Effects of nitrogen addition on the soil detachment in the typical grasslands of the Loess Plateau

Nitrogen deposition will alleviate the nitrogen limitation in terrestrial ecosystems and greatly affect vegetation growth, thereby soil erosion. It is important to clarify the effects of nitrogen addition to the plant roots and soil properties on the soil erosion process. A nitrogen addition experiment was conducted in the grassland dominated by Bothriochloa ischaemum (Linn.) Keng (BI), which has received 0, 2.5, 5, and 10 g N m^-2 yr^-1 (N₀, N_2.5, N₅ and N₁₀, respectively) for three consecutive years. Then, a total of 150 undisturbed soil samples were collected (including bare soil control) and subjected to flowing water to test their soil detachment capacities under six shear stress levels (10.2 Pa to 29.9 Pa). Three-year nitrogen addition increased the soil bulk density, soil cohesion and nitrate nitrogen while decreasing the saturated hydraulic conductivity, soil water-stable aggregates, soil organic carbon, total nitrogen and ammonium nitrogen. The root mass density and root diameter decreased with nitrogen addition. And the root length, surface area and volume density of the N₀ and N₅ treatments were larger than those of the other treatments, while the plant roots were significantly inhibited by N₁₀. Additionally, the soil detachment capacity (D_c) and rill erodibility (K_r) of the N₀ and N₅ treatments were significantly less than those of the N_2.5 and N₁₀ treatments, of which the D_c (0.020 kg m^-2 s^-1) of the N₀ treatment was 69.0%, 24.3% and 66.8% less than that of the N_2.5, N₅ and N₁₀ treatments, respectively. The K_r of the N₀ treatment was 0.0012 s m^-1, which was 72.1%, 25.0% and 70.0% less than that of the others. This study implies that an increase in nitrogen addition likely exacerbates soil erosion in the early (approximately 2.5 g N m^-2 yr^-1) and late phases (more than 10 g N m^-2 yr^-1). However, when the nitrogen addition rate is approximately 5 g m^-2 yr^-1, soil erosion may be inhibited because of the responses of the plant roots and soil to nitrogen addition.