Methane and carbon dioxide fluxes from poorly drained adjacent cultivated and forest sites

Methane and carbon dioxide fluxes at the soil surface were measured from April to November 1992 in Ottawa, on adjacent cultivated (corn) and forest (temperate woodland) sites using closed chambers (10 chambers per site). The objectives were to quantify the spatial and temporal variability of gas exchange rates, and to determine the effects of soil temperature and moisture on the fluxes. On the forest soil, rates of CO 2 emissions and CH 4 uptake ranged from 2.27 to 14.82 g m −2 d −1 and from 0.04 to 1.10 mg m −2 d −1 , respectively. On the cultivated soil, the measured CO 2 fluxes varied from 0.27 to 7.07 g m −2 d −1 while methane uptake ranged from 0 to 0.13 mg m −2 d −1 . There was a positive correlation between soil surface CO 2 fluxes and soil temperature for the forest (R 2 = 0.74, s(ŷ) = 1.77 g m −2 d −1 ) and the cultivated (R 2 = 0.48, s(ŷ) = 1.10 g m −2 d −1 ) sites. Temperature had little effect on methane uptake by the forest soil suggesting that gas diffusion was rate limiting. This was further substantiated by the observation that methane uptake showed a strong negative correlation with soil water content (R 2 = 0.79, s(ŷ) = 0.12 mg m −2 d −1 ). The spatial variability for methane uptake in the forest soil was found to be much larger than that previously observed for soil carbon dioxide production but is lower than that reported for nitrous oxide production. For fluxes larger than 0.15 mg m −2 d −1 , the number of sites necessary to estimate the average flux with a precision of 10% (α = 0.05) ranged from 7 to 452. Key words: Greenhouse gas, methane oxidation, soil respiration, spatial variability