Wetland greenhouse gas emissions

Wetlands are one of the most biologically productive and biologically diverse ecosystems (even more productive than tropical rain forests) (1). Wetlands act as sinks, sources and transformers of materials including carbon (C) as one of the most important component of these nutrient fluxes (2, 3). Wetlands are known as an important C sinks in the global scale, because almost one third of world soil carbon pool is stored in wetland soils (400-750 GT). Peatlands, as a specific type of wetlands, have the highest C storage potential (4, 5, 6). They cover only 3% of the Earth surface but they store huge amount of C representing 30-50% of C in the atmosphere.

Why wetlands have so high C storage capacity? The explanation is hidden in carbon cycling in wetlands and factors influencing this cycle (7, 8, 9) . Main income of C into the system is via photosynthesis and losses of C via autotrophic (plant) and heterotrophic (microbial) respiration and leaching. Main condition for ecosystem to act as a C sinks is to produce more biomass than its decomposed. It is true for many wetlands that their primary production exceeds the decomposition and thus the excess of organic matter is accumulated in soil profile (10). It is because the substrate (soil) is permanently or periodically flooded and anaerobic conditions with low availability of oxygen prevails there. Anaerobic conditions considerably reduce the decomposition rate and mostly only the easy decomposable organic matter (sugars, amino acids) is decomposed and recalcitrant fractions as lignin, cellulose are accumulated there. Therefore CO2 from atmosphere accumulates in wetlands in the form of undecomposed plant biomass, which is in case of peatland called peat. Due to this “CO2 absorption”, wetlands have a cooling effect on climate.

On the other site, wetlands are one of the largest natural methane (CH4) source to the atmosphere. It is estimated that wetlands emit 20-25% of current global methane emissions. Methane has 25times stronger global warming potential than CO2. Thus wetlands have dual impact on climate – CO2 sink x CH4 source. Methane is end product of anaerobic decomposition. Methane production and emissions are driven mostly by hydrology, trophic status and plant species composition. Hydrological regime influences persistence of anaerobic conditions, which are needed for methanogenic archaea producing methane. If the water table fluctuates or decreases too much, then oxygen is present in soil profile and methane production is interrupted. Therefore not every wetland is a methane source.

Wetlands are an important long-term sinks of CO2 from the atmosphere but on the contrary they emit CH4 into the atmosphere. Both these gases are known as greenhouse gases – it means that they absorb long-wave infrared radiation and their increasing concentration in the atmosphere lead to the increasing temperature. The temperature increase and climate change may influence primary production, C sequestration, decomposition, plant species composition and methane emissions, therefore there exist many feedbacks between wetlands and climate. This can be reflected in changing C storage potential of wetlands and their functioning in global C cycling.  Projected changes in temperature show that the most severe increase in temperature should be in high northern latitudes where are the most of peatlands situated (12). However, it is still unknown what will happen with wetland functioning under the current climate changes (13).