Methane climate change risk suggested by proof of redox cycling of humic substances

Wetlands, such as peatlands, have a high content of organic-compound rich humic soils, which are formed during the breakdown of biological organisms such as plants. When water levels are high, these soils remain wet, and so are starved of oxygen, causing the organic compounds to accept electrons from respiring bacteria -- rather like the charging of a giant battery. This prevents these electrons from being used by methane-forming bacteria, and so prevents these bacteria from releasing methane into the atmosphere.

The researchers now found that this giant electrical reaction can also go into reverse, like the charging and discharging of a giant battery. As water levels drop due to periodic variations in weather, then the humic soils will tend to release the electrons to oxygen forming harmless water. However, there is a danger is that climate change could lead to higher water levels, causing these soils remaining submerged for a greater period, and so and the discharging process disappears thus leading to more methane formation.

Potentially even worse, increasing global temperatures may cause permafrost to melt, leading to the soils being charged by microbial activity with electrons but not discharged. This could release vast quantities of methane from the wetlands. This in turn causes a greater greenhouse effect, and so on in a potentially vicious cycle.

One of the two lead researchers, Andreas Kappler (Tubingen), who is also the secretary of the European Association of Geochemistry, said: "From a scientific point of view, our study is a step forward in understanding the geochemistry of how the humic material in the world's wetlands store electrons and prevent the release of a massive amount of methane. It also shows that reversible electrochemical processes have the potential to have a large effect on the environment. There are uncertainties as to the exact extent, but we estimate anywhere between an extra 10% up to an extra 166% methane could be released. What it also shows that these are fragile ecosystems and that slight changes in their geochemical conditions could have dramatic consequences for the environment."

The world's wetlands cover around 5% of the planet's ice-free land surface, meaning that they are roughly equivalent in size to Australia: but they have a disproportionate effect on the planet's methane metabolism -- contributing between 15 and 40% of the global methane flux into the atmosphere. Wetlands include areas such as marches, swamps, bogs, and fens, and in fact the largest wetlands in the world are swamp forests of the Amazon and the peatlands of Siberia.

Commenting on the potential climate effect, Professor Ralf Conrad (Marburg, Germany) said: "The study shows that humic substances can act as rechargable oxidants in flooded environments in the same way as it is known for inorganic nitrogen, sulfur, iron and manganese. Humic substances are ubiquitous constituents in the terrestrial biosphere and form a huge carbon reservoir. Now it has been shown that humic substances also control the microbially mediated electron flux in wetland ecosystems. This role has so far not been considered in models of global carbon cycling and climate change, but has a potentially great impact."