Contribution of microbial photosynthesis to peatland carbon uptake along a latitudinal gradient
What did we study and why?
Peatlands store important amounts of carbon due to an imbalance between atmospheric carbon (C) uptake and release, i.e., the fixation of atmospheric C through photosynthesis is higher than the decomposition of organic matter. While plant -and especially Sphagnum species- are considered to drive C fixation, peatlands also provide a habitat to myriads of photosynthetic microbes. The contribution of these photosynthetic microbes to peatland C fixation is however unknown. Do photosynthetic microbes significantly contribute to peatland C uptake? Is this contribution limited by environmental conditions?
What did we do?
To find out, we studied photosynthetic microbes in five peatlands distributed along a latitudinal gradient in Europe. We sampled Sphagnum from which we extracted microbial communities. In each peatland, we investigated the diversity, abundance and photosynthetic rates of photosynthetic microbes by means of DNA sequencing, microscopy, and measures of photosynthesis.
What did we find?
We found that the community structure and the composition of photosynthetic microbes shifted along the latitudinal gradient under the effects of water availability ad plant composition. Despite these shifts, we showed that microbial C-fixation rates remained similar in the different peatlands, and that photosynthetic microbes accounted for approximately 10% of Sphagnum C uptake.
What does this mean ?
This experiment teaches us that photosynthetic microbes significantly contribute to peatland C uptake at all latitudes. We roughly estimated that photosynthetic microbes take up around 75 MT CO2 per year in northern peatlands, an amount being in the same range than projected peatland C loss due to climate warming. This amount reveal that photosynthetic microbes play an important role in the peatland C cycle. However, this conclusion comes with a warning. The becoming of microbial-fixed C in peatlands remains poorly known. Therefore, our study outlines and encourages to further explore the role of photosynthetic microbes in the peatland C dynamic.
Come rain, come shine: peatlands carbon dynamics shift under precipitation shifts
What did we study & why
Most peatlands occur in the northern parts of our planet. Although peatlands only cover 2-3% of the global land surface, they store about 30% of all the carbon locked-up belowground. Peatlands thank their carbon storage capacity to peat mosses (Sphagnum) and some other specialized plants. But this specialized vegetation strongly depends on water. In the past decade, scientists have observed that rainfall in Europe and North-America becomes heavier, but occurs less often. These extreme rainfall trends are expected to continue according to the latest climate models. How will the carbon storage capacity of peatlands be affected by changes in the rainfall patterns?
What did we do?
To find out, we conducted a rainfall experiment. We sampled intact blocks of peat in the French Pyrenees and created mini-peatlands in pots. We simulated 6 different rainfall patterns ranging from normal-extreme. During 2.5 months in spring, we measured how the water tables, and the carbon storage processes reacted to the rainfall treatments. When summer started, we set the rainfall patterns back to normal and measured how the mini-peatlands recovered.
What did we find?
We found that the extreme rainfall treatments made the water levels in the pots much more variable compared to the normal treatments. Changes in rainfall and water level reduced the amount of carbon that was lost from the mini-peatlands. When the water table was deep and fluctuated a lot, the carbon uptake by the plants was larger than the release of carbon by the plants and belowground soil organisms combined. We think this is because some plants, like grasses, grow better and take-up more CO2 when the water tables are deep. Also, the amount of methane released from the peat was smaller under extreme compared to normal rainfall patterns. We expected this, because less methane is produced when water tables are deep.
When rainfall patterns returned to normal, we noticed that the rates of methane emissions responded immediately and returned to normal as well. However, the rates of CO2 uptake and release did not and kept showing signals from the springtime rainfall treatments. What probably happened is that the grasses that grew well during spring still influenced the exchange of CO2 during summer and caused the lagged effect in our observations.
What does this mean?
This experiment teaches us that changes in rainfall patterns will influence the carbon dynamics in peatlands. It seems that more extreme rainfall will improve the carbon storage of peatlands, but this conclusion comes with a warning. The improved carbon uptake may be because the vegetation is changing, and this can trigger a whole series of processes that can destabilize the peatland carbon balance. While our mini-peatlands are small and are not full copies of real peatlands, our results are similar to findings of other scientists. The new knowledge we gain from our experiment is that the fluctuation of the water table is an important factor determining peatlands responses to climate change and that scientists have to take this into account when studying peatland carbon in- and outputs. Finally, peatlands do not have a reset-button: changes in the rainfall patterns that occur at the time that plants grow the most (Spring) are not simply erased when rainfall returns to normal.
Free access to the publication https://www.frontiersin.org/articles/10.3389/fenvs.2021.659953/full