Biotic and abiotic factors on production and emission of greenhouse gases from terrestrial ecosystems and aquatic ecosystems

 

1.   Greenhouse gas emissions from forest soils

Carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) are the most important greenhouse gas (GHG) emitted from agricultural and forest soils, contributing 60, 15 and 5%, respectively, towards enhanced global warming. The radiative forcing of GHGs has led to an increase in the average global surface temperature of 0.6°C since the late 19th century. Consequently, changes in the amount, distribution and intensity of rainfall/precipitation are also expected to occur. CH₄ and N₂O have a large global warming potential (GWP) that is respectively 25 and 298 times greater than CO₂ over a 100 yr period. Since pre-industrial times, increasing emissions of GHGs due to human activities have led to a marked increase in atmospheric GHG concentrations (IPCC, 2013). Between 1970 and 2010, total GHG emissions increased by 8 GtCO₂eq over the 1970s, 6 GtCO₂eq over the 1980s, and by 2 GtCO2 over the 1990s, with an annual growth rate over these decadal periods of 2.0%, 1.4%, 0.6%, and 2.2%, respectively (IPCC, 2013). GWP weighted territorial GHG emissions increased from 27 to 49 GtCO2eq, an 80% increase in forty years. The emissions of these gases have increased at different rates. Between 1970-2010, global anthropogenic fossil CO₂ emissions more than doubled, and represented 75% of total anthropogenic GHG emissions in 2010, while CH₄ and N₂O each increased by about 45%.

Currently, Agriculture, Forestry, and Other Land Use (AFOLU) accounts for approximately a quarter of anthropogenic GHG emissions, largely deriving from deforestation and livestock, soil and nutrient management (IPCC, 2014). CO₂ emissions from deforestation and forest degradation have been estimated to account for about 12-20% of global anthropogenic CO₂ emissions (IPCC, 2007). These estimates can be improved as the magnitude of gas flux from the agricultural and forest sectors still has large knowledge gaps. In particular, estimates of N₂O and CH₄ emissions from forest ecosystems are far from to be exhaustive.

The decision no 529/2013/eu of the European Parliament and of the council of 21 may 2013 stated that “Member States shall prepare and maintain accounts that accurately reflect all emissions and removals resulting from the activities on their territory falling within the following categories: a) afforestation, b) reforestation, c) deforestation, d) forest management”, “... covering emissions and removals of the following greenhouse gases: a) CO2, b) CH₄, c) N2O”. Thus, accounting of emissions and removals of CO₂, CH₄ and N₂O is fundamental in order to meet EU targets. Although deforestation is the main source, forest degradation contribute to atmospheric GHG emissions through decomposition of remaining plant material and soil carbon (C). These larger emission are no more balanced by the C storage capacity in woody biomass and soil, due to unstable structural conditions of the degraded stands. Deforestation and forest degradation are important contributors to global GHG emissions, but if these processes are controlled, forests can significantly contribute to climate change mitigation. Forest degradation, implying a decrease in canopy cover and regeneration, as well as forest fragmentation, will affect the annual increment of C sequestration, reducing the potential of these forests to act as a sink or transforming them into a source of GHGs. CO2 emissions from deforestation and forest degradation have been estimated to account for about 12-20% of global anthropogenic CO₂ emissions (IPCC, 2007).

The potential of a broad range of forest-related activities (including protection from natural disturbance, improved silviculture, savannah thickening, restoration of degraded lands, and management of forest products) at 0.6 GtC/yr over six regions in the temperate and boreal zone (Canada, USA, Australia, Iceland, Japan, and EU).

2. Processes involved and main drivers

Processes involved in GHG production and emission are complex and different, depending on the gas considered. Main processes include autotrophic and heterotrophic respiration, metahnogenesis, CH₄ oxidation, nitrification and denitrification.

CO₂

CO₂ emissions from soils are greater than all other terrestrial-atmospheric C exchanges, with the exception of gross photosynthesis. An equivalent of almost 10% of CO₂ contained in the atmosphere passes through soils each year, which is more than 10 times the amount of CO₂ released by fossil fuel combustion. In European forests, about 55% of photosynthetically fixed C finds its way back into the atmosphere via belowground respiration. Due to the magnitude of this flux and the large stock of C present in soils, any change in soil C emissions in response to environmental changes could constitute a significant feedback on CO2 concentration in the atmosphere.

Mechanisms responsible for CO₂ production are the result of two distinct processes: i) breakdown of root-derived C (root and rhizosphere respiration) and ii) decomposition of soil-derived C (heterotrophic respiration of SOM). The rhizosphere respiration includes belowground autotrophic respiration and heterotrophic respiration of C substrates originating from newly assimilated C, e.g. root exudates and recent dead root biomass. In terrestrial ecosystems, about 35-80% of C fixed through photosynthesis is transferred belowground to fuel root activity, mycorrhizal networks and root exudates. Root productivity and photosynthetic activity are thus the main factors controlling below-ground C allocation, and therefore the CO₂ efflux from soils. Thus, soil respiration (SR) results from activity of a multi-organism network of oxidation pathways, where individual root/rhizosphere and heterotrophic components may respond to environmental constraints in contrasting ways. Indeed, one of the main problems with predicting soil respiration is that it is influenced by a multitude of interacting factors including soil temperature, moisture, soil C or litter quality, root density, microbial community structure and size, physical and chemical soil properties and vegetation type, nutrient status and growth rate. Consequently, in most ecosystems the rate of soil respiration is highly temporally and spatially variable.

2.2 N₂O

Reduction of N₂O emissions from terrestrial ecosystems is particularly challenging due to the number and complexity of N₂O production processes occurring in soil. Main processes include: i) chemodenitrification; ii) nitrification; iii) denitrification; iv) nitrifier denitrification, v) nitrate ammonification. All these mechanisms are responsible for N₂O emissions and can occur simultaneously in soil in different micro-niches. Nitrification is a microbial oxidative process that lead to the release of nitrate, via nitrite, starting from reduced forms of N, typically ammonia with a two steps reaction: 1) NH₄ ⁺ + 3/2O₂ → NO₂⁻ + 2H+ + H₂O + E

2) NO₂⁻ + 1/2O₂ → NO₃⁻ + E

The first, limiting step of nitrification is the ammonia oxidation carried out by a relatively restricted number of autotrophic chemilithotrophic bacteria. In aerobic systems, nitrification is one of the main mechanism responsible for N2O production, which is favored by high soil NH₄ + concentrations, high soil temperature and water filled pore space lower than 60%.

Several microbial processes compete for NO₃⁻ released in soil: denitrification, dissimilatory NO₃⁻ reduction to NH₄ + , and anaerobic NH₄ + oxidation. Denitrification is a respiratory process in which NO₃⁻ is reduced stepwise to dinitrogen (N2) via nitrite, nitric oxide and nitrous oxide intermediates. In bacteria, this process typically occur under low O₂ or under anoxic conditions, with water filled pore space higher than 60%. In forest ecosystems the loss of NO₃ - from root zone represents the loss of an important plant nutrient while the incomplete soil denitrification can lead to release of N₂O to the environment. NO₃ - reductions are catalysed stepwise by four different reductase encoded by several genes nitrate reductase (narG, napA), nitrite reductase (nirS, nirK), nitric oxide reductase (cnorB, qnorB) and nitrous oxide reductase (nosZ). The composition of the nitrifying and denitrifying communities in soil and their functional diversity may be crucial in regulating N₂O emissions to the atmosphere.

2.3 CH₄

CH₄ production is the microbial end product of the anaerobic mineralization of soil organic matter (SOM) degradation performed by microorganisms of Archaea domain in anoxic environments, including submerged soils. The two main types of methanogenic pathways are acetate- and H₂/CO₂-dependent methanogenesis:

a) CO₂ + 4H₂ → CH₄ + 2H₂O

b) 4HCOOH → CH₄ + 3CO₂ + 2H₂O CH₄ production is suppressed when other alternative electron acceptors (O₂, NO₃ - , Fe(III), and SO₄ 2-) are present and typically occur at redox potential lower than 200 mV. CH₄ is emitted in atmosphere through three main mechanisms: transport through plants aerenchyma, diffusion and ebullition. Aerenchyma transport is responsible for most of CH₄ emitted from terrestrial ecosystems and act as a pipe for the CH4 present in groundwater in the presence of deep roots. Thus, wetland soils (swamps, bogs, etc.) and rainforests are the main natural source of CH4 with an estimated emission of 100-200 Tg year^-l. Inverse process is CH₄ oxidation, which is performed by aerobic methanotrophic microorganisms. Forest in oxic and upland soils are efficient CH₄ sinks and are estimated to consume about 10% of atmospheric CH4 (IPCC, 2007). Soil water content is therefore the main driver of CH₄ production/consumption, influencing the presence of alternative electron acceptors and redox potential. Soil physical properties (such as texture, aggregation status, diffusivity...) and soil organic matter strongly affect CH4 production and emissions, by altering O₂ and substrate availability.

3. Mitigation potential

Forest-based strategies offer a cost-effective means to mitigate climate change, so appropriate forest management can help both to reduce emissions from deforestation and forest degradation and to increase C removals. With the 20-20- 20 targets, the EU has set itself the objective of reducing emissions by 20% until 2020 (European Commission, 2012). The main mitigation options within AFOLU (Agriculture, Forest and Other Land Use) involve one or more of these three strategies: i) reduction/prevention of emissions to the atmosphere by conserving existing C pools in soils or vegetation that would otherwise be lost or by reducing emissions of CH₄ and N₂O; ii) sequestration – enhancing the uptake of C in terrestrial reservoirs, and thereby removing CO₂ from the atmosphere; and iii) reducing CO₂ emissions by substitution of biological products for fossil fuels or energy-intensive products. This work will focus on the three strategies, giving an overview of management options able to mitigate GHG emissions from soil.

The capacity of ecosystems to store C depends on the balance between net primary productivity (NPP) and heterotrophic respiration. Whether a particular ecosystem is functioning as sink or source of GHG emission may change over time, depending on its vulnerability to climate change and other stressors and disturbances.

Forest ecosystems generally represent a net sink for CO₂ and have the potential to offset from 2% to 30% of expected emissions during this century, as confirmed by inventory measurements in both managed and unmanaged forests in temperate and tropical regions. It has been argued that conservation of forests by using good selvicultural practice and through tree planting can enhance strongly the C sink provided by terrestrial ecosystems.

Although data on C sequestration potential are widely accessible in most part of the world, less is known about the potential for GHG emission reduction with proper management strategies. Moreover, even if the importance of CH₄ and N₂O emissions is recognized, scientific research has largely focused on CO₂. Reducing GHG emissions and GWP is a fundamental aspect of climate change mitigation strategies and strongly depends on the adopted management options. N₂O emissions reduction is particularly challenging due to the complexity of processes involved and their interactions, thus the result may be achieved only if the different aspects of processes involved are considered.

3.1 Afforestation/reforestation intervention

Conversion of degraded soil from agricultural to forest use can accrue the pool of C stored into soil, with a positive balance between GHG emissions and C accumulation. A decrease of CO₂ emissions is the result of lower C mineralization rates due to minor or absent soil disturbance, which increase physical protection of C. This reduction is maximum in case of conversion from agricultural to natural forests, while can be partial in case of plantations. Contrarily, conversions from pasture to forests can bring to net losses of C, mainly because of lower turnover rates of soil organic matter, in particular in case of pine afforestation.

Less studies focused on changes of CH₄ and N₂O emissions after afforestation/reforestation intervention. Available results seems to indicate a tendency towards lower N₂O emissions because of lower N input from fertilization or animal dejections. Potential reduction of CH₄ emissions mainly depends on water conditions of soil before intervention. Main benefits have been found in peats ecosystems following drainage and water uptake by plants. Impact of stand ages on GHG emissions have been less studied but first results seems indicate a trend toward lower CH₄ and higher N₂O emissions with the forest age.

3.2 Forest degradation

The United Nations Framework on Climate Change (UNFCCC), at its thirteenth meeting in 2005 (COP11), agreed to start a work program to explore a range of policy approaches and positive incentives for Reducing Emissions from Deforestation and Degradation (REDD). This process was further encouraged in the 2007 COP-13 with the explicit consideration of REDD activities as a means to enhance mitigation action by developing countries in the future. As widely used by forest scientists, forest degradation implies a long-term loss of productivity, which thereby lower the capacity to supply products and/or services, including C storage capacity in vegetation and soil, changes in tree vigor and quality, species composition, soils, water, nutrients and the landscape.

Forest-based strategies offer a cost-effective means to mitigate climate change, so appropriate forest management can help both to reduce emissions from deforestation and forest degradation and to increase C removals. Increasing the C pool in vegetation and soil can be accomplished by protecting secondary forests and other degraded forests whose biomass and soil C densities are less than their maximum value and allowing them to sequester C by natural or artificial regeneration and soil enrichment. In this context, the conversion of degraded forest pine plantations to facilitate the introduction of late successional native broadleaves species means to help restoring natural functioning processes (e.g. natural regeneration, or more generally, self-organization), increasing their stability, resilience and self-perpetuating capacity, besides their capacity to mitigate GHg emissions and increase C storage.

3.3 Silvicultural practices

Management of forest ecosystems for climate change mitigation may include several strategies: i.e. fire protection, pest control, less intensive harvest, increasing the length of time to rotation (harvest), limitation of soil compaction, regulation of tree densities, selection of species, biodiversity conservation, residues management following felling. However, these strategies have the strongest and clear effect on C accumulation in forest biomass, while less is known on the impact on GHG emissions. Forest management, such as felling and thinning could potentially change N₂O emission rates by altering the soil water content owing to the absence of trees (felling) or reduction of shading (thinning).

The few studies that investigated effects of clear-felling on GHG fluxes revealed that clear-felling resulted in a pulse of N₂O, NO and CO₂ emissions. Clear-felling has been found to profoundly alter several pedo-climatic properties, which in turn may affect GHG emissions: soil temperature, soil water content, groundwater depth, soil bulk density and compaction. Soil compaction can bring a considerable increase of N₂O e CH₄ emissions because of macropores volume reduction and water saturation, with a tendency towards anaerobic conditions. An alteration of substrate availability is expected after clear-felling, either in terms of decomposable C or N. Above and belowground litter and forest residues are made available for microbial decomposition, thus increasing CO₂ emissions. Moreover, N₂O emissions can be affected by clearfelling through modification in N availability: in fact, in the absence of plant uptake, the excess of N can trigger nitrification and thus N₂O emissions, favored also by the higher temperatures.

An increase of denitrification and methanogenesis has been found after clear-felling in high moisture environments. Thinning operation may affect GHG emission indirectly by altering soil temperature and moisture conditions. However, residues management can be extremely important in order to provide or remove organic matter available for decomposition, thus a proper strategy should be adopted depending on pedo-climatic condition of the site.

3.4 Fast growing plantations

Fast growing plantations or short rotation forests respond to the objective of substituting biological products for fossil fuels or energy-intensive products, thereby reducing CO₂ emissions. Their role is becoming more and more important in climate change mitigation strategies, but still their management should be accurately planned to reduce GHG emissions from soil. In particular, irrigation, fertilization and tillage are common practices in plantations, which may strongly affect GHG emissions. The influence of excess N or water has been already discussed: inappropriate or excessive fertilization can provoke peaks of N₂O, while an excess of water can induce methanogenesis.

Tillage operations before the implant or during tree growth significantly affect GHG emissions, directly by favoring diffusion rates into soil and providing substrates for decomposition through aggregates breaking and indirectly altering temperature and mositure conditions for microbial processes.

An inventory was made to quantify the emission of the waterborne greenhouse gases CH₄, N₂O and CO₂ from fresh surface waters and wetlands in the Netherlands. The aim of the study was to give an overview of the existing fluxes and balances and to quantify the role of aquatic and semi-aquatic habitats in the total emissions of these gases in the Netherlands. The CO₂ uptake of surface waters and wetlands turned out to be very low. Only 0.6% of the total annual CO₂ emission from the Netherlands is absorbed by the ecosystems in these habitats. However, they are significant sources for N₂O and CH₄; respectively, 18% and 19% (including sewage treatment plants) of the total annual emission in the Netherlands. It is indicated that especially in the case of N₂O and CH₄ the reduction of the eutrophication will also reduce the output of these gases from the surface waters and the wetlands. However, attention should be paid to the operation of sewage treatment plants because reducingthe discharge of nitrogen by extending the denitrification capacity could easily lead to more nitrous oxide release from these plants.

Atmospheric concentrations of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) are predicted to increase as a consequence of fossil fuel emissions and the impact on biosphere–atmosphere interactions. Forest ecosystems in general, and forest soils in particular, can be sinks or sources for CO₂, CH₄, and N₂O. Environmental studies traditionally target soil temperature and moisture as the main predictors of soil greenhouse gas (GHG) flux from different ecosystems; however, these emissions are primarily biologically driven.

Measurement of  net CO₂, CH₄ and N₂O fluxes after 5 years of experimental warming (+3.4°C), and 2 years of ≈45% summer rainfall reduction, in two forest sites in a boreal–temperate ecotone under different habitat conditions (closed or open canopy) in Minnesota, USA.

Climo-edaphic factors (soil texture, canopy, seasonality, climate, and soil physicochemical properties) driving GHG emissions.

Changes in CO₂ fluxes were predominantly determined abiotically by temperature and moisture, after accounting for bacterial abundance. Methane fluxes on the other hand, were determined both abiotically, by gas diffusivity (via soil texture) and microbially, by methanotroph pmoA gene abundance, whereas, N₂O emissions showed only a strong biotic regulation via ammonia-oxidizing bacteria amoA gene abundance. Warming did not significantly alter CO₂ and CH₄ fluxes after 5 years of manipulation, while N₂O emissions were greater with warming under open canopy.

Soil GHG emissions result from multiple direct and indirect interactions of microbial and abiotic drivers

Soil greenhouse gas (GHG) fluxes are the result of biological processes leading to their production and/or consumption in terrestrial ecosystems. However, the majority of field-based studies focus on the importance of abiotic (soil physicochemical properties), rather than biotic (microbial communities) factors in driving carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes. These studies have attributed flux responses to effects of soil oxygen levels, water content, pH, temperature and substrate availability on microbial community activity without a direct measure of how such abiotic determinants change microbial communities in ways that might alter GHG fluxes. At the soil microsite level, CO₂, CH₄ and N₂O fluxes are primarily driven by microbial pathways, controlled at the gene and cellular level; however, soil abiotic properties are capable of indirectly affecting flux rates into the atmosphere by regulating microbial abundance and activity, but also simultaneously, by affecting gas diffusion rates into the soil profile or to the atmosphere.

 At the landscape level, soil physicochemical properties are strongly affected by soil texture, climatic conditions, vegetation type and land-use. Therefore, lack of empirical evidence of the degree of regulation by biotic or abiotic factors from smaller to larger environmental scales still exists and needs to be clarified. This current limitation of our understanding on the role of soil microbes in controlling soil functioning directly impacts the prediction of the direction, magnitude and duration of GHG emissions. Forests ecosystems are particularly important because they consume on average more CH4 than all other terrestrial ecosystems and are major contributors to carbon (C) storage in soil and aboveground vegetation. In the case of CO₂, heterotrophic respiration (a broad microbial function) in forest soils is a major contributor to CO₂ efflux from these ecosystems, together with autotrophic root respiration. In contrast to CO₂ production, specific microbial groups are responsible for CH₄ and N₂O production and consumption and thus these fluxes are considered specialized ecosystem processes.

Anaerobic methanogenic archaea carry out CH₄ production, whereas aerobic methanotrophic bacteria are responsible for CH₄ consumption. The oxidation of atmospheric CH₄ by aerobic soils serves as a significant global CH₄ sink in terrestrial ecosystems. In the case of N₂O, multiple specialized microbial groups are responsible for N₂O production, namely (1) aerobic ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) through nitrification-mediated pathways (ammonia oxidation and/or nitrifier denitrification); or (2) denitrifying microorganisms through the multistep process of heterotrophic denitrification. To date, this multistep reaction is also the only one known to be responsible for the sink of N₂O in the soil, carried out by specialized N₂O-reducing bacteria. In fact, there is evidence that 30%–80% of the N₂O produced from deeper soil layers may be reduced to N₂ before diffusion into the atmosphere. By the late 21st century, global mean annual temperatures are predicted to increase between 1.2 and 4.8°C, with more uncertainty associated with how intensity and frequency of precipitation patterns will change (IPCC, 2013). However, despite recent advances, only a few manipulative long-term field studies in forest ecosystems have directly assessed the combined effects of warming and reduced summer rainfall on CO₂, CH₄ and N₂O emissions. Climate change may potentially alter the relative importance of biotic and abiotic factors in driving GHG emissions (e.g. shifting microbial abundance), however little is known about the impacts of climate change on GHG emissions via abiotic and biotic factors.

Modelling studies suggest the interaction between warming and soil moisture are a significant determinant of ecosystem responses to the ongoing changing climate due to a regulation of biological responses. Studies including the combined effects of warming and changing rainfall patterns are therefore imperative because they are expected to occur simultaneously and thus lead to different effects on soil biotic and abiotic properties in comparison to their individual effects. In this study, we investigated the long-term impact of warming (+3.4°C) and reduced summer rainfall manipulation (≈45% exclusion) on soil CO₂, CH₄ and N₂O emissions in a boreal-temperate ecotone, the Boreal Forest Warming at an Ecotone in Danger (B4WarmED), 5 years after the beginning of the experiment. We aimed to determine (1) whether these biogenic GHG fluxes were primarily explained by changes in soil physicochemical characteristics (abiotic) or by changes in microbial community abundances (biotic), regardless of whether these differences arose because of climate change treatments (i.e. experimental warming and rainfall manipulation), variation between sites (reflected in different soil texture), habitat (presence or absence of canopy) or seasonality (difference in ambient climate between monthly measurements); and (2) the long-term effects of climate treatments on CO₂, CH₄ and N₂O emissions responses. In addressing these aims we hypothesized first, that abiotic factors, such as soil temperature and/ or moisture, would be the main drivers of CO₂, CH₄ and N₂O fluxes, by indirectly affecting microbial gene abundance and/or gas diffusion, irrespective of site, habitat, seasonal variation, warming and rainfall manipulation. Second, we hypothesized that warming and reduced rainfall would individually increase CO₂ and N₂O emissions and CH₄ uptake by favouring aerobic conditions, whereas the two climate treatments combined would have an offsetting effect on GHG feedback responses.

As two important greenhouse gases (GHGs) contributing to climate warming, methane (CH₄) and nitrous oxide (N₂O) are receiving more and more attention. Methane, with a relative global warming potential 25 times that of CO₂ at a 100‐yr time horizon, has increased by more than 100% since 1800 and contributes approximately 20% to the global radiative forcing. The dominant sources of CH₄ are natural wetlands, anthropogenic activities, and biomass burning. while upland soils are the major CH₄ sink. Nitrous oxide has a relative global warming potential 298 times that of CO₂ at a 100‐yr time horizon, and contributes approximately 7% to the radiative forcing. Atmospheric N₂O has increased by 18% compared to the preindustrial level, with a linear increasing rate of 0.26% per year during the recent few decades. The observed increase in atmospheric N₂O concentration was primarily attributed to reactive nitrogen inputs from synthetic nitrogen fertilizer and animal manure applications, cropland expansion, and processes associated with fossil‐fuel combustion and biomass burning. The production and consumption of N₂O in soils involves both biotic and abiotic processes. Comparing to global CH₄ emission estimate, global N₂O emission remains largely uncertain, ranging from 6.7 to 36.6 Tg N/yr.

Many environmental factors influence the production and consumption of CH₄ and N₂O. global warming might increase net CH₄ uptake in upland ecosystems due to stimulation of microbial CH₄ oxidation and higher CH₄ diffusivity with lower soil moisture. Global warming results in increased N₂O emissions in most land ecosystems due to stimulations of nitrifiers and denitrifiers activity and nitrogen supply through mineralization; however, global warming may also reduce N₂O emissions through soil drying and stimulation of plant growth and nitrogen uptake. Elevated atmospheric CO₂ might either increase or decrease CH₄ uptake in the upland ecosystems, but increase CH₄ emissions in water‐saturated ecosystems through increased photosynthesis and carbon input to the soil, which stimulate the methanogenic bacteria growth. Under the condition of high nitrogen supply, elevated atmospheric CO₂ could significantly increase N₂O emission due to the increase of soil moisture and soil labile carbon. Under the condition of high nitrogen supply, elevated atmospheric CO₂ could significantly increase N₂O emission due to the increase of soil moisture and soil labile carbon while the increasing effect could be small or even negative in nitrogen‐limited ecosystems since increased plant growth may result in less nitrogen availability for nitrifiers and denitrifiers.

Tropospheric ozone (O₃) pollution could cause losses of photosynthesis, and thus reduce CH₄ emissions while either stimulate or reduce N₂O emissions by influencing the litter mass and quality. Nitrogen fertilization could dramatically decrease CH₄ consumption in grassland and forest and either decrease or increase CH₄ emissions in rice fields, depending on nitrogen fertilizer amount,  while increasing N₂O emissions.  In addition, interactions among multiple factors also play an important role. For example, nitrogen deposition and elevated atmospheric CO₂ were reported to interactively reduce CH₄ emission from wetland and increase CH₄ uptake in upland soil and another study concluded that temperature and elevated atmospheric CO₂ interactively changed seasonal variation of CH₄ emissions. Atmospheric CO₂and nitrogen deposition can interactively increase soil available nitrogen and labile carbon, thus greatly increase soil N₂O emissions.

The most rapid increase in CH₄ emission was found in natural wetlands and rice fields due to increased rice cultivation area and climate warming. N₂O emission increased substantially in all the biome types and the largest increase occurred in upland crops due to increasing air temperature and nitrogen fertilizer use. Clearly, the three major GHGs (CH₄, N₂O, and CO₂) should be simultaneously considered when evaluating if a policy is effective to mitigate climate change. 

African aquatic systems such as streams, rivers, wetlands, floodplains, reservoirs, lagoons, and lakes can be significant sources of GHG . Differences in regional setting and hydrology mean that emissions are highly spatially and temporally variable, and when combined with the paucity of studies, it is challenging to identify clear control factors (Table 3). Studies found SSA aquatic systems can be significant sources of GHG emissions. In Ivory Coast, three out of five lagoons were oversaturated in CO2 during all seasons and all were CO2 sources (3.1–16.2 g CO₂ m⁻² d⁻¹ ) due to net ecosystem heterotrophy and inputs of riverine CO₂-rich waters. In the flooded forest zone of the Congo River basin (Republic of Congo) and the Niger River floodplain (Mali), high CH4 emissions (5.16 × 1020– 6.35 × 1022 g CH₄ m⁻² d ⁻¹ ) were recorded on flooded soils. In the Nyong River (Cameroon), CO2 emissions (5.5 kg CO2 m⁻² yr⁻¹ ) were 4 times greater than the flux of dissolved inorganic carbon (Brunet et al., 2009). In the Zambezi River (Zambia), 38 % of the total C in the river is emitted into the atmosphere, mostly as CO₂ (98 %). The source of CH₄ to the atmosphere from Lake Kivu corresponded to ∼ 60 % of the terrestrial sink of atmospheric CH₄ over the lake’s catchment. A recent study of 10 river systems in SSA estimated water–air CO₂, CH₄, and N₂O fluxes to be 8.2 to 66.9 g CO2 m−2 d −1 , 0.008 to 0.46 g CH4 m⁻² d ⁻¹ , and 0.09 to 1.23 mg N₂O m⁻² d ⁻¹ , respectively. The authors suggested that lateral inputs of CO₂ from soils, groundwater, and wetlands were the largest contributors of the CO₂ emitted from the river systems. The magnitude of GHG emissions from SSA aquatic systems varied with type and location. Streams and rivers in savannah regions had higher CO₂ emissions (46.8–56.4 g CO₂ m⁻² d⁻¹ ) than swamps (13.7– 16.3 g CO₂ m⁻² d⁻¹ ) and tropical forest catchments (37.9–62.9 g CO2 m⁻² d ⁻¹ ) in the Congo Basin. The average CH₄ flux in river channels (0.75 g CH₄ m⁻² d ⁻¹ ) was higher than that in floodplains and lagoons (0.41–0.49 g CH₄ m⁻² d⁻¹ ) in the Okavango Delta. Methane emissions from river deltas were substantially higher (∼ 103 mg CH⁴ m⁻² d⁻¹ ) than those from non-river bays (< 100 mg CH4 m⁻² d ⁻¹ ) in Lake Kariba (Zambia/Zimbabwe). Methane fluxes were higher in river deltas (∼ 103 mg CH₄ m⁻² d ⁻¹ ) compared to non-river bays (< 100 mg CH₄ m⁻² d ⁻¹) in Lake Kariba (Zambia/Zimbabwe). While CO₂ and CH₄ concentrations in the main channel were highest downstream of the floodplains, N₂O concentrations were lowest downstream of the floodplains in the Zambezi River (Zambia and Mozambique). Greenhouse gas emissions from Dambos in Zimbabwe varied with catena position. Upland dambos were important sources of N₂O and CO₂, and a sink for CH₄, while those in a mid-slope position were a major source of CH₄ but a weak source of CO₂ and N₂O, and those at the bottom were a weak source of all GHGs. The concentration and flux of GHGs are strongly linked to hydrological characteristics such as discharge, but clear patterns have not yet been identified. Surface CO₂ flux was positively correlated with discharge in the Congo River, while in Ivory Coast, rivers were often oversaturated with CO₂ and the seasonal variability in partial pressure of CO₂ (pCO₂) was due to dilution during the flooding period. Similarly, CO₂ fluxes show a very pronounced seasonal pattern strongly linked to hydrological conditions in the Oubangui River in the Central African Republic. Although higher CH₄ concentrations were found during low-discharge conditions, N₂O concentrations were lowest during low-discharge conditions.

In Lake Kivu, seasonal variations of CH₄ in the main basin were driven by deepening of the mixolimnion and mixing of surface waters with deeper waters rich in CH₄. In the Zambezi River (Zambia and Mozambique), interannual variability was relatively large for CO₂ and CH₄ and significantly higher concentrations were measured during wet seasons. However, interannual variability in N₂O was less pronounced and generally higher values were found during the dry season.

In Kampala, Uganda, precipitation was a major driver for seasonal variation of CO₂, CH₄, and N2O fluxes in subsurface flow wetland buffer strips due to its potential influence on hydraulic saturation affecting oxygen fluctuation. Studies found the concentration and flux of GHGs are also strongly linked to environment and water quality but clear patterns have not yet been identified. In the Okavango Delta (Botswana), CH₄ emissions were highest during the warmer, summer rainy season and lowest during cooler winter season, suggesting the emissions were probably regulated by water temperature. They also found the lowest N₂O values were observed at the highest pCO2 and lowest % O2 levels, suggesting the removal of N2O by denitrification. In Lake Kivu (East Africa), the magnitude of CO₂ emissions to the atmosphere seems to depend mainly on inputs of dissolved inorganic carbon from deep geothermal springs rather than on the lake metabolism

Ecology of Greenhouse Gas Emissions from Coastal Wetlands

Overview

Wetlands have the potential to absorb large amounts of carbon dioxide via photosynthesis, and flooded soils have low oxygen levels which decrease rates of decomposition to promote the retention of soil carbon. However, the type of greenhouse gases emitted from wetlands varies by wetland type and soil condition. A suite of approaches are being used to assess  fluxes of greenhouses gases, like methane, carbon dioxide, and nitrous oxide. 

The Science Issue and Relevance: Wetlands provide a particularly good environment for carbon sequestration, which has elevated them as potential hotspots for ameliorating increased CO₂ and CH₄ concentrations in the atmosphere. Not only does healthy wetland vegetation have the potential to take up large amounts of CO₂ through photosynthesis, but also flooded soils have low oxygen levels which reduce rates of decomposition to promote retention of soil carbon. However, mass fluxes of carbon are only part of the story; while some wetlands can store large amounts of carbon, the biogeochemical state of the soil affects the balance among the types of greenhouse gases emitted. Some types of gases have greater radiative forcing and are more deleterious than others. For example, greater amounts of CH₄ are often emitted from freshwater wetlands, and CH₄ is much more persistent in the atmosphere than CO₂. Thus, along with understanding the net ecosystem exchange of carbon, it is also critical that the primary suite of greenhouse gases (CO₂, CH₄, N₂O) are considered together along environmental gradients. While many studies of greenhouse gas emissions are underway, our group is particularly interested in assessing fluxes accurately through combinations of various techniques and describing the nuances associated with each assessment.

Methodologies for Addressing the Issue: Greenhouse gases are emitted from the soil, belowground roots, and soil-emergent plant structures, while CO₂is taken up by photosynthetic tissue. The scale selected for greenhouse gas assessments matter tremendously. We use a suite of approaches to assess greenhouse gas fluxes, including dark static flux chambers, clear static flux chambers, infrared gas analyzers, laboratory-based and portable gas chromatography, and eddy covariance systems. We also measure greenhouse gas fluxes from a range of wetland types differing in natural and managed hydrology, salinity, latitude, and geography in order to understand how greenhouse gas emissions are regulated in each environment (e.g., water level, soil temperature, river discharge, etc.) and to determine mass fluxes of greenhouse gases on a molar, molecular, elemental, and global warming potential basis, depending on application.

 

Salvador WMA, Louisiana

Future Steps: This project builds on a number of different funding sources, and we anticipate that our greenhouse gas research will become increasingly more applied. Projects are currently underway in created salt marshes of North Carolina, drained tall Pocosin wetlands in Virginia, natural marshes in Louisiana, and managed marshes in China.

To understand how Earth’s climate will change in the future, scientists need to know how much heat-trapping gas is going into the atmosphere today. However, oceans’ emissions of two major greenhouse gases, methane (CH₄) and nitrous oxide (N₂O), vary dramatically in time and space. With a centralized digital resource, the Marine Methane and Nitrous Oxide (MEMENTO) database, information on CH₄ and N₂O concentration measurements from around the globe are collected to help researchers more precisely quantify these oceanic emissions.

Oceanic CH₄ can arise from shallow sediments, and both CH₄ and N₂O are produced by ocean-dwelling microbes. Although only a relatively small fraction of global CH₄ emissions—around 2%—come from the ocean (including coastal areas), oceans are a major source for atmospheric N₂O, providing around 25% of the total. When it reaches the stratosphere, N₂O attacks ozone, destroying it on a global scale.

Estimates of oceanic emissions are based on extrapolations of concentrations measured at the ocean’s surface or results from model studies. However, the fluxes of N₂O and CH₄ can vary substantially from day to day and from place to place, meaning that even with recent improvements in measurement techniques and increased measurements, global emission estimates are still highly uncertain. Millions of measurements taken at different times and covering the globe are needed for researchers to more precisely estimate how much gas is being emitted.

MEMENTO Ups the Game

MEMENTO, an initiative that began in 2009, is the first attempt to systematically compile all global data on oceanic CH₄and N₂O measurements. MEMENTO, an initiative that began in 2009, is the first attempt to systematically compile all global data on oceanic CH₄ and N₂O measurements. It archives data taken not only at the ocean surface but also from the deep ocean. As curators of the data set, our goals are to see how oceanic concentrations of the gases vary in time and space and to provide more precise global emission estimates of oceanic CH₄ and N₂O to the climate research community.

MEMENTO already includes original data from more than 180 measurement campaigns, which have provided more than 20,000 CH₄ and more than 100,000 N₂O measurements over the past 50 years (see Figure 1 for sampling locations). These data sets include dissolved gas concentrations along with information on sampling position, sampling depth and time, and, if available, data on ocean temperature and salinity as well as oxygen and nutrient concentrations.

If available, we also include atmospheric measurements from the same campaign, such as air temperature and air pressure, usually sampled a few meters above sea level height. We also add to all submissions the contact information of the researchers who provided the data, their related publications, and if available, a link to the host center of the original data sets.

An Emphasis on Quality

If measurements lack information on sampling position, sampling time, and sampling depth (for oceanographic data), we do not import them into the database. We put all data submissions imported to MEMENTO through a systematic quality control procedure to guarantee that essential metadata are available and to minimize erroneous entries. If measurements lack information on sampling position, sampling time, and sampling depth (for oceanographic data), we do not import them into the database. In addition, we apply a ­first-​­order range check to all imported variables to exclude obviously incorrect data entries, such as negative concentrations, erroneous date formats, or data positioned over land.

CH₄, N₂O, and oxygen data are imported in their original units. In a second data-processing step, we will calculate global surface fields and depth profiles in common units. Missing temperature and salinity data will be supplied from external data sources.

A Work in Progress

We regularly update the database with newly available data sets and continuously improve it by including additional meta-​­information, allowing additional data formats, and implementing new data quality control criteria.

In addition, we are working closely with the recently initiated Scientific Committee on Oceanic Research (SCOR) Working Group 143, entitled “Dissolved N₂O and CH₄ measurements: Working towards a global network of ocean time series measurements of N₂O and CH₄.” As an additional quality flag for our data, we will implement standard procedures that are developed within the working group for measuring N₂O and CH₄.

As we expand ­MEMENTO, we will also build on the experiences researchers have gained from existing databases such as the Surface Ocean CO₂ Atlas (­SOCAT), the Global Surface Seawater Dimethylsulfide Database (GSSDD), and the Halocarbons in the Ocean and Atmosphere Database Project (­HalOcAt). Specifically, we are looking to create best practices on how to structure data archives, methods for checking data quality, and ways to make data archives more user friendly.

A Resource for the Research Community

We intend for ­MEMENTO to serve as a living resource from which researchers can pull ­quality-​­controlled oceanic CH₄ and N₂O data for a variety of purposes. Researchers have already begun using the database to produce important results for N₂O production and consumption processes on global and regional scales. A list of associated publications is available on the MEMENTO website.

MEMENTO data are freely available to interested users, who can access the database via the ­MEMENTO website. We would like to expand our database, so please consider adding your CH₄ and N₂O data. Contact us to obtain the log-in information to the database and information on how to submit your data to ­MEMENTO.