Nitrogen as the key: the overestimated effect of CO2 as a plant fertiliser

Bettina Weber ist Biologin an der Uni Graz und beschäftigt sich mit der biologischen Stickstofffixierung und deren Auswirkung auf den Klimawandel. Foto: Uni Graz/Tzivanopoulos

High levels of atmospheric CO2 can intensify climate change. Having said that, high CO2 also stimulates plant growth which decreases climate change. But only if, among other things, nitrogen is present in sufficient quantities. The actual amount of nitrogen has only recently been reassessed. The “fertilising effect” of CO2 has been significantly overestimated, as a brand-new study involving the University of Graz now shows.

In order to use nitrogen, the element must be fixed in the soil with the help of microorganisms. This biological binding occurs in natural ecosystems, but also on agricultural land. “While this process has been significantly overestimated in nature, it has increased by 75 percent over the past 20 years due to agriculture,” Bettina Weber, biologist at the University of Graz, summarises the findings of a publication from earlier this year.
A new study based on these results now shows that existing calculations on nitrogen fixation in certain Earth System models, which are used, among other things, in the World Climate Report, have been revised. This is the conclusion of the new study, which has been published on 24 November in the scientific journal PNAS.
The publication was led by Sian Kou-Giesbrecht from Simon Fraser University in Burnaby, Canada, as part of a working group on biological nitrogen fixation, of which Bettina Weber is a member. The working group is supported by the U.S. Geological Survey (USGS) John Wesley Powell Centre for Analysis and Synthesis.

“We compared different Earth System models with current nitrogen fixation values and found that they overestimate the nitrogen fixation rate on natural surfaces by about 50 per cent,” Weber explains. Overall, this overestimation of biological nitrogen fixation leads to a reduction in the CO2 fertilisation effect of about 11 per cent.
Weber therefore advocates revising the Earth System models in order to better assess developments. “This is because gases such as nitrogen oxides and nitrous oxide are produced as part of the nitrogen cycle. These can be released into the atmosphere through conversion processes and alter or disrupt climate processes.”

Andreas Schweiger

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