Temporal Changes in Soil Surface Elevation under Different Tillage Systems

Conventional, fixed-depth sampling may lead to errors in quantifying temporal changes in C stocks if soil bulk density (BD) changes. When surface elevation is altered due to a change in BD, sampling depth should be adjusted so that the sampled soil mass remains constant. We evaluated the ability of a topographic survey method (precise level) to quantify temporal changes (2000-2007) in surface elevation following conversion of long-term pasture to arable cropping using conventional tillage (plowing to 20 cm), minimum tillage, or no-till. Measured elevations were compared with values estimated from BD determined for fixed-depth increments (0-7.5, 7.5-15, 15-25, and 25-30 cm). In the first season, soil mass to 25 cm decreased by similar to 270 Mg/ha under conventional cultivation (relative to precultivation mass of 3345 Mg/ha) as BD decreased in the 7.5- to 15- and 15- to 25-cm layers. This represented an upward movement of the soil surface by an estimated 22 mm. Minimum tillage and no-till treatments moved 9 and 2 mm upward, respectively. Subsequently, the soil consolidated and surface elevation declined by an estimated 8 mm in all cultivated treatments. Elevation changes measured by the precise level were very similar to these BD-based estimates. Changes in elevation in conventional, minimum tillage, and no-till treatments averaged 13, 1.5, and -6 mm, respectively (positive values indicate upward movement, negative values subsidence). Corresponding BD-estimated values were 17, 1.1, and -3 mm. Using topographic survey information, sampling depth can be adjusted so that a constant mass is sampled from soils where BD has been changed by tillage or natural processes.