Rochette, P., Desjardins, R. L. andPatteyE. 1991. Spatialandtemporalvariabilityof soilrespiration in agricultural frelds. Can J. Soil Sci. 7l: [189][190][191][192][193][194][195][196]. Chamber measurements of CO2 evolution were made on bare soil, and in maize (1988) and wheat (1989)

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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 CO2 emissions and CH4 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 CO2 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 CO2 fluxes and soil temperature for the forest (R2 = 0.74, s(ŷ) = 1.77 g m−2 d−1) and the cultivated (R2 = 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 (R2 = 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

Soil respiration is an important component of the net carbon dioxide exchange between agricultural ecosystems and the atmosphere, and reliable estimates of soil respiration are required in carbon balance studies. Most of the field measurements of soil respiration reported in the literature have been made using alkali traps. The use of portable CO2 analysers in dynamic closed chamber systems is recent. The introduction of this new technique requires its evaluation against existing methods in order to compare new information with older data. Nine intercomparisons between dynamic systems and alkali traps were made. Measurements of Fc,s obtained by both chambers showed a good agreement in all but two comparisons in which alkali trap measurements were lower than the dynamic chamber by about 22%. This first report of agreement between both techniques suggests that many measurements made in the past using alkali traps may be comparable to the measurements made more recently using the dynamic chambers. Analysis of the soil temperature and CO2 concentration inside the alkali traps failed to explain why the alkali traps occasionally underestimated the fluxes. Soil respiration measured with a dynamic closed chamber were also compared to eddy-correlation measurements. The results did not reveal any consistent bias between techniques but the scattering was large. This dispersion is likely the result of the difference between the areas measured by the two techniques. Key words: Carbon dioxide, greenhouse gases, CO2 flux, soil carbon

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