Carbon goes with the flow

Many people see the carbon cycle as vertical - Carbondioxide moving up and down between soil, plants and the atmosphere.

However, according to a new research at Michigan State University, a new dimension is added to the vertical perspective by showing how water moves massive amounts of carbon laterally through ecosystems - especially during floods. These findings, which analysed more than 1,000 watersheds, covering about 75 per cent of the contiguous US - have implications for climate change and water quality.

Carbon in the environment, specifically dissolved organic carbon or DOC, is a master variable that influences many of our planet's fundamental processes, such as water chemistry, greenhouse gas emissions and pollutant transport across land and water.

"When water flows through ecosystems, it picks up organic carbon from plants and soils, and in many cases, water determines whether the ecosystem is a net carbon source or sink. The massive amount of carbon that leaks out of ecosystems as DOC is about as big as the net amount of carbon taken up from the atmosphere each year. So accurate accounting is crucial when managing the 'carbon bank account," said Jay Zarnetske, the study's lead author.

Researcher's new work suggested a better way to account for the carbon leaving ecosystems as DOC by including data from flood events. Citing logistical and safety concerns, scientists typically give rivers wide berth during floods. As a result, researchers know less about DOC behaviour during floods. When water is flowing fast and brown, though, is when the most carbon is being transported out of most watersheds. In other words, this is a time when more sampling is needed.

What surprised the team of scientists is that floods readily flush carbon from landscapes in diverse ecosystems across North America, spanning from Michigan forests to the Sonoran Desert. They initially thought the DOC would be diluted by floods in many parts of the U.S. Floods, however, lead to the release of large amounts of DOC - or stronger tea, metaphorically speaking - from almost all environments in a relatively short time.

Another important confirmation from the study's massive data set was the significant role wetlands play in our watersheds. The DOC flushing behaviour across the U.S. was primarily related to the acreage of wetlands in a watershed. Wetlands act as buffers or storage zones for DOC in watersheds. If floodwaters rise, water and DOC in the wetlands closest to the river can rapidly spill over.

Consequently, where natural wetlands are located within the watershed is important. Draining natural wetlands and "trading them" for another nearby swamp or building an artificial wetland might look good on paper, but it's going to affect an area's ability to store and release carbon, Zarnetske added.