Changes in rill detachment capacity after deforestation and soil conservation practices in forestlands of Northern Iran

Abstract

Rill erosion is particularly intense after deforestation on hillslopes with steep and long profiles. Since the rill detachment capacity (“Dc”) shows a noticeable variability resulting from different soil properties and vegetation characteristics, there is a need to explore the changes in key soil properties and rill erosion rates under different natural or planted species and a variety of soil conservation practices after deforestation. This study evaluates the changes in four key soil properties (organic carbon content, bulk density and water-stable aggregates of soil, and weight density of plant roots) and rill detachment capacity in forest sites with natural tree species as well as in areas subjected to reforestation and soil conservation treatments in comparison to deforested sites. To this aim, 2000 soil samples have been collected in forestlands of Northern Iran. Sampling was done in deforested areas (assumed as reference condition), and in forest sites 14 plant species (natural or after reforestation). Deforested samples were subjected to five soil conservation treatments (using additives or hydromulching). On all soil samples, Dc has been measured in a laboratory flume at five water flow rates (0.27 to 0.69 L/m s−1) and five soil slopes (5.9 to 31.7 %). All soil properties, when compared to the reference condition, were significantly different (between −50 % to 124 %) among the three soil conditions (natural forests, reforested soils and deforested and treated sites), the natural forests and the treated sites showing a better quality compared to planted forests. Dc was noticeably lower in all conditions (0.022 ± 0.021 kg m−2 s−1) compared to the deforested and untreated sites (0.046 ± 0.023 kg m−2 s−1). Natural and planted forests showed a similar decrease in rill erodibility (−70 to 80 %), while a much lower reduction in Dc (−36 %) was measured for the treated areas. Overall, the study demonstrates that the changes in soil properties due to plant species and soil management as well as the associated variability in rill detachment are noticeably site-specific, and greatly depend on soil conservation treatments. The results of this study may give landscape planners clear indications about the relationships between rill detachment and the associated soil properties among treated and untreated (natural or reforested) soils.