From Carbon Sink to Carbon Source
The Sanjiang Plain in China's Heilongjiang province is the nation's largest freshwater marsh. Historically, this vast, 10.88 million-hectare wetland acted as an enormous carbon reservoir. For thousands of years, waterlogged, anaerobic conditions in the soil allowed partially decayed plant matter to accumulate as peat, effectively locking away atmospheric carbon. This process created a regional carbon pool estimated to contain 0.26 petagrams of carbon (Pg C)—equivalent to 260 billion metric tons.
Starting in the 1950s, a massive agricultural development campaign began to drain the wetlands. By 2015, the marshland had shrunk by approximately 47%, from over 18,247 square kilometers to just 9,720 square kilometers, primarily converted to cropland. This land-use change had a deep effect on the region's carbon cycle. Exposing the carbon-rich peat soils to oxygen in the atmosphere triggered rapid microbial decomposition. The process switched the entire region from a net carbon sink to a significant carbon source. One study estimated that the conversion of marshland to cropland between the 1950s and 2000s resulted in the loss of 204 teragrams (204 million metric tons) of soil organic carbon. This release of greenhouse gases, including carbon dioxide and methane, has had measurable effects on the regional climate.
Watching the Wetland Breathe
In recent decades, conservation efforts have focused on restoring the damaged wetlands. Since 2000, 18 new nature reserves have been established to protect the remaining marshes and the biodiversity they support. Large-scale restoration projects aim to re-flood drained areas, allowing native wetland vegetation like Carex lasiocarpa and reeds to re-establish.
To understand the effectiveness of these restoration efforts, scientists are now monitoring the exchange of greenhouse gases between the wetlands and the atmosphere in real time. This is accomplished using a technique called eddy covariance. Instruments are mounted on towers that stand above the wetland canopy. These instruments take high-frequency measurements—as often as 10 times per second—of vertical wind speed and the concentration of gases like CO2 and methane (CH4). By calculating the covariance between wind velocity and gas concentration, researchers can determine the net ecosystem exchange of carbon. This data reveals whether a section of the wetland is currently acting as a carbon source (releasing carbon) or a sink (absorbing carbon).
This precise monitoring shows that re-flooded areas can transition back to being net CO2 sinks relatively quickly once vegetation cover becomes established. However, the picture is complicated by methane, a more potent greenhouse gas that is released from anaerobic wetland soils. The data from these flux towers is used for calibrating climate models and for guiding management strategies that maximize carbon sequestration while minimizing the release of other greenhouse gases.