Effects of temperature and soil moisture on methyl halide and chloroform fluxes from drained peatland pasture soils

Abstract

A series of laboratory-based incubations using a stable isotope tracer technique was applied to measure the net and gross fluxes of CH₃Cl and CH₃Br as well as the net fluxes of CHCl₃ from surface soils of the Sacramento-San Joaquin Delta of California. Annually averaged flux measurements show that these mineral/oxidized peat soils are a net source of CH₃Cl (140 ± 266 nmol m⁻² d⁻¹) and CHCl₃ (258 ± 288 nmol m⁻² d⁻¹), and a net sink of CH₃Br (−2.3 ± 4.5 nmol m⁻² d⁻¹). Gross CH₃Cl and CH₃Br fluxes are strongly influenced by both soil moisture and temperature: gross production rates of CH₃Cl and CH₃Br are linearly correlated with temperature, whereas gross consumption rates exhibit Gaussian relationships with maximum consumption at soil moisture levels between 20 and 30% volumetric water content (VWC) and a temperature range of 25 to 35 °C. Although soil moisture and soil temperature strongly affect consumption rates, the range of gross consumption rates overall is limited (−506 ± 176 nmol m⁻² d⁻¹ for CH₃Cl and −12 ± 4 nmol m⁻² d⁻¹ for CH₃Br) and is similar to rates reported in previous studies. CHCl₃ fluxes are not correlated with methyl halide fluxes, temperature, or soil moisture. The annual emission rates of CHCl₃ from the Sacramento-San Joaquin Delta are found to be a potentially significant local source of this compound.