Measurements of soil CO 2 flux in the absence of living plants can be used to evaluate the effectiveness of soil management practices for C sequestration, but field CO 2 flux is spatially variable and may be affected by soil compaction and the percentage of total pore space filled with water (%WFPS). The objectives of our study were: (i) to evaluate the effect of wheel traffic compaction on CO 2 flux at two landscape positions with differing soil properties; and (ii) to examine the relationship of CO 2 flux and %WFPS under field conditions and a wide range of soil porosities. Carbon dioxide flux was measured near Ames, IA, in a no-till system without living plants using the closed chamber method on nine cylinders inserted into the soil at each measurement site and evenly spaced across three rows, an untracked interrow, and a tracked interrow. Flux, volumetric water contents, and soil temperature were measured on 12 or 13 d between day of the year (DOY) 164 and 284 in 2001, 2004, and 2005. Bulk density, soil organic C concentration, and soil texture were determined after DOY 284. On most days, CO 2 flux was less in the tracked interrow than in the row or untracked interrow positions. In all 3 yr, the cumulative flux of the tracked position was significantly less than one or both of the other positions. Landscape position did not affect the response of CO 2 flux to traffic. Percentage water-filled pore space was not a good predictor of surface CO 2 flux in the field. The effect of wheel traffic compaction on CO 2 flux should be considered when soil CO 2 flux is used to compare management practices.
