Peat bogs – CO2 traps – must be protected

Too great a quantity of nitrogen, associated with higher temperatures, is a serious threat to the existence of peat bogs. Yet the latter retain a large portion of the carbon stored in soils worldwide, which would be released into the atmosphere with the destruction of these fragile environments. This major threat has been highlighted in an article which was recently published in the journal New Phytologist, written by Alexandre Buttler, professor at the Laboratory of Ecological Systems at EPFL, in cooperation with Luca Bragazza, professor at the University of Ferrara in Italy and at the WSL site in Lausanne. Within the framework of a consortium of scientists created to better understand the challenges associated with peat, they have compiled and cross-referenced the results of 29 studies conducted in 14 countries in North America and Eurasia.

Since the end of the Pleistocene era (approx. 12,000 years ago), peat bogs have had a significant influence on the Earth’s atmosphere. Although their surface only covers 3% of the land, it is estimated that they retain about one-third of the carbon stored in the soils of the planet. Mostly, they result from an accumulation of partly decomposed organic matter of a type of moss called “sphagnum”.

“These plants have particular characteristics: they have antibiotic properties, which means that micro-organisms decompose them very slowly”, explains Alexandre Buttler. These peat-moss bogs are also very poor in nutrients. Thus, their survival relies essentially on what the atmosphere can supply them with. Therefore, any qualitative or quantitative changes in rainfall can seriously modify the capacity of these sites to absorb CO2 from the atmosphere. As sphagnum cannot regulate their water content – unlike vascular plants which are more evolved – they need excess liquid to survive and thrive. A period of drought could therefore spell the end of them.

So what about the effects of nitrogen, an important element in the growing process of plants, but which can disrupt the balance between plants if it is present in too great a quantity in its assimilable form? As a result of human activities such as fossil-fuel combustion and the use of fertilizers in agriculture, the quantities of assimilable nitrogen present at the surface of the Earth have considerably increased. It’s currently estimated that they have doubled in the last 100 years, leading to changes in the distribution of vegetal species in the peat bogs, and in particular encouraging the development of vascular plants to the detriment of sphagnum moss. Because vascular plants decompose more easily than sphagnum, there is a risk that the peat mass could diminish rapidly.

“Whereas the increase in nitrogen emissions in an area where it is scarce encourages the growth of or, at least, does not affect the production of sphagnum, we have been able to show, on the other hand, that much higher quantities are harmful to this production, and all the more so if the region is already laden with it”, continues Alexandre Buttler.

One of the most unexpected results of this research is that the increase in the level of nitrogen also interacts with climatic factors. “We have discovered that higher temperatures further intensify this negative effect”, adds Luca Bragazza. “Our calculations have shown that an increasing supply of nitrogen, combined with a warmer climate, could severely reduce the capacity of peat-moss bogs to trap carbon.”