Sources and sinks of methyl bromide and methyl chloride in the tallgrass prairie: Applying a stable isotope tracer technique over highly variable gross fluxes

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) are important stratospheric ozone depleting compounds, but their natural terrestrial sources and sinks have large uncertainties. Gross fluxes of these compounds were measured during the growing season using a stable isotope tracer technique at a temperate tallgrass prairie in northeastern Kansas, United States. Results show that the tallgrass prairie acts as both a source and sink for CH3Br and CH3Cl, with large emissions associated with Amorpha spp. shrubs and a smaller soil sink than expected. Net flux behavior is not significantly altered by the addition of stable isotope tracers. Four models employed to calculate gross fluxes are largely in agreement with each other except at sites with high CH3Cl emissions, where one model most robustly quantifies gross consumption. Gross production rates may increase at Amorpha sites following simulated rainfall, but gross consumption shows no clear response, suggesting that late season microbial uptake of methyl halides is not strongly influenced by sudden changes in soil moisture. Key Points Gross fluxes are measurable with improved stable isotope tracer technique In the tallgrass prairie, Amorpha bushes are large methyl halide sources Grassland soil uptake rates are likely smaller than prior estimates