Soil erodibility influenced by natural restoration time of abandoned farmland on the Loess Plateau of China

Natural restoration age of abandoned farmlands has significant effects on near soil surface characteristics and thus affects soil erodibility. However, few studies have been conducted to investigate the potential effects of natural restoration age on soil erodibility on the Loess Plateau where many slope farmlands have been abandoned for soil erosion control in the past decades. This study was performed to quantify the effects of natural restoration age on soil erodibility reflected by soil cohesion (Coh), saturated conductivity (Ks), the number of drop impact (NDI), the mean weight diameter of soil aggregates (MWD), soil penetration resistance (PR), and soil erodibility K factor. One slope farmland (as the control) and six abandoned farmlands restored for 3 to 33 years were selected for soil indicators measurements. A weighted summation method was used to produce one comprehensive soil erodibility index (CSEI) to demonstrate comprehensively the temporal variation in soil erodibility with natural restoration age. The results showed that Coh, Ks, NDI, and MWD increased generally when the restoration age < 25 years, and then they were relatively stable till the restoration age of 33 years. While the PR and K decreased gradually with restoration age, and tended to stabilize after 19 years abandonment. CSEI decreased generally with natural restoration age and gradually leveled off after restored for 25 years. Compared to the control, soil erodibility (reflected by CSEI) of abandoned farmlands restored for 3, 6, 12, 19, 25, and 33 years decreased on average by 15.9%, 44.6%, 59.4%, 82.7%, 96.5%, and 100%, respectively. The temporal variation in soil erodibility was controlled greatly by the changes in biological soil crust thickness, plant litter density, root mass density, bulk density, texture and organic matter content driven by natural restoration. The natural restoration is an effective measure for decreasing soil erodibility on the Loess Plateau.