It’s far from certain that peat bogs can continue absorbing carbon dioxide at current rates

Analyses of cellulose in peat cores collected by collaborating scientists working in five continents indicate that a carbon dioxide-driven increase in photosynthesis of mosses is strongly dependent on the water table, which may change the species composition of peat moss communities. The findings will help predict how peatlands will react to a changing climate. Understanding peatlands is crucial as while they cover only three per cent of the global land surface, they store a third of the global soil carbon.

Henrik Serk and colleagues from five continents investigated the response of the photorespiration to photosynthesis ratio in Sphagnum mosses to rising carbon dioxide in the 20th century. With water, photosynthesis pulls carbon dioxide from the air and fixes it in plants. However, as well as photosynthesis, there’s another process, photorespiration. This process occurs when rubisco, an enzyme that picks up carbon dioxide, picks up oxygen and converts it to carbon dioxide. “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 author Jürgen Schleucher, Professor at Umeå University, Sweden, in a press release.

For the study, researchers collected peat cores from ten locations worldwide. They used nuclear magnetic resonance spectroscopy to compare distributions of the stable hydrogen isotope deuterium in cellulose of modern and century-old peat mosses. This allowed them to reconstruct changes in photosynthetic efficiency during the 20th century by estimating the impact of photorespiration.

The analysis revealed that increasing carbon dioxide during the last century had reduced photorespiration, probably boosting carbon storage in peatlands to date and dampening climate change. However, increasing atmospheric carbon dioxide only reduced photorespiration in peatlands when water levels were intermediate, not when conditions were too wet or too dry. Unlike vascular plants, mosses cannot transport water, so the water table level controls their moisture content, affecting their photosynthetic performance. So, models based on vascular plants’ physiological responses cannot be applied.

The importance of the water table can have significant consequences for peatland species composition, as only mosses that grow at an intermediate distance from the water table level benefit from the higher atmospheric carbon dioxide concentration. Changes in rainfall could have a massive effect on peat mosses ability to store carbon as the water table rises or falls.

“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.

RESEARCH ARTICLE

Serk, H., Nilsson, M.B., Bohlin, E., Ehlers, I., Wieloch, T., Olid, C., Grover, S., Kalbitz, K., Limpens, J., Moore, T., Münchberger, W., Talbot, J., Wang, X., Knorr, K.-H., Pancotto, V. and Schleucher, J. (2021) “Global CO2 fertilization of Sphagnum peat mosses via suppression of photorespiration during the twentieth century,” Scientific Reports. https://doi.org/10.1038/s41598-021-02953-1

Updated 18 Jan 2022 with a less ambiguous image of Sphagnum moss.