Abstract

Abstract. Ecosystems in alpine mountainous regions are vulnerable and easily disturbed by global environmental change. Alpine swamp meadow, a unique grassland type in the eastern Tibetan Plateau that provides important ecosystem services to the upstream and downstream regions of international rivers of Asia and other parts of the world, is undergoing severe degradation, which can dramatically alter soil hydraulic properties and water cycling processes. However, the effects of alpine swamp meadow degradation on soil hydraulic properties and the corresponding influencing mechanisms are still poorly understood. In this study, soil moisture content (SMC), field capacity (FC) and saturated hydraulic conductivity (Ks) together with several basic soil properties under lightly degraded (LD), moderately degraded (MD) and severely degraded (SD) alpine swamp meadow were investigated; the variations in SMC, FC and Ks with alpine swamp meadow degradation and their dominant influencing factors were analysed. The results showed that SMC and FC decreased consistently from LD to SD, while Ks decreased from LD to MD and then increased from MD to SD, following the order of LD > SD > MD. Significant differences in soil hydraulic properties between degradation degrees were found in the upper soil layers (0–20 cm), indicating that the influences of degradation were most pronounced in the topsoils. FC was positively correlated with capillary porosity, water-stable aggregates, soil organic carbon, and silt and clay content; Ks was positively correlated with non-capillary porosity (NCP). Relative to other soil properties, soil porosity is the dominant factor influencing FC and Ks. Capillary porosity explained 91.1 % of total variance in FC, and NCP explained 97.3 % of total variance in Ks. The combined effect of disappearing 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 provides a more comprehensive understanding of the soil hydrological effects of vegetation degradation. Further hydrological modelling studies in the Tibetan Plateau and similar regions are recommended to understand the effects of degraded alpine swamp meadows on soil hydraulic properties.

Highlights

  • Soil moisture plays a critical role in land surface processes and hydrological cycles

  • Field capacity (FC) was positively correlated with capillary porosity (CP), water-stable aggregates (WSAs), Soil organic carbon (SOC), and silt and clay content, but was negatively correlated with bulk density (BD) and sand content

  • Based on nine plots representing alpine swamp meadow of different degradation degrees, this study mainly investigated the changes in soil hydraulic properties with alpine swamp meadow degradation, and analysed the influencing mechanism of grassland degradation on FC and saturated hydraulic conductivity (Ks)

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Summary

Introduction

Soil moisture plays a critical role in land surface processes and hydrological cycles. It directly participates in soil hydrological processes and influences vegetation growth and even modifies weather processes and local climate (Legates et al, 2011; Shein, 2010; Vereecken et al, 2015). Field capacity (FC) and saturated hydraulic conductivity (Ks) are two key soil hydraulic properties that jointly affect soil water storage, transmission and distribution (Cassel and Nielsen, 1986; Marshall et al, 2014). Knowledge of how FC and Ks vary and of their influencing factors is essential for a better understanding of soil hydrological processes. T. Pan et al.: Variation of soil hydraulic properties with alpine grassland degradation in Tibetan Plateau

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