Investigating Aggregates Stability Against Raindrops Impact in Some Soils of a Semi-Arid Region, North West of Zanjan

Aggregate stability is one of the most important soil physical properties which can affect the soil disruption by raindrops. This research was conducted to study the aggregates breakdown under raindrops impact and its relationship with aggregate stability in some semi-arid region soils of Zanjan province, Iran. Thirty soils samples with three replications were taken from thirty dry-farming lands at North West of Zanjan in 2011. Aggregate samples with diameters of 6-8 mm were taken from each farm soil and put in a flat box with dimensions of 24 cm × 30 cm and 6-cm depth. The soil boxes were separately affected by four simulated rainfall events with intensity of 60 mm h^-1 for 30-min duration. The amount of aggregate breakdown in each soil box under each rainfall event was then determined by comparing of the mean weight diameter (MWD) of soil aggregates before and after each rainfall event. Aggregates stability was determined using two methods: wet-sieving and water-drop test. Results indicated that the amount of aggregate breakdown on the basis of MWD values significantly varied among 30 soil samples (p£ 0.001), due to rainfall impact. Aggregate breakdown had no significant correlation with the aggregate stability determined using the wet-sieving method, whereas it was negatively correlated to the aggregate stability obtained based on the water-drop test method (R²= 0.29, p£ 0.05). Aggregate breakdown in the soils was significantly related to geometric mean diameter of mineral particles, gravel and organic matter (R²= 0.65, p£ 0.001).