Samantha K. Chapman, Matthew A. Hayes, Brendan Kelly, and J. Adam Langley. 2019. Exploring the oxygen sensitivity of wetland soil carbon mineralization. Biology Letters 15: 20180407. http://dx.doi.org/10.1098/rsbl.2018.0407
Blue carbon sinks are coastal ecosystems such as mangroves, marshes, and seagrass beds, that store carbon from the world’s oceans. Because these ecosystems store tons of carbon, it is important to understand how they will be altered by future environmental conditions. Oxygen availability may be an important driver in wetlands, which are primarily anoxic (little to no oxygen) ecosystems. With sea level rise, increased droughts, and shifting plant species, oxygen availability could increase respiration rates in these ecosystems and positively feedback to climate change. In this study, a literature search and an experimental mesocosm explored how changing oxygen availability may impact soil respiration rates.
What is blue carbon?
Carbon is part of all living organisms and is essential to life on Earth! Carbon cycles from living organisms, being released as carbon dioxide during respiration (breathing) and the decomposition of organic matter (dead stuff), and is taken up by plants during photosynthesis. Carbon is in the biomass of humans, animals, trees, and plants, and is stored for long periods of time in the soil. Humans have been impacting the carbon cycle for many years, emitting carbon dioxide back into the atmosphere through fossil fuel combustion. Because carbon dioxide is a greenhouse gas that helps warm our planet, the more that is emitted and accumulates in the atmosphere, the hotter our planet will be. This global phenomenon is called climate change.
The term “blue carbon” has been used in recent years to refer to coastal ecosystems such as mangroves, salt marshes, and seagrass beds that store carbon from the world’s oceans. Why are these ecosystems so important? They are known to store more carbon than other ecosystems, such as forests, and they can help us combat the impacts of climate change. If the carbon is stored, than it’s not in the atmosphere as carbon dioxide contributing to climate change. Though scientists have come far in understanding the mechanisms that drive carbon storage in blue carbon sinks, little is known about how these mechanisms will be altered in the future.
Oxygen, carbon storage, and wetlands
Scientists believe that coastal ecosystems, or wetlands, are able to store so much carbon because they lack oxygen. Wetlands, such as marshes, are thought to be anoxic (contain little or no oxygen) because their soils are saturated by water (they are on the coast, of course!). Oxygen allows decomposition to occur more efficiently, meaning more carbon dioxide is released into the atmosphere than without oxygen: microbes love oxygen! What would happen if oxygen availability in these ecosystems change?
A group led by Samantha Chapman from Villanova University wanted to explore what would happen to blue carbon dynamics (in soils) if oxygen availability changes in the future. Because climate change will cause “increased droughts, higher sea levels, and shifting dominant wetland plant species,” oxygen availability and decomposition, and therefore carbon storage, could be altered. In order to do this, they explored the literature and set up an experimental mesocosm manipulating soil oxygen availability in “intact soil” incubations.
Mesocosms that mimic realistic soil conditions
Lab incubations are contamination-free, controlled experiments, and very common in the field of ecosystem ecology. There have been many studies using lab incubations to address how oxygen availability changes soil respiration rates. However, very few studies have done incubations that use intact soil cores with constant oxygen treatments to mimic “real life” soil conditions. The experimental mesocosms in this study attempted to do just that.
The scientists took five intact soil monoliths (soil profiles) from a brackish, tidal wetland in Maryland to use for oxic and anoxic mesocosms in the lab. In order to make sure the oxygen treatments were the same across mesocosms, they set up a flow-through system using porewater that was either oxygenated or deoxygenated, cycling it back into the columns. They assessed the amount of carbon dioxide the oxic and anoxic mesocosms produced every two weeks during a six week period.
Analyzing the results and the literature
Once all the data were collected from the experimental mesocosm, the scientists derived an oxic to anoxic (O:A) ratio to compare carbon dioxide respiration rates between oxic and anoxic soils. Essentially, they wanted to see if these rates were higher in oxygenated soils, considering microbes are more active with oxygen present. What does a high O:A ratio mean? The higher the ratio, the more sensitive decomposition will be to oxygen.
They performed a literature search in order to compare the O:A ratios between their mesocosm and other studies across wetlands in different biomes. They found that in the literature (and in their experiment) the O:A ratios were very variable (they differed a lot!). This mesocosm was the first to report an O:A ratio for a coastal wetland, which fell in the range of other freshwater wetlands. However, the ratio of 4.1 they derived was lower than other wetlands reported, which landed on a range of 0.8 to 33. They hypothesized this was because of experimental error and oxygen leakage.
It’s very difficult to assess how sensitive blue carbon in soils is to oxygen availability, especially because most of the data come from lab incubations that are often error-prone and difficult to mimic “real life” environmental conditions. The results of this study may have underestimated how “sensitive blue carbon is to oxygen availability and how stable it is under anoxic conditions.”
Continuing to explore how oxygen availability will drive blue carbon dynamics in wetlands ecosystems is crucial as global change will continue to alter environmental conditions. Further, creating lab experiments that more closely mimic realistic conditions and oxygen flow will give a better representation of oxygen sensitivity than just a comparison of oxic and anoxic conditions in wetlands soils.