Slumping dynamics in tilled sandy soils under natural rainfall and experimental flooding

Abstract

Compaction of tilled layers under the single effect of rainfall or irrigation was called slumping. Slumping affects strongly root development and plant biomass production. It has been observed in different soil types, but sandy soils appear particularly prone to this physical degradation. Our objectives in this study were (i) to measure in the field the changes in soil structure and water status simultaneously, (ii) to study the effects of rainfall and management practices on slumping, and (iii) to propose a conceptual model for sandy soil slumping. An experimental site was selected in Northeast Thailand and we studied the effect of tillage depth and initial water content on slumping dynamic. Plots (9 m × 15 m each) were tilled at (i) two depths (20 and 40 cm, called S and D respectively) in dry conditions, (ii) at 20 cm depth in dry or wet conditions (called Y and W respectively). These plots were submitted to natural rainfall for 20 or 61 days to get different total rainfall amounts (114 and 212 mm respectively). In addition, smaller plots (0.24 m 2 each) were used for experimental flooding irrigation (similar to measured rainfalls, i.e. 100 and 200 mm). Soil bulk density, soil surface elevation, soil water content and matric potential were measured. A decrease in soil elevation was observed in all treatments. In the absence of erosion it was interpreted as a loss of porosity which resulted from slumping. Bulk density increased in all layers of the tilled profile (from 1.38 to 1.57 g cm −3). In the surface layer (0–5 cm) this increase was systematically higher compared to deeper layer. No significant difference in final bulk density was found between the S and W treatments, and between the Y and W treatments. Bulk density increased more rapidly in the Y and W treatments than in the S and D treatments, even though the cumulative rainfall was lower. After the flooding experiments, bulk density was higher than after natural rainfall despite similar amounts of water added to the soil.