How the world’s largest wetland contributes to climate change

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A scientific study has found that Pantanal, the world’s largest wetland, located in South America, contributes significantly to the global greenhouse gas budget. The study focused on the dynamics of nitrous oxide (N2O), an often overlooked yet extremely important greenhouse gas. N2O is 300 times more potent than carbon dioxide (CO2) and also acts as an ozone depleting gas. The scientific study found that Pantanal, on its own, contributes close to 2% of global N2O emissions.

 

Watch a You Tube video on Pantanal, the world’s largest wetland (courtesy of AdrianJPhoto):

 

The dynamics of the global N2O budget still remains unresolved. Studies found that there was a significant concentration of atmospheric N2O in tropical regions with an anomalously high concentration over tropical South America (Kort et al 2011). Furthermore, it was believed that uncultivated (i.e. natural) tropical soils contribute approximately 25% to the global N2O budget (D’Amelio et al 2009).

Wetlands are highly diverse and productive ecosystems. Pantanal, which is mainly found in Brazil but also crosses over into Paraguay and Bolivia, is a UNESCO World Heritage Listed Area due to its incredible landscapes and high biological diversity (for example, there are around 650 species of birds – about the same number of bird species in all of Australia).  N2O is a biogenic greenhouse gas, where its release or depletion from the atmosphere is primarily driven by biological processes. Therefore, a large, highly productive, wetland is potentially a significant source or sink of N2O.

Scientists from Denmark, Brazil, Australia and Singapore hypothesised that the wetting and drying of soils in wetlands could play an important role in the production and reduction of atmospheric N2O. The process is mainly driven by microbes performing denitrification or nitrification. The process is summarised in the following figure which is a conceptual drawing of a wetland site over a one-year period:

A conceptual drawing of a one year period depicting how nitrifying and denitrifying microbes produce and consume N2O with wetting and drying cycles. Image source: Liengaard et al (2013), Figure 11.

A conceptual drawing of a one year period depicting how nitrifying and denitrifying microbes produce and consume N2O with wetting and drying cycles. Image source: Liengaard et al (2013), Figure 11.

 

Liengaard et al (2013, page 11) described the process:

“During flooding, intense nitrogen fixation accompanies the growth of floating meadows dominated by water hyacinths (Eichhornia crassipes). As the water retreats, the dense, decaying mats release ammonium, and obscure light, preventing growth of other plants. As the soil is drained and aerated, O2 becomes available for intense nitrification in the soil while rain showers frequently deplete the O2 and elicit denitrification with bursts of N2O until the drained season ends with re-flooding of the soil.”

The water hyacinth,

The water hyacinth (Eichhornia crassipes).

 

The wetting and drying cycles of wetland soils, caused by rainfall, flooding and drainage, are driving microbial nitrification and denitrification which, in turn, is contributing to N2O emissions.

Liengaard et al (2013) used a number of scientific techniques to demonstrate these dynamics including N2O and oxygen (O2) microsensors. The N2O microsensors have measurement tip diameters as small as 25um and are ideal for quantifying processes at the microbial scale. Liengaard et al sampled soils from various sites around the Pantanal wetland and returned them to the laboratory. They then employed the microprofiling method to show how N2O and O2 change in the soil under various experimental conditions.

The following figure shows one such experimental condition where the soil cores were experimentally wetted to simulate rainfall. Figures A and B are the N2O and O2 microprofiles 4 hours after wetting, and figures C and D are the microprofiles 10 hours after wetting. This figure demonstrates that shortly after rainfall the soil profile is well oxygenated and there is little presenance of N2O. However, following 10 hours after the rainfall event, O2 is depleted and N2O has increased significantly.

Microprofiles of N2O and O2 in tropical wetland soil from Pantanal after a simulated rainfall event. (Image source: Figure 8, Liengaard et al 2013)

Microprofiles of N2O and O2 in tropical wetland soil from Pantanal after a simulated rainfall event. (Image source: Figure 8, Liengaard et al 2013)

 

 

Liengaard et al also used the N2O microprofiles to calculate N2O flux from the soils. N2O flux can be calculated from the concentration gradient in the water layer on the soil surface using Fick’s first law. The authors found that the mean daily N2O flux was 0.43 ± 0.03 mmol N2O m−2 day−1. With these, and other calculations, Liengaard et al concluded that the Pantanal wetland is contributing approximately 1.7% of global N2O emissions.

By using advanced scientific techniques and instrumentation, the scientists in this study were better able to understand the dynamics of N2O and the importance of its contribution to the overall greenhouse gas budget.

 

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references

D’Amelio, M.T.S., Gatti, L.V., Miller, J.B., and Tans, P. (2009). Regional N2O fluxes in Amazonia derived from air craft vertical profiles. Atmos. Chem. Phys. 9, 8785–8797.

Kort, E.A., Patra, P.K., Ishijima, K., Daube, B.C., Jimenez, R., Elkins, J., et al. (2011). Tropospheric distribution and variability of N2O: evidence for strong tropical emissions. Geophys. Res. Lett. 38, L15806.

Liengaard L, Nielsen LP, Revsbech NP, Priemé A, Elberling B, Enrich-Prast A, and Kühl M (2013) Extreme emission of N2O from tropical wetland soil (Pantanal, South America). Front. Microbio.  3:433. doi: 10.3389/fmicb.2012.00433

 

 

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