Response of a clay loam agricultural soil to mechanical stress from tractor traffic with and without slip

摘要

Agricultural wheeled tractors apply traction force by means of a stress-strain interaction between tyres and topsoil. The traction force and wheel slip of the tractor exert a noticeable effect on the soil stress state, and hence on soil deformation. In this study, changes to soil physical and hydraulic properties owing to deformation caused by the passage of a mechanical front wheel drive (MFWD) tractor (65 kW engine power), both when self-propelled without wheel slip and with high traction force and wheel slip, were compared in a clay loam (Albic luvisol) agricultural field. An analytical soil-tyre interaction model, adapted for a MFWD vehicle, was used to simulate the stress state at the soil-tyre contact surface. Effects of the different stress states on the soil were analysed in terms of changes in total porosity (TP), pore size distribution (PSD), and saturated water conductivity K_(sat). Results clearly showed that the stress state at the soil-tyre contact significantly rose when the tractor moved with slip than when it moved without slip, mostly in terms of shear stress (from 6.0 to 61.6 kPa at the soil-rear tyre contact surface). As a consequence, the severity of soil degradation owing to tractor traffic increased significantly. Changes in soil structure and soil hydraulic properties were more pronounced in the first 0.15 m, where TP decreased from 53.8% to 43% and macro-pores (Φ> 50μm) fell from 2% to 0% due to the effect of high wheel slip. The K_(sat) at soil surface (0-0.04 m depth) decreased from 10.0 mm/h in the trafficked site without slip, to 0.5 mm/h in the trafficked site with slip, with no significant reduction below this depth. Because macropores play a key role in controlling water movement in the soil and in reducing surface runoff, the change in structure and degradation of hydraulic properties exert a great impact on soil erosion and soil management. The considerable influence of the tractor working condition on soil deformation and degradation needs to gain more attention in the future in order to better develop a proper soil management and prevent soil erosion.