Seeing the sink beneath the source: an improved stable isotope tracer method for measuring highly variable gross fluxes of methyl halides
Abstract
Measuring methyl bromide (CH3Br) and methyl chloride (CH3Cl) fluxes in terrestrial ecosystems is complicated by the presence of simultaneous production (typically associated with plants and/or fungi) and consumption (typically associated with soils). Thus, specific sites within an ecosystem can act as either a net source or net sink, depending on season, soil conditions, or vegetative cover. To interpret the highly variable net fluxes found in many of these ecosystems, a stable isotope tracer technique has been developed to measure gross fluxes of CH3Br and CH3Cl. This method entails adding small amounts of 13CH3Br and 13CH3Cl to an incubation chamber, monitoring the headspace concentration changes of both 13C and 13C isotopologues, and applying a box model to simultaneously solve for gross production and consumption. Over the last decade, this technique has been successfully applied to laboratory soil incubations and field studies from a variety of ecosystems, including boreal forest, annual grasslands, shortgrass steppe, oak-savanna woodland, and Arctic tundra. These studies demonstrate that gross uptake rates are strongly affected by soil moisture within ecosystems but are on average much lower than previously estimated, and that gross production rates are highly dependent on plant species enclosed, with minor production within the soils as well. Measuring gross uptake rates is more challenging in ecosystems with large net emissions of methyl halides, such as coastal salt marshes, rice fields and certain grassland sites. Using the tallgrass prairie of Kansas as a case study, four slightly different models to calculate gross fluxes are compared. These models are largely in agreement except at sites with large emissions (i.e., sites with Amorpha shrubs), where one of the models most robustly quantifies gross consumption. This improved stable isotope tracer method is used to track the separate responses of gross production and gross consumption of methyl halides associated with simulated rainfall in the tallgrass prairie.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.B54A..08R
- Keywords:
-
- 0315 ATMOSPHERIC COMPOSITION AND STRUCTURE / Biosphere/atmosphere interactions;
- 0412 BIOGEOSCIENCES / Biogeochemical kinetics and reaction modeling;
- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 0490 BIOGEOSCIENCES / Trace gases