Stable Isotopic Signatures of CO Uptake and Emission by Soil

Abstract:

CO is important for atmospheric chemistry, is a pollutant, and it has been recognized as an important indirect greenhouse gas. Globally, soil uptake is one of main sinks of CO. On local scale, soil can be either a net sink or a net source of CO, due to the fact that both consumption and production of CO take place in soil concomitantly. These two phenomena are independent: while the uptake is microbial, the production is from abiotic oxidation of organic matter.

In order to determine the isotopic signature of the exchange of CO between soil and atmosphere, soil chamber experiments were performed at a forest site in the Netherlands. Flaks samples were filled from the soil chamber, and analyzed for the stable isotopes ¹³C and ¹⁸O using the high precision measurement facility at IMAU. We found that the uptake of CO by soil is associated with a small positive fractionation, i.e. the lighter CO is taken up faster. Although the soil at this site was a strong sink for CO, the isotopic data show that a small emission flux was also present in all cases. The isotopic composition of the emitted CO is depleted in ¹³C compared to atmospheric CO, and compatible with a source from plant and soil organic matter oxidation.