Seasonal patterns of methane uptake and carbon dioxide release by a temperate woodland soil

Abstract

Direct measurements of CH 4 and CO 2 atmosphere/soil exchange with a drained upland inceptisol were made over a 2‐year period in a mixed hardwood forest in New Hampshire. Soil gas concentrations of CH 4 and CO 2 were also monitored over the same period. Soil incubation experiments were used to characterize the depth variation and the temperature response of the consumption and production reactions. CH 4 was always taken up by the soils after spring thaw in April. Maximum rates of consumption were 4.8 mg CH 4 m−2 d−1 in 1989 and 4.9 mg CH 4 m−2 d−1 in 1990. CO 2 efflux was much higher with rates over 25 g CO 2 m−2 d−1 measured in July 1990. Annually, about 600 mg CH 4 were consumed and approximately 2 kg CO 2 emitted per square meter. There was an apparent negative correlation between soil CH 4 and CO 2 concentrations. Gas samples taken in the surface litter layer showed CH 4 to be depleted and CO 2 enhanced within 1 cm of the surface. Below the surface, CH 4 decreased and CO 2 increased with depth. At 15 cm, CH 4 was never greater than 0.3 ppm and was usually less than 0.2 ppm. At the same depth, CO 2 concentrations ranged from 1100 to over 7500 ppm. Incubation experiments indicated that CO 2 was being produced throughout the top 15 cm of surface soil with a similar temperature response. Significant CH 4 oxidation was measured only in a zone at the top of the mineral soil layer. Both activities could be stopped by autoclaving. CO 2 flux from the ground throughout the year was driven by biological activity, mainly soil and root respiration. CH 4 uptake, on the other hand, was more complicated. Biological activity controlled the establishment of soil concentration gradients, and so in spring, CH 4 influx was tightly linked to rates of consumption. However, in summer and fall, diffusive supply of CH 4 to its site of consumption in the soils limited flux rates.