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Grazing cows affect the emissions of organic compounds

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Although half of the world's agricultural land is grazing land, grazing-induced flux measurements of volatile organic compounds (VOC) have not been reported yet. Researchers from BIRA-IASB therefore investigated the exchange of VOCs between a cattle-grazed grassland and the atmosphere at a Walloon ICOS site in Dorinne. Grazing-induced fluxes differed markedly from those in absence of grazing and typically lasted for 2 or 3 days. They were typically one to two orders of magnitude lower than harvest-induced emissions.
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Despite of being highly relevant ecosystems at the global scale, information on BVOC emissions from grasslands is still sparse.

Therefore, the Mass Spectrometry group of BIRA-IASB and Gembloux Agro-Bio Tech (University of Liège) recently conducted a long-term field study of VOC exchanges between a managed grassland and the atmosphere at the ICOS Terrestrial Observatory in Dorinne (Belgium) in the framework of the CROSTVOC project (FNRS).

Ecosystem-scale BVOC, CO2 and H2O flux measurements were performed with the disjunct eddy covariance technique (see Figure 1) and alternated by automated dynamic chamber flux measurements (see Figure 2). In both approaches a fast and sensitive on-line proton transfer reaction mass spectrometry (PTRMS) instrument was used as VOC analyser.

Rotational cow grazing

As cow grazing is expected to induce reactive “wound VOCs” in plants, the chambers were used to study the effect of this kind of biotic stress on BVOC emissions under natural field conditions. The measurements revealed induced emissions of several alcohols, aldehydes and ketones which, however, only lasted for a few days following the grazing events.

Overall it was estimated that rotational grazing was a less important BVOC source compared to grass cutting and drying in the field.

More data in different parts of the world are clearly needed to properly assess the impact of these grassland management practices on BVOC fluxes.

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Figure 2: Dynamic chamber set-up showing two parcels, which were alternately used for grazing by cows and as a control. Fences were opened and cuvettes were temporarily removed prior to grazing and measurements started about two hours after grazing.
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Figure 3: Eddy covariance mast with a sonic anemometer and gas sampling lines.