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How far do experimentally elevated CO2 levels reach into the surrounding? – An example using the 13C label of soil organic matter as an archive

During the last two decades, free air CO2 enrichment (FACE) studies have been conducted to study the effects of rising atmospheric CO2 concentrations on ecosystems. The distances between fumigated and control plots differ widely among those projects, but no experimental data are available how far into the surrounding area an effect of CO2 fumigation can be detected. As the CO2 gas added to the fumigated plots has a different 13C label than ambient atmospheric CO2, its carbon can be traced into plants and soil organic matter (SOM). The Swiss FACE in Eschikon had been conducted for 10 years on a grassland site. After it had ended, we analysed soil samples from three transects extending from the plots to the surrounding area for their organic carbon (Corg) content and carbon isotopic signature. We determined the maximum spatial extension to which carbon originating from the fumigation was incorporated into SOM. A budget of the fumigation gas‐derived Corg in the upper 10 cm of the soil showed that approximately 50 kg C were stored within the plots, and an additional 31 kg C were stored in their immediate surroundings up to a distance of 9 m from the gas pipes. The presented approach provides us with a method to determine a posteriori the extension to which the CO2 fumigation treatment contaminated its immediate surroundings during a FACE experiment. In the presented example, this showed that the distances between plots could have been reduced significantly. Although not generalizable to other experimental settings, the finding indicates that optimizing the spatial layout, e.g. by modelling gas dispersion, will be useful when planning future large‐scale FACE infrastructures. Our approach provides a solid basis to test such gas‐dispersion models on existing FACE sites before planning new sites.