Dynamics of soil surface Bulk Density: Role of Water Table Elevation and Rainfall Duration

Measurements of soil bulk density profi les combined with thin-section analysis have been suggested to assess the structural seedbed degradation caused by rainfall. The effects of water table elevation and rainfall duration on surface sealing and seedbed slumping were studied on a repacked silt loam soil. Two initial water table elevations (0.3 and 0.7 m below the soil surface)and three simulated rainfall durations (15, 30, and 40 min at 30.5 mm h−1 followed by 180 min at 7 mm h−1) were used. Seedbed bulk density profi les were generated using x-radiography of resin-impregnated soil slices. Macroporosity measurements using image analysis and thin-section observations showed that infi lling of eroded particles in interaggregate voids and compaction of the infi lled particles were the main sealing processes. Below the seal, the seedbed exhibited coalescence and welding of aggregates into larger units, which affected mainly macroporosity. A model of sealing, exponential decrease in bulk density with depth, and slumping, linear increase in bulk density with depth, adequately reproduced the measured bulk density profi les (regression RMSE range 0.0570.106 Mg m3). The change in surface bulk density increased with rainfall duration, whereas this factor did not signifi cantly affect slumping. The highest initial water table elevation led to the highest soil surface and internal seedbed bulk densities. It was suggested that high values of soil water content led to a decrease in aggregate cohesion. Moreover, the number of wetting and drying cycles and the water content during these cycles were shown to increase the magnitude of slumping.