Abstract
Under rainfall conditions, aggregates may suffer breakdown by different mechanisms. Slaking is a very efficient breakdown mechanism. However, its occurrence under rainfall conditions has not been demonstrated. Therefore, the aim of this study was to evaluate the occurrence of slaking under rain. Two soils with silt loam (SL) and clay loam (CL) textures were analyzed. Two classes of aggregates were utilized: 1–3 mm and 3–5 mm. The aggregates were submitted to stability tests and to high intensity (90 mm·h−1) and low intensity (28 mm·h−1) rainfalls, and different kinetic energy impacts (large and small raindrops) using a rainfall simulator. The fragment size distributions were determined both after the stability tests and rainfall simulations, with the calculation of the mean weighted diameter (MWD). After the stability tests the SL presented smaller MWDs for all stability tests when compared to the CL. In both soils the lowest MWD was obtained using the fast wetting test, showing they were sensitive to slaking. For both soils and the two aggregate classes evaluated, the MWDs were recorded from the early beginning of the rainfall event under the four rainfall conditions. The occurrence of slaking in the evaluated soils was not verified under the simulated rainfall conditions studied. The early disaggregation was strongly related to the cumulative kinetic energy, advocating for the occurrence of mechanical breakdown. Because slaking requires a very high wetting rate on initially dry aggregates, it seems unlikely to occur under field conditions, except perhaps for furrow irrigation.
Highlights
Under rainfall conditions, the seedbed structure changes due to aggregate breakdown, leading to the development of a crust, and facilitating overland flow and erosion
The silt loam (SL) presented a smaller mean weighted diameter (MWD) for all stability tests when compared to the clay loam (CL)
The CL 3–5 mm size aggregates were more sensitive to FW than to mechanical breakdown (MB) and SW, while the CL 1–3 mm size aggregates were more sensitive to MB and SW than to FW
Summary
The seedbed structure changes due to aggregate breakdown, leading to the development of a crust, and facilitating overland flow and erosion. Rainfall erosivity depends mainly on rain intensity, raindrop size distribution, raindrop impact velocity and kinetic energy [1,2,3,4]. The main rain characteristics responsible for soil disaggregation are kinetic energy and rainfall intensity. These characteristics were combined together to produce erosivity indexes, such as EI30 [5]. Aggregate breakdown depend on the characteristics of the soil such as texture, mineralogy, organic matter, initial moisture and initial aggregate size distribution, aggregate size and stability, vegetation cover and soil management [6,7,8,9,10,11,12]
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