Characteristics and controls of solute transport under different conditions of soil texture and vegetation type in the water-wind erosion crisscross region of China's Loess Plateau.

The analysis of solute transport characteristics in soil is of great significance in understanding nutrient cycling and pollutant migration in the Earth's Critical Zone. The objective of this study was to investigate the transport characteristics and the influencing factors of Cl- in soils with different textures (sandy-S and loamy-L), and covered by different vegetation types (arbor-AR, shrub-SH and grass-GR) in the water-wind erosion crisscross region of the northern Loess Plateau of China. Results showed that the initial penetration time (TS: 12-80 min), entire penetration time (TE: 75-480 min), average flow velocity in the pore (V: 0.52-1.98 cm h-1) and the hydrodynamic diffusion coefficient (D: 0.75-2.55 cm2 h-1) of Cl- varied with different soil textures and vegetation types, and at different soil depths. The V and D associated with Cl- transport were highest in the 0-20 cm soil layer and decreased with increasing depth, while the opposite trend was observed for TS and TE. For the 0-1 m soil profile of the same texture but covered by different vegetation types, the average V and D followed the order of S-AR > S-GR > S-SH and L-AR > L-SH > L-GR, while the average TS and TE exhibited the exact opposite order. This behavior is caused by the varying distributions of root biomass under different vegetation types that affect the number of macropores, the connectivity density and the preferential flow paths in the soil. For the 0-1 m soil profiles of different textures covered by the same vegetation type, the average V and D followed the order of S-AR > L-AR; S-SH > L-SH; and S-GR > L-GR, while the average TS and TE showed the opposite trend. This is because the pore size and distribution in soil are significantly affected by soil mechanical composition. There are significant correlations between soil properties (e.g., bulk density, number of macropores, pore connectivity density, saturated hydraulic conductivity, soil organic carbon content and particle composition) and the transport parameters (e.g., V, TS, and TE). The pedotransfer functions using readily available soil properties can adequately predict V of Cl- transport under different conditions of soil texture and vegetation type. These results provide guidance for the rational configuration of artificial vegetation in different textural soils with respect to reduce nutrient loss and improve ecosystem functions in the northern Loess Plateau of China.