Temporal Changes in Soil Surface Elevation under Different Tillage Systems

Abstract

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 ?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.