METHANE AND CARBON-DIOXIDE FLUXES FROM POORLY DRAINED ADJACENT CULTIVATED AND FOREST SITES

Methane and carbon dioxide fluxes at the soil surface were measured fr om April to November 1992 in Ottawa, on adjacent cultivated (com) and forest (temperate woodland) sites using closed chambers (10 chambers p er site). The objectives were to quantify the spatial and temporal var iability of gas exchange rates, and to determine the effects of soil t emperature and moisture on the fluxes. On the forest soil, rates of CO 2 emissions and CH4 uptake ranged from 2.27 to 14.82 g m-2 d-1 and fro m 0.04 to 1.10 mg m-2 d-1, respectively. On the cultivated soil, the m easured CO2 fluxes varied from 0.27 to 7.07 g m-2 d-1 while methane up take ranged from 0 to 0.13 mg m-2 d-1. There was a positive correlatio n between soil surface CO2 fluxes and soil temperature for the forest (R2 = 0.74, s(y) = 1.77 g m-2 d-1) and the cultivated (R2 = 0.48, s(y) = 1.10 g m-2 d-1) sites. Temperature had little effect on methane upt ake by the forest soil suggesting that gas diffusion was rate limiting . This was further substantiated by the observation that methane uptak e showed a strong negative correlation with soil water content (R2 = 0 .79, s(y) = 0.12 Mg M-2 d-1). The spatial variability for methane upta ke in the forest soil was found to be much larger than that previously observed for soil carbon dioxide production but is lower than that re ported 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% (alpha = 0.05) ranged from 7 to 452.