Temperate grasslands are considered to be a significant sink for CH3Br, although large uncertainties exist about the magnitude of this sink because of a paucity of field measurements. Here, we report the results of a combined field and laboratory study that investigated the effects of water, temperature, and plant community composition on CH3Cl and CH3Br fluxes in a semiarid temperate grassland. A novel stable isotope tracer technique was also employed to deconvolute simultaneous production and oxidation of CH3Cl and CH3Br. Net and gross fluxes were measured from different landforms (ridges, floodplains) and cover types (grass-dominated, shrub-dominated) to capture a representative range of hydrologic regimes, temperatures, and plant communities. In field experiments, net CH3Cl and CH3Br uptake was observed at all grass-dominated sites (-400 +/- 77 nmol CH3Cl m(-2) day(-1) and -3.4 +/- 0.9 nmol CH3Br m(-2) day(-1)), while net CH3Cl emission (439 +/- 58 nmol CH3Cl m(-2) day(-1)) was observed at sites dominated by the shrub Atriplex canescens, indicating that this plant is a strong CH3Cl producer. Gross CH3Cl and CH3Br oxidation were comparable with estimates from other dryland ecosystems (507 +/- 115 nmol CH3Cl m(-2) day(-1) and 9.1 +/- 2.2 nmol CH3Br m(-2) day(-1)), although CH3Br oxidation rates were at least five times lower than those observed in more mesic temperate grasslands. We suggest that estimates of the temperate grassland CH3Br sink should be reduced by >= 19% (>= 1.8 Gg yr(-1)) to account for the weaker sink strength of semiarid environments. Identification of A. canescens as a 'new' CH3Cl source may have important ramifications for the global atmospheric budget of CH3Cl, given the global distribution of this plant and its congeners and their widespread presence in many dryland ecosystems. Laboratory experiments revealed that soil water was the chief regulator of CH3Cl and CH3Br oxidation, while temperature had no observed effect between 14 and 26 degrees C. Oxidation rates rose most rapidly between 0.4% and 5% volumetric water content, suggesting that methyl halide-oxidizing bacteria respond strongly to small inputs of water under the very driest conditions. Soil drying and rewetting experiments did not appear to affect the oxidation of CH3Cl and CH3Br by soil microorganisms, which are presumably adapted to frequent wet/dry cycles.
- soil texture
- methyl halide biogeochemistry
- isotope tracers
- carbon isotope fractionation
- semiarid temperate grassland