عنوان مقاله [English]
Background and objectives: Soil aggregate stability is an important physical indicator of the soil’s susceptibility to water erosion. Aggregate stability can vary, depending on the aggregate size. Some methods including the dry-sieving, wet-sieving and water-drop test were currently used to evaluate the stability of aggregates in the worldwide. Mean weight diameter of stable aggregates was used for the dry-sieving and wet-siewing method. In the water-drop test, aggregate stability is evaluated using the number of water drops needed for disrupting the aggregates. These indices are used to evaluate the soil structural stability for given size of aggregates. However, there are different sizes of aggregates in the soil. So, application of these indices may cause some errors in evaluating the soil’s susceptibility to water erosion processes. Therefore, this study was conducted to develop a proper aggregate stability for different aggregate sizes in view point of interrill erosion in a semi-arid soil sample.
Materials and Methods: Four aggregate size classes including < 2, 2-4, 4-8, 8-11 mm were collected from an agricultural soil with texture of clay loam in west of Zanjan, north west of Iran. A-600 kg aggregate sample was taken from 0-30 cm surface soil with about 10 m3 in volume for each aggregate sizes by sieving the aggregates in the field. The aggregate samples were packed to the erosion plots with 120 cm × 130 cm in dimensions installed in a 9% uniform slope. A total of twelve plots were investigated using the randomized complete block design for four aggregate size classes with three replications. The plots were exposed to seven simulated rainfalls with 70 mm h-1 in intensity for 30-min with 7-day interval. Soil loss resulted by interrill erosion from each aggregate size was determined during each rainfall simulation. The stability of each aggregate size against mechanical impact (MWDdry), wetting force (MWDwet) and rainderop impact (WDT) was determined using the dry-sieving, wet-sieving and water-drop test methods for each aggregate size class, respectively. Additionally, the aggregate stability per aggregate mass were computed and defined as MWDwet-m, MWDdry-m and WDTm, respectively. Beside this, other physicochemical properties including particle size distribution, gravel, bulk density, saturated hydraulic conductivity, organic carbon and calcium carbonate were determined using the conventional methods in the lab.
Results: Based on the results, significant positive correlations were found between the aggregate size and the stability of aggregates determined using the methods of dry-sieving (r= 0.99), wet-sieving (r= 0.89) and water-drop-test (r= 0.93). The aggregate stability determined using all methods increased with an increase in the aggregate size. Newetheless, evaluating the aggregate stability per aggregate mass indicated that negative correlations existe between the aggregate size and MWDwet-m (r= -0.95), MWDdry-m, (r= -0.88) and WDTm (r= -0.88). Although the coarse aggregates rather than smaller aggregates are resitant against external stresses such as mechanical impacts, wetting force and rainderop impact but their stability per their mass is small. Contrary to our expectation, soil loss by interrill erosion of each aggregate size classes increased with increasing the aggregate stability determined using the dry-sieving, wet-sieving and water-drop-test methods whereas it decreased with increasing the aggregate stability determined using these methods on the basis of the aggregate mass.
Conclusion: This study revealed that MWDwet, MWDdry and WDT are not the proper indices to evaluate the stability of aggregate size with the view point of its resistance to interrill erosion. The aggregate stability determined in these methods per aggregate mas is a new approch to evaluate the susceptibility of various aggregate sizes of a soil to interrill erosion. Among these indices, MWDwet-m is the best indicator in this field.