Rill Erosion and Soil Quality in Forest and Deforested Ecosystems with Different Morphological Characteristics

Rill detachment capacity is a key parameter in concentrated flow erosion. Rill erosion generally turns into gully erosion with severe environmental impacts. Changes in land use and human activities can have heavy effects in rill formation, particularly in forests subject to deforestation; soil morphology plays a significant role in these effects. However, literature reports few studies about rill detachment rates and their implications on soil quality in forest and deforested soils with different morphological characteristics. To fill these gaps, this study has evaluated the rill detachment capacity (Dc) and the main soil quality indicators in three areas (upper, middle and lower slope) of forest and deforested (for 10 years) hillslopes exposed to the north and south in Northern Iran. The variations of Dc have been measured on soil samples under laboratory conditions through a flume experiment at three slope gradients (12 to 19%) and five flow rates (0.22 to 0.67 L m−1 s−1) with four replications. The large and significant (p < 0.05) difference (about 70%) detected for Dc between forest and deforested hillslopes was associated to the higher organic matter content of forest areas; as a consequence, these areas also showed higher aggregate stability, porosity, root weight density, microbial respiration and available water. In the deforested hillslopes exposed to the south, the soil erodibility was higher by 12% compared to those exposed to the north. The differences in the monitored soil quality indicators were instead less noticeable and not always significant (p < 0.05). Conversely, Dc did not significantly change (p < 0.05) among the upper, middle and lower hillslope areas investigated in this study. Simple but accurate models to predict the rill detachment capacity, erodibility and critical shear stress of soils from indicators of soil quality or the unit stream power using regression equations are suggested. Overall, the results can support land planners in prioritizing the actions for soil conservation in deforested hillslopes exposed to the south as well as in the extensive application of the proposed equations in erosion prediction models.