Temperature Effects on Forest Soil Greenhouse Gas Emissions: Mechanisms, Ecosystem Responses, and Future Directions

Abstract

Forest soil greenhouse gas emissions play a critical role in global climate change. This review synthesizes the mechanisms of temperature change impacts on forest soil greenhouse gas (CO₂, CH₄, N₂O) emissions, the complex response patterns of ecosystems, and existing knowledge gaps in current research. We highlight several critical mechanisms, such as the high temperature sensitivity (Q₁₀) of methane (CH₄) and CO₂ emissions from high-latitude peatlands, and the dual effect of chronic nitrogen deposition, which can cause short-term stimulation but long-term suppression of soil CO₂ emissions. It emphasizes how climatic factors, soil characteristics, vegetation types, and anthropogenic disturbances (such as forest management and fire) regulate emission processes through multi-scale interactions. This review further summarizes the advancements and limitations of current research methodologies and points out future research directions. These include strengthening long-term multi-factor experiments, developing high-precision models that integrate microbial functional genomics and isotope tracing techniques, and exploring innovative emission reduction strategies. Ultimately, this synthesis aims to provide a scientific basis and key ecological threshold references for developing climate-resilient sustainable forest management practices and effective climate change mitigation policies.

1. Introduction

1.1. Global Significance of Forest Soil Greenhouse Gas Emissions

Forest ecosystems, as crucial components of the terrestrial carbon pool, play a vital role in global carbon cycling and climate change regulation [1]. Globally, forest soils store vast amounts of organic carbon, estimated at approximately 3012 Pg (1 Pg = 10¹⁵ g), far exceeding the carbon in the atmosphere (879 Pg) and vegetation (600 Pg) combined [2,3,4,5]. This enormous carbon pool is not only large but also highly active; annually, about 61 Pg of carbon moves from vegetation to the soil, while a similar amount is released from the soil to the atmosphere as greenhouse gases (primarily CO₂) [3,6]. This means that approximately 7% of the atmospheric carbon pool cycles through photosynthesis and soil respiration processes each year, highlighting the central role of forest soils in the global carbon balance.

Recent studies indicate that soils are major contributors to global greenhouse gas emissions, significantly impacting climate warming [7]. Among the greenhouse gases released from soils, carbon dioxide (CO₂) accounts for 74% of the total soil-source warming, followed by nitrous oxide (N₂O, 17%) and methane (CH₄, 9%) [7]. The dynamic changes in forest soil greenhouse gas emissions directly affect the stability of the global climate system. It is estimated that human activities leading to soil greenhouse gas emissions account for 15% of total anthropogenic greenhouse gas emissions, underscoring the importance of forest soil management in climate change mitigation strategies [5,7].

Forest ecosystems can act as both carbon sinks and carbon sources. As carbon sinks, forests absorb CO₂ from the atmosphere through photosynthesis and sequester it in biomass and soil organic matter. As carbon sources, forest soils release greenhouse gases such as CO₂, CH₄, and N₂O through processes like microbial decomposition and soil respiration [3]. Against the backdrop of global warming, an increase in forest soil greenhouse gas emissions could lead to a positive feedback effect, where climate warming promotes soil greenhouse gas emissions, and the increased emissions further exacerbate climate warming [8]. Recent research predicts that, by 2030, deforestation alone could lead to emissions of 3990 to 4529 metric tons of CO₂, while forest fires will contribute an additional 750 metric tons of CO₂ equivalent emissions [9,10,11].

The global significance of forest soil greenhouse gas emissions is also reflected in their close links to multiple Sustainable Development Goals. The protection and enhancement of forest soil carbon pools not only contribute to mitigating climate change but also promote biodiversity conservation, water resource management, and food security [12]. Furthermore, research on forest soil greenhouse gas emissions provides crucial scientific support for developing Nature-based Solutions and achieving the global temperature rise control targets set by the Paris Agreement [13,14,15,16,17].

With the increase in global warming, research on forest soil greenhouse gas emissions is attracting more attention from the scientific community and policymakers. A deep understanding of the mechanisms, influencing factors, and response patterns of forest soil greenhouse gas emissions under different management practices is of great theoretical and practical significance for optimizing forest management strategies, enhancing the carbon sink function of forest ecosystems, and mitigating global climate change [9,13,18].

1.2. Main Types and Characteristics of Forest Soil Greenhouse Gas Emissions

Forest soils are significant sources of and sinks for atmospheric greenhouse gases, primarily involving three gases: carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) [15,16]. These three gases have different production mechanisms, emission characteristics, and global warming potentials (GWPs), collectively forming the complete picture of forest soil greenhouse gas emissions [17].

1.2.1. Carbon Dioxide (CO₂)

Carbon dioxide is the primary greenhouse gas emitted from forest soils, accounting for 74% of the warming effect from soil-source greenhouse gases [7]. CO₂ emissions from forest soils mainly originate from the decomposition of soil organic matter and the respiration of soil biota (including microorganisms, soil fauna, and plant roots) [18]. Soil CO₂ flux exhibits significant spatiotemporal variability, influenced by multiple factors such as temperature, moisture, organic matter content, and microbial activity [18,19,20,21,22].

Studies have shown that soil CO₂ emissions vary significantly among different forest types. Annual CO₂ emissions from temperate forest soils typically range from 10 to 20 Mg CO₂ ha⁻¹, while tropical forests, due to higher temperatures and microbial activity, can have soil CO₂ emissions of 25–40 Mg CO₂ ha⁻¹ [22]. Forest soil CO₂ emissions also show distinct seasonal variations, with significantly increased emissions during the growing season (when temperatures are higher) and reduced emissions during the dormant season (when temperatures are lower) [20].

It is noteworthy that forest soil CO₂ emissions are closely related to the carbon cycle. Healthy forest ecosystems usually act as carbon sinks, meaning the amount of carbon fixed through photosynthesis is greater than the amount released through respiration. However, when forests are disturbed (e.g., by logging or fire), soil CO₂ emissions may exceed carbon fixation, turning the forest into a carbon source [23,24,25,26].

1.2.2. Methane (CH₄)

Methane is the second most important greenhouse gas, with a global warming potential 28 times that of CO₂ over a 100-year scale [27,28,29]. Unlike CO₂, the role of forest soils concerning CH₄ is more complex; they can be both a source of and a sink for CH₄, depending on the soil’s oxygen status and microbial community composition [24].

Under aerobic conditions, forest soils typically act as CH₄ sinks, with methanotrophic bacteria (MOB) oxidizing atmospheric CH₄ to CO₂ and water. Studies indicate that global forest soils can absorb approximately 30 Tg of CH₄ annually, accounting for about 6% of the global CH₄ sink [28]. The CH₄ uptake capacity of temperate and boreal forest soils usually ranges from −1.5 to −5.0 kg CH₄ ha⁻¹ year⁻¹ [24,30].

However, under anaerobic conditions (such as in wetland forests or seasonally waterlogged areas), methanogenic archaea (MPA) produce significant amounts of CH₄. CH₄ emissions from these areas can reach 5–50 kg CH₄ ha⁻¹ year⁻¹, becoming an important source of atmospheric CH₄ [31]. The spatial heterogeneity of CH₄ fluxes in forest soils is extremely high; even within the same forest, CH₄ fluxes at different micro-topographic locations can vary by several orders of magnitude [24,32,33].

1.2.3. Nitrous Oxide (N₂O)

Nitrous oxide is one of the most potent major greenhouse gases, with a warming effect 265 times that of CO₂ over a 100-year scale [27]. Although its emission volume is relatively small, N₂O contributes 17% to the warming effect of soil-source greenhouse gases [7]. Forest soil N₂O is primarily produced through nitrification and denitrification, processes that occur under aerobic and anaerobic conditions, respectively [32,34,35].

Forest soil N₂O emissions typically range from 0.5 to 3.0 kg N₂O ha⁻¹ year⁻¹, but can be as high as 5–10 kg N₂O ha⁻¹ year⁻¹ in areas with severe nitrogen deposition [32,36]. N₂O emissions exhibit high spatiotemporal variability, often characterized by “hotspots” (spatially) and “hot moments” (temporally), where emissions from a few locations or during short periods can account for a large proportion of total emissions [37].

Forest soil N₂O emissions are particularly sensitive to soil moisture content. N₂O emissions reach their maximum when soil moisture is within the range of 60%–80% water-filled pore space (WFPS), as both nitrification and denitrification can occur simultaneously under these conditions [32]. Furthermore, soil freeze–thaw cycles are important triggers for N₂O emissions from forest soils, especially in temperate and boreal forests [35,38,39].

1.2.4. Interrelationships Among the Three Greenhouse Gases

The emission processes of CO₂, CH₄, and N₂O in forest soils are not independent but involve complex interactions [40]. For example, the decomposition of soil organic matter provides the carbon source for CO₂ emissions and also supplies energy substrates for methanogens and denitrifying bacteria. Nitrogen-cycling processes (such as nitrification) not only produce N₂O but may also affect CH₄ oxidation [34,41].

Research indicates that the overall effect of forest soil greenhouse gas emissions depends on the relative contributions of these three gases and their respective global warming potentials. When assessing the impact of forest management measures on climate change, it is necessary to consider the emission changes of all three gases comprehensively, rather than focusing on a single gas [38,39].

1.3. Key Factors Influencing Forest Soil Greenhouse Gas Emissions

The emissions of greenhouse gases (CO₂, CH₄, and N₂O) from forest soils are influenced by a variety of environmental and anthropogenic factors. These factors regulate the production, consumption, and transport processes of greenhouse gases by altering soil physicochemical properties, microbial activity, and vegetation characteristics [15,16]. Understanding these key influencing factors is crucial for predicting the response of forest soil greenhouse gas emissions to global climate change and for developing effective mitigation strategies [41].

1.3.1. Climatic Factors

Climatic factors, particularly temperature and precipitation, are the primary drivers affecting forest soil greenhouse gas emissions [42]. Temperature directly influences soil microbial activity and enzyme activity, thereby affecting organic matter decomposition rates and greenhouse gas production rates. Studies have shown that soil CO₂ emissions generally increase with rising temperature, with a temperature sensitivity (Q₁₀ value) typically ranging from 1.5 to 3.5 in different forest types [20]. However, under extremely high temperatures, microbial activity may be inhibited, leading to a slowdown in CO₂ emissions [43].

Soil moisture conditions primarily regulate greenhouse gas production and consumption by affecting oxygen availability. Optimal soil moisture is conducive to microbial activity and organic matter decomposition, promoting CO₂ emissions. Conversely, excessively high soil moisture leads to anaerobic environments, inhibiting CO₂ production but promoting CH₄ production and N₂O emissions [43,44,45,46,47]. Research has found that N₂O emissions reach their maximum when the soil moisture content is within 60%–80% of the water-filled pore space [32].

Extreme weather events induced by climate change, such as droughts, heavy rainfall, and freeze–thaw cycles, also significantly impact forest soil greenhouse gas emissions. For instance, rainfall events following a drought often lead to short-term pulses of soil CO₂ and N₂O emissions (the Birch effect) [48,49,50,51]. Freeze–thaw cycles are important triggers for N₂O emissions in boreal forest soils [35].

1.3.2. Nitrogen Deposition

Atmospheric nitrogen deposition is a significant anthropogenic factor affecting forest soil greenhouse gas emissions [46]. European forests have long been subjected to high levels of nitrogen deposition (approximately 20 kg N ha⁻¹ year⁻¹). Although there has been a decline in recent years, these levels are still significantly higher than the global average (6 kg N ha⁻¹ year⁻¹) [51,52,53,54,55]. Nitrogen deposition affects greenhouse gas emissions through various pathways.

Firstly, nitrogen deposition increases soil available nitrogen, promoting plant growth and carbon fixation, but it may also accelerate soil organic matter decomposition, affecting the CO₂ emission balance [52,56,57,58]. Secondly, nitrogen deposition inhibits the CH₄ oxidation capacity of forest soils, weakening their function as CH₄ sinks. Studies have shown that long-term nitrogen addition can reduce CH₄ uptake by forest soils by 15%–30% [53,54]. Lastly, nitrogen deposition significantly increases N₂O emissions from forest soils because the additional nitrogen input provides substrates for nitrification and denitrification processes [55,59,60,61,62]. Global-scale studies indicate that nitrogen deposition has led to an increase in global N₂O emissions by approximately 0.8–1.5 Tg N year⁻¹ [56,63,64,65,66].

Furthermore, nitrogen deposition can lead to soil acidification, affecting the structure and function of soil microbial communities, thereby altering the production and consumption processes of greenhouse gases [50,67,68,69,70]. Long-term nitrogen deposition may lead to nitrogen saturation in forest ecosystems, causing them to shift from nitrogen sinks to nitrogen sources, increasing nitrogen losses and N₂O emissions [55,59,71,72,73,74].

1.3.3. Forest Management

Forest management practices, such as thinning, tending, harvesting, and regeneration, significantly affect forest soil greenhouse gas emissions by altering stand structure, microclimatic conditions, and soil properties [61] (

Table 1. Effects of stand structure on forest soil greenhouse gas emissions.

Stand Structure Parameter	CO2 Emissions	CH4 Oxidation	Underlying Mechanisms	Management Applications
Tree Density	CO2 emissions are higher in dense stands and decrease with density reduction.	CH4 oxidation is lower in dense stands and optimal at moderate densities.	Underlying mechanisms include the contribution of root respiration, soil temperature regulation, rates of organic matter input, and soil moisture balance.	Management applications involve density management prescriptions, stocking level guidelines, and spacing recommendations.
Spatial Arrangement	CO2 emissions are heterogeneous in clustered arrangements and more uniform in regular spacing.	CH4 oxidation is enhanced in gap-cluster arrangements and reduced in uniform spacing.	Mechanisms involve microclimate variation, root distribution patterns, resource competition gradients, and heterogeneity in soil development.	Applications include variable density thinning, gap creation strategies, retention forestry approaches, and enhancement of structural complexity.
Vertical Structure	CO2 emissions are higher with complex vertical structures and lower in simplified structures.	CH4 oxidation is variable, depending on understory conditions and soil properties.	Factors at play are light penetration effects, rainfall interception, diversity in litter quality, and variation in rooting depth.	Management strategies encompass multi-cohort management, understory development, canopy stratification, and enhancement of vertical diversity.
Species Composition	CO2 emissions are higher in conifer-dominated stands and lower in broadleaf or mixed stands.	CH4 oxidation is higher in mixed stands and lower in pure conifer stands.	Differences arise from litter quality variations, root exudate composition, mycorrhizal associations, and phenological complementarity.	Applications include mixed-species silviculture, conversion strategies, species selection criteria, and enhancement of functional diversity.
Age Structure	CO2 emissions are higher in younger stands and more stable in multi-aged stands.	CH4 oxidation is lower in younger stands and higher in mature and multi-aged stands.	This is driven by differences in growth rates, carbon allocation patterns, the stage of soil development, and overall ecosystem stability.	Management approaches involve uneven-aged management, age class distribution considerations, rotation length decisions, and structural retention strategies.

).

Thinning, by reducing tree density, changes light, temperature, and moisture conditions within the forest, thereby influencing soil microbial activity and organic matter decomposition. Studies indicate that moderate-intensity thinning (removing 30%–40% of trees) typically causes a temporary increase in soil CO₂ emissions, but the long-term effect depends on thinning intensity and recovery time [63,75,76,77]. Thinning may also affect CH₄ and N₂O emissions by altering soil moisture content and oxygen status [78,79,80] (

Table 2. Temporal dynamics of forest soil greenhouse gas emissions post-thinning.

Recovery Phase	Timeframe	CO2 Emissions	CH4 Oxidation	Key Mechanisms	Management Implications
Initial Reduction	0–2 years	CO2 emissions decrease by 15%–45% depending on thinning intensity.	CH4 oxidation decreases by 10%–40% in trafficked areas, with minimal change in undisturbed areas.	Key mechanisms include reduced root biomass and autotrophic respiration, decreased microbial activity, and altered soil physical conditions.	Management implications involve planning for reduced carbon cycling, monitoring soil physical recovery, and considering timing relative to seasonal cycles.
Recovery	2–5 years	CO2 emissions show a gradual return to baseline, which is faster in lightly thinned stands.	CH4 oxidation shows progressive improvement, especially in well-drained soils.	Mechanisms driving recovery are root system expansion from residual trees, understory development, microbial community adaptation, and residue decomposition.	This phase offers an opportunity for understory management, is a critical period for soil remediation if needed, and serves as an important monitoring phase.
Enhancement	5–8 years	CO2 emissions may exceed baseline by 10%–20% in moderately thinned stands.	CH4 oxidation may exceed baseline by 15%–30% in well-drained soils.	Enhanced individual tree growth, changes in root system architecture, altered resource availability, and priming effects are key mechanisms.	Management should consider the timing of subsequent entries, recognize this as a period of maximum carbon cycling, and note potential trade-offs with carbon sequestration.
Stabilization	8+ years	CO2 emissions converge with unthinned conditions despite structural differences.	CH4 oxidation stabilizes at or slightly above pre-treatment levels.	Stabilization is achieved through stand density recovery, soil organic matter stabilization, ecosystem adaptation, and the establishment of a new equilibrium.	This phase indicates appropriate timing for subsequent thinning, provides a baseline for long-term carbon accounting, and serves as a reference for adaptive management.

).

The impact of harvesting methods (e.g., clear-cutting and selective cutting) on greenhouse gas emissions varies significantly. After clear-cutting, soil CO₂ emissions usually increase temporarily and then gradually decrease due to increased surface temperature and reduced organic matter input. Simultaneously, soil disturbances and changes in microbial communities may lead to reduced CH₄ uptake and increased N₂O emissions [60]. In contrast, selective cutting has a smaller impact on soil greenhouse gas emissions and helps maintain the forest’s carbon sink function [4,17,61,79,81].

Additionally, forest regeneration and tree species selection are important management factors affecting greenhouse gas emissions. Different tree species influence soil organic matter decomposition and greenhouse gas production through the quality and quantity of litter, root exudates, and mycorrhizal types [66]. For example, coniferous forest soils generally have a higher CH₄ uptake capacity and lower N₂O emissions than broadleaf forest soils [67,80,82,83].

1.3.4. Soil Physicochemical Properties

Soil physicochemical properties, including texture, pH, organic matter content, and nutrient status, are fundamental factors affecting greenhouse gas production and transport [38].

Soil texture influences water retention capacity, gas diffusivity, and microbial habitat. Clayey soils are prone to forming anaerobic micro-sites, which favor CH₄ production and N₂O emissions, while sandy soils have good aeration, favoring CH₄ oxidation and CO₂ emissions [69,84,85,86]. Soil pH directly affects microbial community composition and enzyme activity. Generally, neutral or slightly alkaline conditions are conducive to microbial activity and organic matter decomposition, promoting CO₂ emissions, whereas acidic conditions inhibit methanotroph activity, reducing CH₄ uptake capacity [70,87,88].

Soil organic matter is the energy and carbon source for microbial activity, and its content and quality directly influence CO₂ production. High organic matter content usually implies higher microbial activity and CO₂ emission potential [13]. Furthermore, soil nutrient status, particularly the carbon-to-nitrogen ratio (C/N ratio), significantly affects nitrogen-cycling processes and N₂O emissions. Soils with a low C/N ratio (<20) often have higher N₂O emission potential [67,74,89,90,91,92].

In summary, forest soil greenhouse gas emissions are comprehensively influenced by multiple factors, including climatic factors, nitrogen deposition, forest management, and soil physicochemical properties. Complex interactions exist among these factors, collectively determining the spatiotemporal patterns of forest soil greenhouse gas emissions and their response to global climate change [38,93,94,95]. A thorough understanding of these key influencing factors and their mechanisms is crucial for accurately assessing the role of forest ecosystems in climate change and for formulating effective forest management strategies [61].

1.4. Research Objectives and Content Framework

1.4.1. Research Background and Significance

With the intensification of global climate change and the expanding impact of human activities, research on greenhouse gas emissions from forest soils is increasingly attracting the attention of the scientific community and policymakers [72]. As previously mentioned, forest soils are important sources of and sinks for atmospheric greenhouse gases, and their emission dynamics directly affect global carbon and nitrogen cycles and the climate system [1]. However, many uncertainties still exist in the current research on forest soil greenhouse gas emissions, particularly regarding the regulatory mechanisms of different influencing factors (such as restoration methods, forest management, nitrogen deposition, and temperature sensitivity) on emission processes. For instance, the specific pathways by which chronic nitrogen deposition alters soil microbial enzyme activity to suppress long-term CO₂ emissions, and how the relative temperature sensitivities of methanogenesis versus methanotrophy shift across different wetland ecosystems, remain highly uncertain [16,46,89].

Forest restoration and sustainable management have become important strategies for addressing climate change, but the impacts of different restoration methods and management measures on soil greenhouse gas emissions have not yet been systematically evaluated [14]. Moreover, against the backdrop of changing global nitrogen deposition patterns and climate warming, the response mechanisms of forest soil greenhouse gas emissions urgently require in-depth research [75]. Therefore, systematically reviewing the research progress on soil greenhouse gas emissions from forest ecosystems is of significant scientific importance and practical value for improving theories of forest carbon and nitrogen cycling, optimizing forest management practices, and formulating effective climate change mitigation strategies. Previous reviews have focused primarily on a single gas or biome type. This review integrates multi-gas, multi-driver, and ecosystem-specific insights, particularly the regulatory role of soil fauna, to offer a more holistic framework [12,72].

1.4.2. Research Objectives

Based on the above background, we aimed to systematically review the research progress on soil greenhouse gas (CO₂, CH₄, and N₂O) emissions from forest ecosystems, focusing on the following aspects:

• To resolve the contrasting outcomes of post-fire restoration, we analyzed the impacts and mechanisms of different restoration methods on forest soil greenhouse gas emissions, providing a scientific basis for managing degraded forest ecosystems [73].
• To clarify the inconsistent effects of forest thinning, we evaluated the regulatory effects of management on soil CH₄ and CO₂ emissions, offering theoretical support for optimizing forest operational management practices [61].
• To uncover complex biogeochemical interactions, we explored the impact of nitrogen deposition and its interaction with soil fauna on forest soil greenhouse gas emissions, deepening our understanding of nitrogen-cycling processes [75].
• To better predict high-latitude feedback, we elucidated the relationship between temperature sensitivity and greenhouse gas emissions from high-latitude wetland forest soils, assessing future emission trends under climate warming [96].
• To guide future scientific inquiry, we identified existing knowledge gaps and propose targeted future research directions, providing new perspectives for research on greenhouse gas emissions from forest ecosystems [17,96].

1.4.3. Content Framework

To achieve the above research objectives, we adopted a systematic review method based on the published scientific literature. Our synthesis is primarily based on over 150 high-quality, peer-reviewed articles from the Web of Science and Scopus databases, with a focus on meta-analyses and long-term field studies from the last decade that provide mechanistic insights and data synthesis.

Part One is introductory and presents the global significance of forest soil greenhouse gas emissions, their main types and characteristics, key influencing factors, and the research objectives and content framework, laying a theoretical foundation for subsequent parts.

Part Two focuses on restoration methods and post-fire soil greenhouse gas dynamics, analyzing the differential impacts of natural and artificial restoration on forest soil greenhouse gas emissions and exploring the patterns of change and driving mechanisms of greenhouse gas emissions during forest recovery after a fire disturbance [73].

Part Three discusses the impact of forest management on soil CH₄/CO₂ emissions, primarily evaluating the regulatory effects of different thinning intensities and tending measures on forest soil greenhouse gas fluxes and analyzing their mechanisms and environmental effects [61].

Part Four investigates the regulatory roles of nitrogen deposition and soil fauna, elaborating on the direct and indirect impacts of nitrogen deposition on forest soil greenhouse gas emissions and the regulatory role of soil fauna (such as millipedes) in greenhouse gas emissions under nitrogen deposition [75].

Part Five analyzes the temperature sensitivity of and greenhouse gas emissions from high-latitude wetland soils, focusing on the impact of climate warming on greenhouse gas emissions from high-latitude forest wetland soils and exploring the sensitivity of greenhouse gas emissions to temperature changes as well as their spatial patterns and driving factors [96].

Part Six provides a comprehensive discussion, summarizing the research progress on forest soil greenhouse gas emissions, identifying existing knowledge gaps in current research, and proposing future research directions and management recommendations [61,96].

Through the above content framework, we systematically reviewed the research progress on soil greenhouse gas emissions from forest ecosystems, providing a scientific basis for a deeper understanding of forest soil greenhouse gas emission mechanisms, predicting their response to global climate change, and formulating effective forest management strategies [78,96] (Figure 1).

2. Nitrogen Deposition and Soil Fauna Regulation

2.1. Effects of Nitrogen Deposition on Forest Soil Greenhouse Gas Emissions

Nitrogen deposition represents one of the most significant anthropogenic alterations to the global nitrogen cycle, with profound implications for forest ecosystem functioning and greenhouse gas dynamics [75,79]. Atmospheric nitrogen deposition has more than doubled globally since pre-industrial times, primarily due to fossil fuel combustion, agricultural intensification, and industrial processes [75]. Current deposition rates exceed 20 kg N ha⁻¹ year⁻¹ in many industrialized regions of North America, Europe, and East Asia.

2.1.1. Nitrogen Deposition Patterns and Trends

The spatial distribution of nitrogen deposition exhibits substantial heterogeneity, reflecting patterns of industrialization, agricultural intensity, and atmospheric circulation. Ref. [82] documented that East Asia currently receives the highest nitrogen deposition rates globally, with total (wet + dry) deposition exceeding 40 kg N ha⁻¹ year⁻¹ in some regions of eastern China [82]. Western Europe and eastern North America also experience elevated deposition rates (10–20 kg N ha⁻¹ year⁻¹), though recent emission control measures have stabilized or slightly reduced deposition in these regions [83]. In contrast, much of the Southern Hemisphere receives relatively low deposition rates (<5 kg N ha⁻¹ year⁻¹), though increasing trends have been observed in rapidly developing regions of South America and Africa [51].

Temporal trends in nitrogen deposition vary regionally. Ref. [72] reported that, while nitrogen deposition has stabilized or declined in North America and Western Europe since the early 2000s, it continues to increase in East and South Asia, with annual growth rates of 1%–3% [83]. Projections suggest that global nitrogen deposition will continue to increase through mid-century, with the most substantial increases expected in developing regions of Asia, Africa, and South America [51,83].

2.1.2. Effects on Soil CO₂ Emissions

Nitrogen deposition exerts complex and sometimes contradictory effects on soil CO₂ emissions from forest ecosystems. The direction and magnitude of these effects depend on the forest type, soil properties, and duration and intensity of nitrogen inputs.

In nitrogen-limited forest ecosystems, short-term nitrogen addition typically stimulates soil CO₂ emissions by enhancing plant productivity and accelerating the decomposition of labile carbon compounds. Refs. [17,79,81] observed a 5%–15% increase in soil CO₂ efflux following nitrogen addition in nitrogen-limited temperate forests, attributed primarily to enhanced root respiration and accelerated turnover of labile carbon pools [1].

However, chronic nitrogen deposition often leads to reduced soil CO₂ emissions. This reduction can be attributed to several mechanisms, including the inhibition of microbial activity, particularly lignin-degrading enzymes, by high nitrogen levels; increased soil acidification, which can negatively affect decomposer communities; and enhanced carbon stabilization through the formation of recalcitrant organic matter complexes. Specifically, this inhibition is linked to the suppression of lignin-degrading enzymes (e.g., manganese peroxidase) and a shift in the microbial community composition toward faster-growing, less carbon-use-efficient bacteria that are less capable of decomposing complex polymers [50,52,58]. A meta-analysis by [52] found that, on average, nitrogen addition significantly decreased soil respiration by about 7% across various forest ecosystems [52].

2.1.3. Effects on Soil CH₄ Fluxes

Forest soils are generally considered net sinks for atmospheric CH₄, primarily due to the activity of methanotrophic bacteria. Nitrogen deposition typically reduces CH₄ uptake by forest soils, thereby diminishing their sink strength.

Several mechanisms contribute to the inhibitory effect of nitrogen deposition on CH₄ oxidation. Increased availability of ammonium (NH₄⁺) from nitrogen deposition can competitively inhibit methane monooxygenase, the key enzyme responsible for CH₄ oxidation by methanotrophs [53]. Additionally, nitrogen-induced soil acidification can create unfavorable conditions for methanotroph populations [50]. Long-term nitrogen enrichment may also alter microbial community structure, favoring nitrifying bacteria over methanotrophs, further reducing CH₄ uptake capacity [41]. Meta-analyses have consistently shown that nitrogen addition significantly decreases CH₄ uptake in forest soils, with reductions ranging from 15% to over 50% depending on the ecosystem and nitrogen load [53,55].

2.1.4. Effects on Soil N₂O Emissions

Nitrogen deposition almost invariably leads to increased N₂O emissions from forest soils. N₂O is a potent greenhouse gas produced primarily through the microbial processes of nitrification and denitrification, both of which are often substrate-limited by nitrogen availability in natural forest ecosystems [32].

Increased nitrogen inputs from deposition alleviate this limitation, stimulating both nitrification (conversion of NH₄⁺ to NO₃⁻, with N₂O as a byproduct) and denitrification (conversion of NO₃⁻ to N₂, with N₂O as an intermediate product) [55]. The magnitude of N₂O emissions is influenced by factors such as soil moisture, temperature, pH, and carbon availability, in addition to nitrogen input levels [32]. Studies across various forest types have demonstrated a strong positive correlation between nitrogen deposition rates and N₂O fluxes [56]. For instance, a global synthesis reported that N₂O emissions increase exponentially with increasing nitrogen inputs, particularly in temperate and tropical forests [56].

2.2. Interaction of Nitrogen Deposition and Soil Fauna

Soil fauna, encompassing a diverse array of organisms from microscopic protozoa to larger invertebrates like earthworms and millipedes, play crucial roles in soil processes, including organic matter decomposition, nutrient cycling, and soil structure formation. The interaction between nitrogen deposition and soil fauna can significantly modulate greenhouse gas emissions from forest soils, yet this area remains less understood compared with the direct effects of nitrogen deposition.

2.2.1. Effects of Nitrogen Deposition on Soil Fauna Communities

Nitrogen deposition can alter the abundance, diversity, and community composition of soil fauna, with responses varying among different faunal groups and being influenced by the level of nitrogen input and soil properties.

Generally, increased nitrogen availability can lead to an increase in the abundance of certain microbial-feeding fauna, such as some nematodes and microarthropods, due to enhanced microbial biomass. However, detrimental effects are often observed for other groups, particularly larger decomposers. For example, studies have reported declines in earthworm populations and diversity under high levels of nitrogen deposition, often attributed to soil acidification and changes in litter quality [50]. Millipedes, important litter fragmenters, may also exhibit varied responses, with some species declining due to altered food resources or soil chemical conditions [85,86,87,88].

2.2.2. Soil Fauna Mediation of Nitrogen Deposition Effects on CO₂ Emissions

Soil fauna influence CO₂ emissions primarily through their roles in decomposition. By fragmenting litter, grazing on microbial populations, and mixing organic matter into the soil, fauna can stimulate microbial activity and accelerate carbon mineralization, leading to increased CO₂ release [97].

Under nitrogen deposition, changes in faunal communities can alter these processes. A reduction in key decomposer fauna, such as earthworms and millipedes, due to nitrogen-induced acidification or unpalatable litter could slow down decomposition rates and potentially reduce CO₂ emissions in the long term, counteracting some of the direct stimulation from nitrogen [97]. Conversely, if nitrogen deposition favors certain microbial-feeding fauna that stimulate microbial turnover, CO₂ emissions might be enhanced. The net effect is complex and depends on the specific changes in the faunal community and their functional roles.

2.2.3. Soil Fauna Mediation of Nitrogen Deposition Effects on CH₄ Fluxes

The role of soil fauna in mediating CH₄ fluxes under nitrogen deposition is not well-established. However, fauna can influence soil structure and aeration, which are critical for methanotrophic activity. For instance, the burrowing activities of larger fauna like earthworms can improve soil aeration, potentially enhancing CH₄ oxidation [97]. If nitrogen deposition negatively impacts these ecosystem engineers, it could indirectly exacerbate the reduction in CH₄ uptake caused by direct nitrogen effects.

Some soil invertebrates, such as termites and certain beetle larvae, are known to harbor methanogenic symbionts in their guts and can be sources of CH₄. While this is well-documented and ecologically significant in many tropical systems, its relevance in temperate or boreal forest soils is considered minor and remains a significant knowledge gap, meriting further investigation.

2.2.4. Soil Fauna Mediation of Nitrogen Deposition Effects on N₂O Emissions

Soil fauna can influence N₂O emissions by affecting soil nitrogen transformations and the physical conditions for nitrification and denitrification. Fauna can stimulate N₂O production by increasing nitrogen mineralization through excretion and by creating anaerobic microsites through their burrowing and feeding activities [97].

Under nitrogen deposition, the interaction with soil fauna can be multifaceted. If nitrogen deposition leads to an increase in faunal biomass or activity that enhances nitrogen cycling rates, N₂O emissions could be further amplified [55,97]. For example, increased nitrogen availability might stimulate the activity of fauna that graze on bacteria, leading to faster turnover of microbial nitrogen and potentially more substrate for N₂O production. Conversely, if nitrogen deposition harms key faunal groups involved in creating aerobic soil conditions, it might inadvertently promote denitrification and N₂O release in wetter soils.

Millipedes, as an example, have been shown to influence nitrogen cycling. Their feeding activities can fragment litter and incorporate it into the soil, and their gut environment can harbor denitrifying bacteria. Studies focusing on how nitrogen deposition specifically alters millipede activity and how this, in turn, affects N₂O emissions are crucial. For instance, if nitrogen-enriched litter is more palatable or processed differently by millipedes, this could alter the C/N ratio of their feces and the subsequent N₂O production from these hotspots. Moreover, the activity of ecosystem engineers like earthworms is known to create anaerobic gut and burrow environments that stimulate denitrification, and nitrogen enrichment of ingested litter could increase the N substrate available for N₂O production within these microsites [89,97].

3. Temperature Sensitivity and Greenhouse Gas Emissions from High-Latitude Wetland Forest Soils

3.1. Characteristics of High-Latitude Wetland Forest Ecosystems

High-latitude regions, encompassing boreal and subarctic zones, contain vast expanses of wetland forests. These ecosystems are characterized by cold climates, short growing seasons, and waterlogged soils, often underlain by permafrost [94]. Dominant vegetation typically includes conifer species like spruce (Picea spp.), fir (Abies spp.), and larch (Larix spp.), along with deciduous species such as birch (Betula spp.) and aspen (Populus spp.), often interspersed with peat-forming mosses (e.g., Sphagnum spp.) and various shrubs [22].

Soils in these wetland forests are typically rich in organic matter due to slow decomposition rates resulting from low temperatures and anaerobic conditions. This has led to the accumulation of large carbon stocks, making these ecosystems globally significant in the carbon cycle [98]. However, these carbon stores are highly vulnerable to climate change, particularly warming temperatures.

3.2. Greenhouse Gas Emission Patterns in High-Latitude Wetland Forests

High-latitude wetland forests are important sources of CH₄ and N₂O and can be sources or sinks of CO₂, depending on hydrological conditions and ecosystem productivity.

CO₂ exchange is driven by the balance between photosynthesis and respiration (autotrophic and heterotrophic). While these forests can be net CO₂ sinks during the growing season, soil respiration, particularly from the large organic matter pool, can release significant amounts of CO₂, especially as temperatures rise [20].

CH₄ emissions are a prominent feature of anaerobic wetland soils, where methanogenic archaea decompose organic matter [31]. Emission rates are highly variable, influenced by water table depth, temperature, substrate availability, and vegetation composition [31]. Drier, aerobic microsites within these landscapes, however, can act as CH₄ sinks.

N₂O emissions from pristine high-latitude wetland forests are generally low due to low nitrogen availability and often acidic conditions that can inhibit nitrification and denitrification [32]. However, disturbances such as permafrost thaw or changes in drainage can increase nitrogen availability and potentially stimulate N₂O production.

3.3. Temperature Sensitivity (Q₁₀) of Greenhouse Gas Emissions

The temperature sensitivity of soil microbial processes, often expressed by the Q₁₀ value (the factor by which the rate increases for a 10 °C rise in temperature), is a critical parameter for predicting the response of greenhouse gas emissions to warming (Figure 2).

3.3.1. Q₁₀ of CO₂ Emissions

Soil CO₂ emissions (soil respiration) are generally highly sensitive to temperature. Q₁₀ values for soil respiration in high-latitude ecosystems typically range from 2 to 4, though values can be higher, especially in lower temperature ranges or for the decomposition of more recalcitrant carbon pools [20,23]. This implies that even modest warming can lead to a substantial increase in CO₂ release from these carbon-rich soils, potentially creating a positive feedback loop to climate change.

3.3.2. Q₁₀ of CH₄ Emissions

CH₄ production by methanogens is also strongly temperature-dependent. Q₁₀ values for CH₄ emissions from wetlands are often high, frequently exceeding 3 and sometimes reaching values greater than 10, particularly in colder environments [31]. This high temperature sensitivity suggests that warming could significantly accelerate CH₄ release from high-latitude wetland forests.

Conversely, CH₄ oxidation by methanotrophs is also temperature-sensitive, typically with Q₁₀ values in the range of 1.5–3 [19]. The net effect of warming on CH₄ flux will depend on the relative temperature sensitivities of production and consumption processes as well as changes in other controlling factors like water table depth.

3.3.3. Q₁₀ of N₂O Emissions

The temperature sensitivity of N₂O emissions is more complex due to the involvement of multiple microbial processes (nitrification and denitrification) and their varying responses to temperature and other environmental factors. Q₁₀ values for N₂O emissions from soils have been reported to vary widely, generally ranging from 2 to 5, but can be much higher during specific events like freeze–thaw cycles [32,84]. In high-latitude systems where nitrogen is often limited, the response of N₂O to temperature may also be constrained by substrate availability.

3.4. Factors Influencing Temperature Sensitivity

Several factors can influence the apparent Q₁₀ of greenhouse gas emissions in high-latitude wetland forests. Substrate availability is a key factor, as the quality and quantity of organic matter can affect the Q₁₀ value; for example, the decomposition of labile carbon may show lower Q₁₀ values compared with more recalcitrant carbon, which might require a higher activation energy and thus exhibit a higher Q₁₀ value [23]. Soil moisture and water table depth interact strongly with temperature, as changes in water table depth can shift soils between aerobic and anaerobic states, fundamentally altering microbial processes and their temperature responses; for instance, drying of wetlands can decrease CH₄ production Q₁₀ but increase CO₂ production Q₁₀ [30]. The composition and adaptation of microbial communities also play a role, as different microbial groups involved in GHG production and consumption have distinct optimal temperatures and temperature sensitivities, and long-term warming may lead to adaptation or shifts in microbial community structure, potentially altering the overall Q₁₀ of the ecosystem over time [46,80]. Nutrient availability, particularly nitrogen and phosphorus, can constrain microbial activity and thus modulate the temperature response of GHG emissions; if nutrients are limited, the full potential temperature response may not be realized [99]. Finally, in regions with permafrost, thaw depth and active layer dynamics are critically linked to temperature, and permafrost thaw can expose previously frozen organic carbon to microbial decomposition, leading to a strong initial pulse of GHG emissions whose apparent temperature sensitivity might be very high due to the sudden increase in substrate availability [100].

3.5. Implications of Climate Warming

Climate warming is projected to be most pronounced in high-latitude regions. The high temperature sensitivity of greenhouse gas emissions from wetland forest soils in these areas suggests a high potential for positive feedback to climate change.

Increased CO₂ and CH₄ emissions due to warming can accelerate the accumulation of greenhouse gases in the atmosphere, further driving warming [8]. Permafrost thaw, in particular, poses a significant risk, as it could release vast amounts of ancient carbon that has been locked away for millennia [73]. While N₂O emissions are currently low in many of these systems, warming-induced changes in nutrient cycling, vegetation, and hydrology (e.g., increased nitrogen mineralization, altered drainage) could potentially increase N₂O releases in the future. Although direct field evidence from warming experiments in these specific ecosystems is still limited, process-based models consistently project an increase in N₂O emissions as rising temperatures accelerate nitrogen cycling, thereby alleviating the current N limitation on nitrification and denitrification [44].

Understanding the temperature sensitivity of these processes and the factors that control them is crucial for developing accurate climate models and for predicting the future trajectory of high-latitude ecosystems under a changing climate [96].

4. Discussion and Future Directions

4.1. Synthesis of Key Findings

This review has synthesized our current understanding regarding the effects of temperature and associated factors on greenhouse gas (CO₂, CH₄, N₂O) emissions from forest soils. Several key themes emerge from this synthesis.

Firstly, forest soils are critical components of global biogeochemical cycles, acting as both sources and sinks for major greenhouse gases. Their response to temperature changes is complex and mediated by a multitude of interacting factors, including soil moisture, nutrient availability (especially nitrogen), vegetation characteristics, and microbial community dynamics [1,15,42].

Secondly, anthropogenic disturbances such as forest management practices (e.g., thinning, harvesting), fire, and atmospheric nitrogen deposition significantly alter the baseline GHG fluxes and their sensitivity to temperature [46,61,73]. Post-fire recovery pathways and restoration methods have distinct impacts on the temporal dynamics of GHG emissions, with implications for the long-term carbon balance, as detailed in

Table 3. Effects of fire severity on forest soil greenhouse gas emissions.

Fire Severity	CO2 Emissions	CH4 Oxidation	N2O Emissions	Recovery Time	Key Mechanisms
Low (surface fire, <50% canopy mortality)	Initial decrease (10%–20%) followed by a return to pre-fire levels within 1–2 years	Reduced by 20%–40%, recovery within 3–5 years	Brief pulse (1–3 months) following first rainfall events	2–5 years	Key mechanisms for low fire severity include partial consumption of the litter layer, limited heating of the soil, and a relatively rapid recovery of vegetation.
Moderate (mixed severity, 50%–80% canopy mortality)	Initial decrease (20%–40%) followed by an increase of 10%–30% above pre-fire levels for 2–3 years	Reduced by 40%–70%, recovery within 5–10 years	Elevated for 1–3 years, particularly following precipitation	5–10 years	Under moderate fire severity, significant combustion of organic matter occurs, accompanied by moderate alterations to the soil structure and an increase in the availability of mineral nitrogen.
High (stand-replacing, >80% canopy mortality)	Initial decrease (40%–60%) followed by variable recovery depending on vegetation establishment	Reduced by 70%–95%, recovery may take 10–20 years	Elevated for 3–5 years with high spatial variability	10–20+ years	High fire severity results in severe loss of organic matter, major alterations to the soil structure, the development of soil hydrophobicity, and a slow process of vegetation recovery.

and

Table 4. Comparison of the effect of different restoration methods on post-fire forest soil greenhouse gas emissions.

Restoration Method	CO2 Emissions	CH4 Oxidation	N2O Emissions	Carbon Sequestration Rate	Implementation Considerations
Natural Regeneration	Natural regeneration typically leads to lower initial CO2 emissions, with gradual stabilization occurring over 3–7 years.	CH4 oxidation recovers gradually at a rate of 10%–15% annually, though this process can be limited by the slow recovery of soil structure.	N2O emissions are often elevated for 2–3 years post-fire and are strongly linked to precipitation events.	The initial carbon sequestration rate is slow, around 0.3–0.8 Mg C ha−1 year−1, but this is followed by steady long-term accumulation.	Implementation considerations for natural regeneration include its low cost and the requirement for viable seed sources, alongside the unpredictability of species composition and a longer overall recovery time.
Active Reforestation	Active reforestation may cause higher initial CO2 emissions due to site preparation activities, but stabilization is faster, typically within 2–3 years.	Recovery of CH4 oxidation is variable and depends on the intensity of site preparation, with long-term rates being species-dependent.	N2O emissions can be elevated if fertilizers are used, but they tend to decline faster with rapid vegetation establishment.	This method yields a higher carbon sequestration rate (0.5–1.2 Mg C ha−1 year−1), and long-term carbon stocks are species-dependent.	Active reforestation involves higher implementation costs and requires seedling production and planting, but offers greater control over species composition and achieves faster canopy closure.
Salvage Logging	Salvage logging results in the highest initial CO2 emissions due to soil disturbance, and stabilization is slower, taking 3–5 years.	Recovery of CH4 oxidation is significantly delayed in trafficked areas, with spatial variability based on equipment impacts.	N2O emissions are variable, depending on soil disturbance and vegetation recovery, and are often elevated in skid trails and landings.	The initial carbon sequestration rate is the lowest (0.2–0.5 Mg C ha−1 year−1), and there is a reduction in long-term potential due to biomass removal.	While allowing for economic timber recovery, salvage logging leads to an increase in soil disturbances, a reduction in coarse woody debris, and an altered microclimate.
Soil Rehabilitation (mulching, amendments)	Initial CO2 emissions are variable depending on the type of amendment used, with potential priming effects from labile amendments.	Soil rehabilitation can enhance CH4 oxidation recovery through improved soil structure, and moisture regulation benefits methanotrophs.	N2O emissions can be reduced through careful C/N ratio management, and mulch can create a moisture barrier that reduces emission pulses.	An enhanced carbon sequestration rate (0.6–1.5 Mg C ha−1 year−1) can be achieved with organic amendments, leading to improved long-term stabilization.	This approach has moderate to high costs, faces scalability challenges, and requires consideration of material sourcing, but offers the potential for immediate erosion control.
Integrated Approaches	Integrated approaches can lead to optimized CO2 emissions through targeted interventions, with spatial variability based on the treatment mosaic.	CH4 oxidation recovery is enhanced through strategic soil protection and spatial targeting of interventions.	N2O emissions are reduced through strategic nitrogen management and spatial and temporal optimization.	Carbon sequestration rates are optimized through complementary methods, leading to enhanced resilience to future disturbances.	Integrated approaches require detailed planning, incur higher initial assessment costs, necessitate a capacity for adaptive management, and aim for optimized resource allocation.

. Forest thinning, depending on its intensity, can modulate soil temperature and moisture, thereby influencing CO₂ and CH₄ fluxes, as shown in

Table 5. Effects of different thinning intensities on forest soil greenhouse gas emissions.

Thinning Intensity	CO2 Emissions	CH4 Oxidation	Soil Temperature	Soil Moisture	Microbial Activity	Root Biomass
Light (20%–30%)	Slight decrease (10%–15%)	Slight increase (5%–10%)	Increase (1–2 °C)	Slight decrease (5%–10%)	Minimal change	Decrease (15%–25%)
Moderate (40%–50%)	Moderate decrease (25%–35%)	Maximum increase (15%–20%)	Moderate increase (2–3 °C)	Moderate decrease (15%–25%)	Increase in diversity	Decrease (30%–45%)
Heavy (60%–70%)	Strong decrease (40%–50%)	Maximum decrease (5%–10%)	Strong increase (3–5 °C)	Strong decrease (25%–40%)	Shift in community composition	Severe decrease (50%–70%)

. Nitrogen deposition generally stimulates N₂O emissions and can suppress CH₄ uptake, with soil fauna potentially mediating these effects through their influence on decomposition and nutrient cycling [55,97].

Thirdly, the temperature sensitivity (Q₁₀) of GHG-producing and GHG-consuming processes is a crucial determinant of how forest soils will respond to climate warming. High-latitude wetland forests, with their vast carbon stores and often high Q₁₀ values for CO₂ and CH₄ emissions (refer to

Table 6. Temperature sensitivity (Q₁₀ values) of soil greenhouse gas emissions in different forest ecosystems.

Forest Ecosystem Type	Dominant Climate Zone	Soil Type	CO2 Emission Q10	CH4 Emission Q10	CH4 Uptake Q10	N2O Emission Q10
Boreal Coniferous Forest	Boreal	Podzol, Histosol	2.0–3.5	2.5–5.0 (source)	1.5–2.5	2.0–4.0
Temperate Broadleaf Forest	Temperate	Alfisol, Inceptisol	1.8–3.0	1.5–3.0 (source)	1.8–3.0	2.5–4.5
Temperate Coniferous Forest	Temperate	Spodosol, Andosol	2.2–3.8	N/A (often sink)	2.0–3.5	2.2–3.8
Tropical Rainforest	Tropical	Oxisol, Ultisol	1.5–2.5	2.0–4.0 (source)	1.5–2.8	1.8–3.5
Montane Forest (High Alt.)	Alpine/Montane	Cambisol, Leptosol	2.5–4.5	Variable	1.2–2.2	3.0–5.0
Forested Wetland (Peatland)	Boreal/Temperate	Histosol	2.0–4.0	3.0–10.0+ (source)	1.0–2.0	2.0–3.5

Note: Q₁₀ values can vary significantly based on the specific site conditions, methodology, and temperature range. Key references for each ecosystem type are as follows: Boreal Coniferous Forest, [19,94]; Temperate Broadleaf Forest, [43,95]; Temperate Coniferous Forest, [101]; Tropical Rainforest, [36,102]; Montane Forest (High Alt.), [103]; Forested Wetland (Peatland), [31,98]. N/A = Not Applicable or typically not a significant source.

for examples), are particularly vulnerable and could provide significant positive feedback to climate change [19,24,98]. However, Q₁₀ itself is not a constant and can be influenced by substrate availability, moisture, microbial adaptation, and nutrient status [23,30,46].

4.2. Knowledge Gaps

Despite considerable research, several knowledge gaps hinder our ability to accurately predict and manage forest soil GHG emissions under changing temperature regimes. The combined effects of multiple global change drivers, such as warming, altered precipitation, elevated CO₂, and nitrogen deposition, on GHG fluxes are poorly understood, as most studies focus on single factors, yet interactions are likely to be non-additive and could lead to unexpected outcomes [40]. Many studies are short-term, making it difficult to extrapolate findings to decadal or centennial timescales. The capacity of soil microbial communities to adapt or acclimatize to sustained warming and other pressures, and how this affects long-term Q₁₀ values and GHG emissions, remains a major uncertainty [45,49]. The influence of soil fauna, beyond a few well-studied groups, on GHG emissions and their interaction with temperature and nitrogen deposition is still largely a “black box”, and a deeper understanding of these interactions is crucial for a holistic view of ecosystem responses [97]. Furthermore, most studies focus on topsoil (0–30 cm), yet deep soil layers (>30 cm) contain substantial carbon stocks and may respond differently to warming, meaning the contribution of deep soil processes to the overall GHG balance is poorly quantified [38]. CH₄ and N₂O fluxes often exhibit high spatial and temporal variability, creating “hotspots” and “hot moments” that make accurate ecosystem-scale estimations challenging, and the mechanisms driving this variability and how to effectively incorporate it into models are not fully understood [31,37]. Finally, current biogeochemical models still have considerable uncertainties in simulating forest soil GHG emissions, particularly in predicting responses to future climate scenarios, making the improvement of model parameterization, validation, and representation of key processes essential [96].

4.3. Future Research Directions

To address these knowledge gaps and improve our predictive capabilities, future research should focus on several key areas. It is vital to establish and maintain integrated long-term experiments that manipulate temperature, precipitation, CO₂ levels, and nitrogen inputs simultaneously to understand interactive effects and long-term ecosystem responses [45]. The employment of stable isotope techniques (e.g., ¹³C, ¹⁵N, ¹⁸O labeling) and advanced molecular tools (e.g., metagenomics, metatranscriptomics, metaproteomics) can help trace element flows, identify key microbial players and pathways, and understand their regulation under changing conditions [43,93]. Studies should be designed to explicitly investigate the role of diverse soil faunal groups in mediating GHG fluxes, particularly their interactions with microbial communities, nutrient cycling, and responses to warming and nitrogen deposition [97,104]. GHG flux measurements and process studies should be extended to deeper soil horizons to better quantify their contribution to ecosystem-scale budgets and their sensitivity to warming [38]. Novel sensor networks and remote sensing technologies should be utilized for high-frequency, spatially explicit monitoring of GHG fluxes and their drivers, alongside the development of robust upscaling methods to integrate point measurements to landscape and regional scales [21,47]. Biogeochemical models need improvement by incorporating more mechanistic representations of microbial processes, faunal interactions, deep soil dynamics, and the effects of multiple interacting global change factors, with rigorous model validation against diverse datasets being crucial [21,96]. Lastly, there should be a focus on evaluating the effectiveness of different forest management practices (e.g., species selection, rotation lengths, residue management, biochar application) in mitigating GHG emissions and enhancing carbon sequestration under future climate scenarios and developing adaptive management strategies that consider the dynamic nature of forest soil responses [4,61].

4.4. Conclusions

Temperature is a primary driver of forest soil greenhouse gas emissions, but its effects are intricately modulated by a host of environmental factors and anthropogenic influences. Forest ecosystems, particularly those in high-latitude regions and wetlands, are highly sensitive to warming, with the potential for significant positive feedback to the climate system through increased CO₂ and CH₄ emissions. For context, global syntheses have estimated that a 1 °C rise in temperature could release tens of petagrams of carbon from soils, with high-latitude regions being disproportionately sensitive. Nitrogen deposition further complicates these dynamics, generally increasing N₂O emissions and reducing CH₄ uptake.

While significant progress has been made in understanding these processes, substantial knowledge gaps remain, particularly concerning long-term responses, interactive effects of multiple stressors, the role of soil biodiversity (especially fauna), and deep soil contributions. Addressing these gaps through integrated, long-term research employing advanced techniques and robust modeling is essential for accurately predicting the future of forest soil GHG fluxes and for developing effective strategies to mitigate climate change while ensuring the sustainable management of forest ecosystems.