Soil water response of plant functional groups along an artificial legume grassland succession under semi-arid conditions

Drought can cause and accelerate the degradation of planted legume grassland, and the changes in plant community composition can further influence soil physical properties. Here, the soil water response of different plant functional groups (legumes, grass, and forbs) were studied along a legume grassland degradation succession according to the above-ground biomass and the proportion of each functional group. In particular, changes in the soil water storage (SWS), soil properties (soil capillary porosity (CP), and soil organic matter) and plant traits (proportion of functional groups and below-ground biomass) were quantified in a continuously cultivated legume grassland for fifteen, sixteen, eighteen, and twenty-two years, without any tillage. The experimental plots were located in the semi-arid region of the Chinese Loess Plateau, in a flat area without runoff contribution. All plant and soil samples, and measurements, were obtained in a few days to minimize the temporal variability. SWS increased at 50-150 cm soil depth when the proportion of legume functional group (LFG) decreased from 70% to 5%, and this increment was ca. 20% at 50-100 cm soil layers. The Shannon-Wiener diversity index (H), the species richness (R), LFG, CP, and the non-capillary porosity (NCP) were significantly correlated with SWS (P < 0.05). The infiltration capacity significantly improved when the proportion of LFG accounted for 5%, which favored the recharge of the soil water reservoir with rainfall. Roots decay apparently improved the soil structure, and favored water infiltration, providing a suitable condition for latter vegetation growth. This study underlined the significant interaction that exists between the plant functional groups and the soil water processes during the artificial legume grassland succession. We proved that plant composition can change the surface hydrologic cycle, and the soil water exchange, in rainfed grassland.