Rising atmospheric CO2 concentrations globally affect photosynthesis of peat-forming mosses

As human CO₂ emissions continue, it is increasingly important to capture CO₂ to mitigate the associated climate change. Peatlands are the largest soil carbon stores globally, but the impact of climate change on peatlands is still unknown. During the 20th century, global atmospheric CO₂ concentrations have increased by nearly 50 per cent and further increases are inevitable according to the Intergovernmental Panel on Climate Change, IPCC, with severe consequences for humanity. So far, uptake of CO₂ by the land biosphere has dampened the CO₂ rise and prevented even more severe effects.

Although peatlands cover only three per cent of the global land surface, they store a third of the global soil carbon. Thus, uptake of CO₂ by peat mosses is important, but little is known about how their physiology is affected by rising CO₂ levels. To understand if peatlands will keep storing carbon and mitigate climate change in the future, the scientists investigated peat mosses' responses to the increase in atmospheric CO₂.

For the study, collaborating researchers from five continents collected peat cores from ten locations worldwide. In a novel use of nuclear magnetic resonance pectroscopy, distributions of the stable hydrogen isotope deuterium in cellulose of modern and century-old peat mosses were then compared. This allowed us to reconstruct changes in photosynthetic efficiency during the 20th century, by estimating the impact of photorespiration, a side reaction of photosynthesis.

"Photorespiration is critical for the carbon balance of plants because it reduces the efficiency of photosynthesis by up to 35 per cent, and it is suppressed by increasing CO₂ but accelerated by increasing temperature," says Jürgen Schleucher, Professor at Department of Medical Biochemistry and Biophysics at Umeå University, Sweden.

The analysis revealed that increasing CO₂ during the last 100 years has reduced photorespiration, which has probably boosted carbon storage in peatlands to date and dampened climate change. However, increasing atmospheric CO₂ only reduced photorespiration in peatlands when water levels were intermediate, not when conditions were too wet or too dry. Unlike higher plants, mosses cannot transport water, so the water table level controls their moisture content, which affects their photosynthetic performance. So, models based on higher plants' physiological responses cannot be applied.

That the effect of CO₂ depends on the water table level can have major consequences for peatland species composition, as only mosses that grow at an intermediate distance from the water table level benefit from the higher atmospheric CO₂ concentration. Moreover, changes in the peatlands' water balance can strongly affect their future carbon balance as too wet or too dry conditions reduce peat mosses' ability to scavenge carbon.

Although peatlands have dampened CO₂-driven climate change so far, the changes have already had devastating effects. If human CO₂ emissions are not strongly reduced, the atmospheric CO₂ concentration will further increase by hundreds of ppm by 2100, and average global temperatures will rise several degrees C above pre-industrial levels. It is unclear how peatlands will be affected by this.

"To get a clearer picture of photorespiration's importance for peat mosses and peat carbon accumulation, the next step is to transfer our data into tailored photosynthesis models to estimate global peatland carbon fluxes. Future CO₂ levels, temperature rises, changes in precipitation and water table levels will all need to be considered to forecast peatlands' fate in a changing climate," says Jürgen Schleucher.