The effect of soil warming on CO2 and CH4 flux from a spruce–fir forest soil was evaluated at the Howland Integrated Forest Study site in Maine, USA from 1993 to 1995. Elevated soil temperatures (∼5 °C) were maintained during the snow-free season (May – November) in replicated 15 × 15-m plots using electric cables buried 1–2 cm below the soil surface; replicated unheated plots served as the control. CO2 evolution from the soil surface and soil air CO2 concentrations both showed clear seasonal trends and significant (P < 0.0001) positive exponential relationships with soil temperature. Soil warming caused a 25–40% increase in CO2 flux from the heated plots compared to the controls. No significant differences were observed between heated and control plot soil air CO2 concentrations which we attribute to rapid equilibration with the atmosphere in the O horizon and minimal treatment effects in the B horizon. Methane fluxes were highly variable and showed no consistent trends with treatment.
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