Variation of soil hydraulic properties with alpine grassland degradation in the Eastern Tibetan Plateau

Ecosystems in alpine mountainous regions are usually more vulnerable and easily be disturbed by environmental change globally. Alpine swamp meadow, a unique grassland type in the eastern Tibetan Plateau which provides important ecosystem services to the upstream and downstream regions of international rivers of Asia even the world, is undergoing severe degradation, which can dramatically alter the soil hydraulic properties and water cycling processes. However, the 15 effects of alpine swamp meadow degradation on soil hydraulic properties are still poorly understood, and exiting findings were inconsistent. In this study, soil field capacity (FC) and saturated hydraulic conductivity (Ks) together with several basic soil properties under light degraded (LD), moderate degraded (MD) and severe degraded (SD) alpine swamp meadow were investigated. Then the variation of FC and Ks with alpine swamp meadow degradation and their dominant influencing factors were analysed. The results show that FC decreased consistently from LD to SD, while Ks decreased from LD to MD 20 and then increased from MD to SD, following the order of LD > SD > MD. Significant differences of soil hydraulic properties between degradation degrees were found at upper soil layers (0-20cm), indicating that the influences of degradation were most pronounced in the top soils. FC was positively correlated with capillary porosity (CP), water-stable aggregates (WSA), soil organic carbon (SOC), silt and clay content, and Ks were positively correlated with non-capillary porosity (NCP). Relative to other soil properties, soil porosity is the dominant influencing factor of FC and Ks. CP explained 25 the 91.1% total variance of FC, and NCP explained that by 97.3% for Ks. The combined effect of vanishing root activities and increasing sand content was responsible for the inconsistent patterns of NCP and Ks. Our findings suggest that alpine swamp meadow degradation would inevitably lead to reduced water holding capacity and rainfall infiltration. This study gives out a more comprehensive understanding of the soil hydrological effects of vegetation degradation. Further hydrological modelling researches in Tibetan Plateau and similar regions are recommended to understand the effects of 30 degraded alpine swamp meadow on soil hydraulic properties.