Peatlands are a unique type of wetland with a complex climate impact. Peatlands form in high-latitude regions with lots of rainfall, like Iceland and Russia. They only cover 3% of the world’s land surface but contain 10-30% of its soil carbon.
Peatlands store a lot of carbon because their soil is saturated with water. In normal soils, water only fills up part of the pore space, and the rest is filled with oxygen and other gases. But in peatland soils, water fills up all the pore space and doesn’t leave much room for gases. Without oxygen, soil microbes like bacteria and archaea decompose dead plants and animals at a slow rate. This dead organic material builds up into carbon-rich soil called peat.
Scientists have shown that peatlands are excellent at carbon storage, but they can also produce gases that cause global warming, like methane, carbon dioxide, and nitrous oxide. These gases are known as greenhouse gases or GHGs because they trap heat within Earth’s atmosphere and raise the planet’s temperature.
Over the last few decades, farmers and developers across the world have drained peatlands to make room for livestock and agriculture. Scientists in the past suggested that draining and farming peatlands could increase GHG emissions by increasing the oxygen in the soil and speeding up microbial activity and plant decomposition. Since peatland soil is normally oxygen-limited, microbes are forced to use other energy sources as they decompose organic matter. For example, microbes that use carbon dioxide for energy and produce methane are called methanogens. Microbes that use nitrogen compounds for energy and produce nitrous oxide are called nitrifiers.
Researchers estimate that microbial decomposition of organic matter is the main source of GHG production in northern peatland soils, producing up to 2 gigatonnes of carbon dioxide and 0.046 gigatonnes of methane each year. However, these estimates are based on only a handful of studies. Scientists argue that they need more measurements of carbon dioxide, methane, and nitrous oxide emissions from both water-saturated and drained peatlands.
A research team from Iceland and Italy wanted to address this knowledge gap by comparing GHG emissions and soil microbial communities between peatland types in South-West Iceland. They studied 4 water-saturated peatlands, 4 peatlands drained for pasture, and 4 peatlands drained for agriculture. The researchers hypothesized that draining the peatlands would change the microbial community and produce more carbon dioxide and nitrous oxide.
The team identified 4 sampling sites in each of the 12 peatlands. They sampled the soil to measure its temperature, moisture, and carbon and nitrogen contents. They also inserted plastic chambers about the size of a gallon bucket into the ground to collect gas vapors released from the soil. They put the gas samples into a machine that uses heat to separate different types of gases, called a gas chromatograph. The researchers used the gas chromatograph to identify the amounts of carbon dioxide, nitrous oxide, and methane in each gas sample. The team also looked for specific genes found in microbial DNA to learn what microbes were present in the soil and how they functioned, a process called amplicon gene sequencing.
The researchers found that the water-saturated peatlands produced 3 times more methane than the drained peatlands. They also found more methanogens in the water-saturated peatlands. Methanogens are essentially allergic to oxygen and die when they are exposed to even small amounts. The team suggested that the higher water levels in the water-saturated peatlands displaced any oxygen and allowed methanogens to thrive and produce more methane.
In addition, they calculated that the drained peatlands produced 2.5 to 4.5 times more carbon dioxide and 2 to 4 times more nitrous oxide than the water-saturated peatlands. The team found more nitrifiers in the drained peatlands that produced nitrous oxide. Since nitrous oxide and carbon dioxide trap more heat in the atmosphere than methane, they inferred that drained peatlands should exacerbate global warming more than water-saturated peatlands.
The team concluded that draining peatlands increases nitrous oxide and carbon dioxide emissions and negatively impacts the climate. They found that microbial communities respond to changes in energy sources, with nitrous oxide-producing microbes becoming more abundant in drained peatlands. The researchers proposed that sustainable land-use practices, like decreasing nitrogen fertilizers, could help mitigate these climate impacts.
