Soil CO
2 concentrations to a depth of 50 cm depth were monitored with Vaisala CO
2 probes from June 2004 to May 2005 in conventionally plowed and no-tillage agricultural plots and found to be generally higher in the former than the latter. Soil CO
2 concentrations were also higher in summer and lower in winter, but were modulated by harvesting and the application of crop residues. Analysis of diurnal variations in soil CO
2 concentrations, soil CO
2 effluxes, and soil temperatures revealed distinct differences between plots. The relative amplitudes of the variation in CO
2 concentrations peaked at a soil depth of 5 cm. Soil CO
2 concentrations were higher when there was higher soil water content after precipitation, while soil CO
2 effluxes were lower under the same circumstances. These two results relate to the lower diffusivity at higher soil water content, which restricts CO
2 efflux, leading to higher soil CO
2 concentrations. Calculations reveal that in the shallower soil layer (0-5 cm), CO
2 production was greater in no-tillage plots, whereas in the deeper layer (15-25 cm), it was greater in conventional plots.
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