When I tell people what I do (“I measure greenhouse gas emissions from agricultural soils.”), the most common answer is “Oh, you are working with cows.” Almost everybody – regardless of their background – seems to know that cows emit the greenhouse gas methane (CH4). However, far less people know that soil is another big player when it comes to CH4. The following figure gives a simplified overview of CH4 in soils.
In soils, CH4 is produced by a specific group of microorganisms, the so-called methanogens. Methane production is the last step in the degradation of soil organic matter under anoxic conditions (i.e. without oxygen). Methanogens convert either small organic compounds or carbon dioxide (CO2) to CH4, and they can only do it under anoxic conditions because they are very sensitive to oxygen (O2). Soils can be permanently anoxic below the groundwater table, or develop temporarily anoxic zones above the groundwater table, e.g. after heavy rain. Most of the CH4 in soils is produced below the groundwater table, which means that wetlands and rice fields are very important CH4 emitters on a regional and also global scale.
Gases will always diffuse from an area of high concentration to an area of low concentration. In the case of CH4, it will diffuse from the anoxic soil zone to the well-aerated (=oxic) soil zone. That zone is inhabited by another group of microorganisms, so-called methanotrophs. They feed on CH4 and O2 and convert these to CO2. Only CH4 which is not consumed by methanotrophs can diffuse all the way to the soil surface and into the atmosphere. An aerated soil layer of 20 cm is already sufficient to completely prevent CH4 emissions from waterlogged soils. Methanotrophs are also able to consume CH4 which diffuses from the atmosphere into the soil. As a result, well-aerated soils can be significant sinks for atmospheric CH4. For more detailed information about the global CH4 cycle and CH4 in soils follow these links: IPCC 5th Assessment report, mini review in Environmental Microbiology.
And now I come along and say that I am going to measure CH4 production in well-aerated soils. Why should that be of any interest? That’s where the cows come into play: Methanogens love to live in the rumen of cows because it is anoxic and there is plenty of organic matter to degrade. In soils, some methanogens use the same trick by living in the gut of scarab beetle larvae, where they don’t have to care at all about O2 in the soil. This is why I call scarab beetles “soil cows”!
There are over 30,000 species of scarab beetle and the number of larvae in soils can be quite large. Therefore, scarab beetle larvae might be a very important source of CH4. I say ‘might’ because, to my knowledge, nobody has ever measured it in the field. I have been studying CH4 production in soils for almost 10 years now and I just learned a year ago that scarab beetle are small “soil cows”. In my project, CH4ScarabDetect, I have chosen the larvae of the common cockchafer and the forest cockchafer as study objects to get a first impression of how important these larvae might be as CH4 sources in soils. Why I have chosen these two species from over more than 30,000 will be the topic of my next post. As a small teaser, find a picture of a forest cockchafer larva below.