting for up to nine years in some cases43. However, other studies show increased emissions following burning such as in tropical organic soils44,45, or no significant changes39,46. Nevertheless, these examples suggest that controlled burning could potentially alter CH4 emissions after rewetting, making it a promising area for further investigation.
Our study found that the combination of controlled burning and rewetting can significantly reduce CH4 emissions after rewetting to levels comparable to, or even lower than, those observed in drained organic soils (Fig. 1). Remarkably, this reduction was observed even under fully flooded conditions, with water levels reaching 8 cm above the soil surface, surpassing the typical recommendation of maintaining the water table within 0–10 cm below the surface for organic soil rewetting11. More importantly, this reduction in CH4 emissions preserves the CO2 mitigation benefits commonly associated with rewetting (Fig. 1). It is essential to recognize that these results are based on laboratory incubation experiments with a 90-day period, capturing only the initial CH4 emission responses. Additional long-term field experiments are needed to validate the burning effects on the sustainability of CH4 reductions. Nonetheless, our initial tests suggest the potential for achieving precise CH4 reduction in localized areas by controlled burning prior to rewetting.
Mechanisms behind post-burn rewetting to mitigate CH4 emissions
The mechanisms behind the effectiveness of controlled burning in mitigating CH4 emissions from rewetted organic soils are likely complex, involving both abiotic and biotic changes within the soil environment. Previous studies have attributed the reduction in CH4 emissions following burning to various factors (as summarized in Supplementary Table 4), including reduced vegetation cover, expanded oxic zones, diminished carbon substrates, and decreased microbial activity. To better understand these processes, we examined the relationships between emissions, changes in soil physicochemical properties, and microbial dynamics.
Controlled burning can reduce vegetation cover, which may limit CH4 production by decreasing root exudates that typically fuel methanogenesis47. The removal of plants with specialized aerenchyma tissues, which act as conduits transporting CH4 directly from soil to atmosphere48, may also contribute to reduced CH4 fluxes. However, these effects are often short-lived as root systems remain intact. In our study, we removed the aboveground biomass, focusing on soil processes and excluding the role of vegetation in CH4 cycling. Furthermore, while post-burning ash in other studies can clog soil pores and expand oxic zones unfavorable to methanogenesis49, this did not apply here as soils were rewetted. Instead, CH4 emission reductions in our study appeared to result from a combination of soil chemical changes and shifts in microbial communities (Fig. 6).
The data is from rewetted soils without burning and rewetted soils with burning. The colour gradients denote Pearson’s correlation coefficients; the line edge width corresponds to correlation strength, and the edge colour denotes whether the correlation is significant or not (p < 0.05). The symbols *, **, and *** indicate significance at p < 0.05, p < 0.01, and p < 0.001, respectively. Abbreviations are explained in Supplementary Table 3.
On the one hand, burning can affect the chemical composition of topsoil carbon, such as increased aromaticity and humification (Fig. 2). These changes suggest that the organic carbon left after burning was more recalcitrant and less easily decomposed by methanogens, thereby reducing CH4 emissions50. The significant correlations observed between CH4 emissions and substrate availability-related indicators (i.e., stability indices SI1-SI4) in the correlation analysis further underscore this relationship (p < 0.05, Fig. 6).
Moreover, soil pH also increased following burning due to the incorporation of alkaline ash and the release of base cations51, which may influence CH4 oxidation52, though the evidence is inconclusive53. The increase in soil electrical conductivity, due to the release of soluble salts54, may further impact microbial processes and CH4 emissions (p < 0.05, Fig. 6). While the precise mechanisms remain unclear, these changes likely reshaped the microbial community structure55.
In addition to these chemical changes, the study observed obvious shifts in the microbial community structure, particularly among methanogens. The relative abundances of key methanogenic archaea, such as Methanobacteriaceae, Methanosarcinaceae, and Methanomassiliicoccaceae, were notably reduced in the rewetted soils with burning compared to the rewetted soils without burning (Fig. 4f). This decline was most pronounced in Methanobacteriaceae, a hydrogenotrophic methanogen, indicating that burning may have disrupted hydrogen-dependent methanogenesis, a dominant pathway in the studied soils. The quantitative analysis of the methanogenic gene mcrA further confirmed the inhibitory impact of burning on methanogen activity, showing significantly lower expression levels in the rewetted soils with burning (Fig. 5d). This disruption from burning significantly impaired methanogenesis processes (p < 0.05, Fig. 6), as also observed in other studies30,56.
The study also highlighted the impact of burning on CH4 oxidation processes. The rewetted soils with burning showed a marked reduction in the activity of methanotrophs compared to the rewetted soils without burning, likely due to the combined effects of reduced CH4 availability57 and the disturbance from burning58. The co-occurrence network analysis revealed a weaker connectivity and a more fragmented microbial network in the burnt soils compared to the unburnt soils (Supplementary Fig. 2, Supplementary Table 5), suggesting that the disrupted microbial structure may have undermined both methanogenesis and CH4 oxidation59. Therefore, from a microbiological perspective, the reduction in CH4 emissions due to burning appeared to be primarily driven by the suppression of methanogenesis rather than an enhancement of CH4 oxidation.
To summarize, the CH4 emission dynamics in the rewetted soils with burning were governed by a more complex interplay of factors. Controlled burning altered soil physicochemical properties, notably increasing soil aromaticity and humification, which reduced the availability of labile carbon substrates crucial for methanogenesis. This chemical shift was accompanied by significant changes in microbial community structure, particularly a reduction in the abundance and activity of methanogens, which were further suppressed by the post-burn soil environment, such as the changes of pH and electrical conductivity. This integrated mechanism effectively mitigated the typical CH4 surge observed in rewetted organic soils.
Nevertheless, as vegetation gradually re-establishes after rewetting, fresh labile carbon inputs in the form of plant litter and root exudates gradually become key substrates for methanogens, which may once again contribute to CH4 production60,61. In parallel, with the changes in nutrient availability, there could be also a reset of microbial communities, which in turn could influence CH4 fluxes62. Long-term field studies are therefore needed to determine whether the suppression of methanogenesis observed in our study persists across multiple growing seasons.
Potential ecological risks of post-burn rewetting
Our study demonstrates that incorporating controlled burning on low-lying organic soils (drainage for agricultural use for many years) before rewetting can significantly reduce CH4 emissions and total GWP in these rewetted areas. However, before scaling up this approach in the field, potential ecological concerns must be carefully considered, such as initial carbon loss, future carbon accumulation, vegetation diversity, smouldering fires, and the release and leaching of nutrients and toxic substances.
One of the main concerns regarding controlled burning is the carbon loss associated with the partial consumption of soil organic layers. This loss could potentially reduce the carbon stock of organic soils, raising questions about the overall climate benefits of this strategy. In our preliminary carbon balance assessment (Paragraph S1 in Supplementary Information), we found that while there was a loss of total carbon due to burning (although not significant, Supplementary Table 1), the subsequent GHG reductions achieved through post-burn rewetting could offset this loss within approximately 2.3 years at 10 °C and 1.1 years at 20 °C, thereby achieving a net cooling effect shortly. Due to the limited number of studies on post-burn rewetting, direct comparisons are challenging. However, Flanagan et al. 29 observed that in low-severity burned organic soils, cumulative CO2 emissions could decrease below those of unburned soils within 1 to 3 years, even without rewetting29. In contrast, our findings for direct rewetting show that high CH4 emissions produce a net warming effect (Fig. 1h), consistent with Ojanen et al. 6, who reported that simple rewetting could delay net cooling benefits for several years for temperate organic soils, even up to approximately 60 years for nutrient-poor, shallow-drained organic soils6. Thus, the initial carbon loss from burning in our study appears insufficient to strongly undermine the GHG reduction benefits achieved through post-burn rewetting.
Concerns can also raise regarding the impact of controlled burning on the long-term carbon accumulation dynamics, given the potential impact of burning on the biogeochemical processes in degraded organic soils63. However, such concerns are often linked to high-frequency or high-intensity fires. Evidence suggests that low-frequency controlled burning (e.g., every 20–30 years) does not significantly affect carbon accumulation rates in organic soils27,64,65. Research on fire-managed peat moorlands indicates that carbon accumulation can be sustained despite periodic burning, as the acrotelm structure remains preserved, thereby minimizing long-term degradation risks27. In some cases, low-frequency rotational burning can even enhance long-term carbon accumulation by expanding the inert carbon pool, including recalcitrant charcoal64,65.
The potential impact on vegetation diversity is another potential consideration because, when implementing such practices in organic soils, it is intuitive to question whether they might negatively affect plant diversity. However, in the context of drained organic soils under agricultural use, such as arable lands dominated by homogeneous crops or species-poor grazing land, rather than ecologically sensitive areas with unique biodiversity, burning does not appear to have a significant negative effect on vegetation diversity. In terms of vegetation succession, previous studies suggest that controlled burning at appropriate intervals can manage vegetation dynamics, such as reducing the dominance of fire-prone species like the shrub Calluna vulgaris and encouraging the establishment of peat-forming species like peat mosses27,66,67,68. This means that if controlled burning is just applied before rewetting, it may effectively clear residual vegetation while creating conditions that promote the establishment of some native species, but what is really happening needs to be investigated in field trials. However, improper management of burning, particularly in heavily drained organic soils, can lead to persistent smouldering fires lasting for weeks or even months69. Such fires can greatly alter soil nutrient availability, such as increasing phosphorus and potassium levels, which may promote the expansion of undesirable shrubs such as willows at the expense of native vegetation70. Therefore, appropriate measures should be taken to avoid the occurrence of such uncontrolled smouldering fires during burning processes.
Generally, unlike unmanaged grasslands, shrublands, or forests, decades of agricultural management in the targeted organic soils, such as croplands and grazing land, have resulted in minimal fuel loads, which naturally reduce the risk of smouldering fires71. Nevertheless, dry soils and residual vegetation can still pose a potential risk. To mitigate this, several precautionary strategies can be applied to limit burning to targeted only on micro-sites and prevent unintentional smouldering fire spread both vertically and laterally. Establishing firebreaks with bare, well-moistened soil around each burning site, combined with metallic fences, would act as physical barriers to limit the fire within designated boundaries72. Additionally, in highly drained soils, pre-wetting the surface before burning can moderate burning intensity and reduce the risk of fires penetrating deeper layers of drained soils73. These measures are especially effective when combined with controlled burning during cooler, wetter periods, such as late autumn or early spring, when higher ambient moisture levels naturally reduce the likelihood of smouldering fires74. Moreover, the burning process would be monitored by trained personnel equipped with fire suppression tools on-site, ensuring that any potential smoldering fires can be promptly detected and extinguished. In fact, applying controlled burning in this context is similar to straw burning practices used in some countries, such as Japan, after crop harvesting75. The risk of smouldering fires after controlled burning would therefore be low if properly managed.
Another potential concern is the release and transport of nutrients from organic soils, especially from those heavily degraded fens with high phosphorus content76. Burning can promote phosphorus release through organic matter mineralization77 and ash incorporation78, which may subsequently leach into the surrounding aquatic systems after rewetting, and this process could partially explain the decline in soil phosphorus levels observed in the post-burn rewetting treatments of our study. While this process could contribute to the risk of eutrophication, the actual extent of this risk requires further investigation.
In addition to the release of nutrients, burning and rewetting may also lead to the release and leaching of pollutants such as fluoride79 and metal(loid)s (e.g., cadmium and arsenic)80,81, which could lead to washout into downstream systems82. It also may cause the leaching of dissolved organic carbon from the burned topsoil83. Changes in the quantity and reactivity of dissolved organic carbon from surface soil may further affect the decomposition of deep carbon, such as potential priming effects84. While these effects may be episodic, they should not be ignored and require further field experiments and long-term observations.
Finally, economic feasibility is also a key factor to consider, as it determines whether controlled burning can be practically implemented on a larger scale as a pre-rewetting strategy. Cost considerations are essential for stakeholders, including land managers and policymakers, to weigh the potential benefits against financial constraints. At present, the main costs appear to be initial site assessments for identifying target areas, as well as labor, equipment, and safety monitoring required for controlled burning. Given the limited scale of these targeted burning sites, expenses remain relatively manageable. Further research on costs across diverse sites and conditions will be beneficial for fully assessing feasibility.
Conclusions
This study demonstrates that controlled burning, when applied as a pre-rewetting strategy, can alter soil chemistry and microbial communities and thus create conditions that suppress CH4 production. However, this study remains conceptual, and further field experiments and long-term monitoring are needed to fully assess its ecological impacts before considering practical applications in organic soil rewetting. Until then, we also do not recommend its use as a supporting tool for habitat re-establishment in organic soil rewetting, although it has been unilaterally implemented as an ecological restoration measure in some regions of the world. Instead, we propose this work as a starting point for rethinking rewetting strategies in organic soils, i.e., more targeted interventions should be explored for CH4 hotspot areas, rather than applying uniform rewetting strategies without considering topographic heterogeneity.
Methods
Site description and soil sampling
Given that a large portion of temperate organic soils has been converted to agricultural use, including grazing land in Denmark, it is crucial to understand the impacts of rewetting strategies on these degraded organic soils. In this study, intact soil cores were collected in August 2023 from degraded organic soils located approximately 5 km north of Aarhus University Viborg, Denmark (56°32’14” N, 9°34’31” E). This area is within a natura 2000 area and mainly classified as extensively managed grazing land, with dominant vegetation including Festuca spp. and Holcus lanatus. The study site was shallowly drained at the time of soil collection. Climate details are provided in Paragraph S2 of Supplementary Information.
Burning experiment
To examine the synergy effects of controlled burning and rewetting on CH4 emissions from degraded organic soils, intact soil cores were randomly divided into three treatment groups: drained organic soils (DOS), rewetted organic soils (ROS), and rewetted organic soils with burning (ROSB). In the ROSB group, the burning duration was set at 10 min85. The specific burning procedure involved placing the soil core in the burning chamber of a garden fireplace (BLUE MOUNTAIN, Denmark) and then performing the burning from approximately 5 cm above the soil surface using a long-arm propane-butane gas torch burner.
Then, small intact soil cores (5 cm diameter, 8 cm height) were extracted from all groups for subsequent incubation experiments, representing the uppermost 8 cm of the soil profile, which, due to prior agricultural activity, is rich in easily accessible organic carbon once rewetted. Notably, to specifically focus on the soil and microbial responses to burning, the aboveground vegetation was removed in the DOS and ROS treatments to minimize the influence of plant-related respiration and plant-mediated gas transport processes41.
Incubation experiment
To compare GHG dynamics and soil characteristics under controlled laboratory conditions, a series of incubation experiments were conducted in climate chambers. The six small intact soil cores of each group were randomly divided into two incubation temperature groups (i.e., 10 °C and 20 °C, three for each). Subsequently, these soil cores were placed into glass jars (inner diameter 10 cm, height 25 cm) equipped with sealed lids fitted with rubber gaskets for headspace gas sampling. These jars were then placed in separate dark and temperature-controlled climate chambers. For the DOS group, the water table was maintained 8 cm below the soil surface to simulate shallow-drained conditions86, and a 1 cm water layer was retained at the base of the soil core to ensure that the lower portion of the core remained saturated. For the ROS and ROSB groups, the water table was maintained 8 cm above the soil surface to simulate wet conditions in low-lying areas of organic soils87. This resulted in six treatments, each with three replicates: DOS at 10 °C (DOS10), DOS at 20 °C (DOS20); ROS at 10 °C (ROS10), ROS at 20 °C (ROS20); ROSB at 10 °C (ROSB10), and ROSB at 20 °C (ROSB20) (Supplementary Table 6).
Gas sampling and measurement
To monitor the dynamics of GHG emissions over time, the headspace gas in glass jars was sampled twice a week during the 90-day incubation period. For each sampling, 10 mL of headspace gas was extracted for CO2 and CH4 concentration analysis. Cumulative gas emission fluxes, emission rates, and the global warming potential (GWP) of the emitted gases were then calculated. Detailed methods are provided in Paragraph S3 of Supplementary Information.
Soil analyses
To analyze the molecular composition of soil carbon and detect specific changes induced by burning, the burnt and unburnt topsoil at 0 – 1 cm depth was extracted from extra soil cores before incubation, then dried and sieved through a 2 mm mesh sieve. The dominant functional groups were subsequently characterized by Fourier transform infrared spectroscopy (INVENIO Bruker, USA). Soil stability indices (SI1-SI4) were then calculated based on changes in the normalized peak heights of key functional groups29,88. The specific measurement and calculation methods are provided in Paragraph S4 of Supplementary Information.
After incubation, each soil core was sieved preferentially with a 2 mm mesh sieve, and any soil that did not pass easily was further processed using a 4 mm mesh sieve for homogenization89. This process would remove large plant roots. A portion of the fresh soil was then allocated for ammonium nitrogen, nitrate nitrogen, and microbial analysis. The remaining soil was dried and used for subsequent physicochemical property analysis, including pH, electrical conductivity, total carbon, total organic carbon, total nitrogen, and elemental contents such as iron, manganese, phosphorus, and sulfur. Detailed methodologies for measuring these indicators are provided in Paragraph S5 of Supplementary Information.
Microbiology
To quantify the abundance of CH4 cycling-related functional genes, DNA was extracted from fresh soils under different treatments and analyzed using real-time quantitative PCR (qPCR). Detailed methodologies (Paragraph S6) and target genes (Supplementary Table 7) are provided in Supplementary Information. High-throughput sequencing for bacterial and archaeal 16S rRNA genes and subsequent data processing were performed by Biomarker Technologies (BMK) GmbH, Münster, Germany. Further details on 16S rRNA gene sequencing are provided in Paragraph S7 of Supplementary Information.
Statistical analysis
Basic statistical analyses were conducted using SPSS software (Version 22.0). Differences between treatments were assessed with a one-way analysis of variance (ANOVA), with significance set at p < 0.05. The Shapiro-Wilk test was employed to assess normality, and non-normal data were transformed (e.g., using logarithmic or square root transformations) to meet this assumption. Levene’s test was used to evaluate the homogeneity of variances, and in cases where this assumption was violated, Welch’s ANOVA was applied. For post-hoc comparisons, the Least Significant Difference was utilized when variances were homogeneous, while Games-Howell was applied for heterogeneous variances. Post-hoc results were only deemed valid if the corresponding ANOVA showed significant differences. Venn diagrams illustrating the differences in OTUs of bacterial and archaeal communities across different treatments were generated using the EVenn platform90. Predictive metagenomic analysis using PICRUSt was applied to infer functional profiles from 16S rRNA gene sequences using the Wekemo Bioincloud (https://www.bioincloud.tech)91. Pearson correlation analysis was employed to explore the relationships between GHG emissions from ROS and ROSB treatments with soil properties using R (Version 4.1.0). The non-numeric variable “burning” was encoded as -1 for ROS treatments and 1 for ROSB treatments, allowing for its inclusion in the correlation analysis.
Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.
Data availability
The supporting data is available in DRYAD.
Code availability
All software packages and online platforms used in the analyses of this study are publicly accessible. No custom-written scripts or codes were generated.
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Acknowledgements
This work was supported by the European Union’s Horizon Europe programme (WET HORIZONS, GA number 101056848) and China Scholarship Council (NO: CXXM20220022). We thank our colleagues and technicians for their help in experiment preparation. We would also like to thank anonymous reviewers for their valuable comments and suggestions.
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Shihao Cui: conceptualization, data curation, investigation, methodology, visualization, writing – original draft, writing – review and editing. Haonan Guo: data curation, methodology, writing – review and editing. Lorenzo Pugliese: conceptualization, supervision, writing – review and editing. Claudia Kalla Nielsen: conceptualization, investigation, methodology, supervision, writing – review and editing. Shubiao Wu: conceptualization, funding acquisition, project administration, supervision, writing – review and editing.
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Cui, S., Guo, H., Pugliese, L. et al. Controlled burning of peat before rewetting modifies soil chemistry and microbial dynamics to reduce short-term methane emissions. Commun Earth Environ 6, 346 (2025). https://doi.org/10.1038/s43247-025-02336-8
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DOI: https://doi.org/10.1038/s43247-025-02336-8
