Abstract
Abstract: Atmospheric nitrogen deposition is one of the most concerned issues in global changes. Grassland communities respond significantly to nitrogen deposition and then have influences on soil erosion. In this study, the typical zonal grass species (Bothriochloa ischaemum (Linn.) Keng) on the Loess Plateau was selected as the research object. The nitrogen deposition process was simulated through nitrogen additions, and the influences of seasonal changes of vegetation communities on overland flow hydrodynamics were explored by the artificial simulated rainfall method. This experiment was conducted at the Institute of Soil and Water Conservation, CAS & MWR (108°04′27.95″ E and 34°16′56.24″ N), Yangling City, Shaanxi Province, China in 2016. Five treatments with different nitrogen addition levels of 0, 2.5, 5, 10 g/(m2·a) (based on N) were designed and they were regarded as N0, N2.5, N5 and N10, respectively. All the treatments were subjected to simulated rainfall under three rainfall intensities of 60, 90 and 120 mm/h monthly from June to September. In addition, from May to September, the coverage of Bothriochloa ischaemum (Linn.) Keng and biological soil crusts before the first rainfall test of each month were monitored. During the rainfall, the mixed samples of runoff and sediment were collected when the flow velocity became stable, and the surface flow velocity and water temperature were measured at the same time. According to the formula of open channel flow, the hydrodynamic parameters such as flow discharge, mean flow velocity, water depth, Reynolds number, Froude number, Darcy-Weisbach resistance coefficient, Manning's roughness coefficient, flow shear stress, stream power and unit energy of water-carrying section were calculated. The results showed that nitrogen additions promoted the coverage of Bothriochloa ischaemum (Linn.) Keng and algal crusts, but inhibited the moss crusts. Compared with no nitrogen treatment (N0), the overland flow resistances values of treatments of N2.5, N5 and N10 were significantly decreased by 68.6% to 71.5% (Darcy-Weisbach resistance coefficient) and by 44.7% to 47.4% (Manning's Roughness coefficient). The mean flow velocity values were accelerated by 32.0% to 44.0% and water depth values were reduced by 25.1% to 28.7%. The flow shear stress and stream power values were increased significantly by 228.7% to 327.4% and 313.5% to 543.2%, respectively. The unit energy of water-carrying section values was reduced by 24.4% to 27.9%. With seasons changing, the resistance values to overland flow in September were increased significantly by 220.2% to 496.9% (Darcy-Weisbach resistance coefficient) and 79.5% to 139.4% (Manning's roughness coefficient), compared with the early-mid period (from June to August). Furthermore, the mean flow velocity values were decreased by 23.5% to 29.7%, the water depth values increased by 36.4% to 66.9%. The flow shear stress and stream power values were decreased by 97.7% to 99.4% and 98.1% to 99.7%, respectively, and the unit energy of water-carrying section values was increased by 20.8% to 64.2%. Moreover, overland flow resistance was decreased with the increase of rainfall intensity, which promoted mean flow velocity, water depth, and stream power. All in all, with the nitrogen addition increasing, the overland flow resistance was significantly reduced, the flow velocity was accelerated, the water depth was decreased, the flow shear stress and stream power were significantly increased, and the unit energy of water-carrying section was reduced. Grassland plays vital roles in regulating overland flow. However, the increase of atmospheric nitrogen deposition may exacerbate soil erosion on the slope. Overall, the results can provide a scientific instruction for grassland vegetation construction and soil erosion control in the Loess Plateau.
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