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

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 (K s ) 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 K s 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 K s 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; K s was positively correlated with non-capillary porosity (NCP). Relative to other soil properties, soil porosity is the dominant factor influencing FC and K s . Capillary porosity explained 91.1 % of total variance in FC, and NCP explained 97.3 % of total variance in K s . The combined effect of disappearing root activities and increasing sand content was responsible for the inconsistent patterns of NCP and K s . 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.

Root Features Determine the Increasing Proportion of Forbs in Response to Degradation in Alpine Steppe, Tibetan Plateau

“…In the present study, we observed a sharp decrease in SWC with enhanced degradation (Table 1), which is another major negative influence of degradation on soils, consistent with previous findings (Wang et al, 2009;Yi et al, 2012). It may be because retrogression induces decreases in field capacity and saturated hydraulic conductivity (Pan et al, 2017), which play important positive roles in key hydraulic courses and functions, such as water holding capacity, soil water retention, and infiltration (Fu et al, 2015). Additionally, due to the decline in plant cover, more soil areas are exposed to stronger radiation, which induces an increase in evaporation-reducing SWC (Wang et al, 2009).…”

Light Grazing Significantly Reduces Soil Water Storage in Alpine Grasslands on the Qinghai-Tibet Plateau

“…Changes in soil particle size and aggregate structure also contribute to soil desiccation. Even in the extremely degraded grasslands that have been reclaimed for conversion into artificial grasslands, there is no significant improvement in water-retention capacity [9]. With an increase in grazing intensity, the soil saturated moisture capacity, the capillary moisture capacity, and the field moisture capacity do not increase linearly; they all increase initially but then decrease, with a peak at the stage of moderate degradation.…”

Approaches for Estimating Soil Water Retention Curves at Various Bulk Densities With the Extended Van Genuchten Model

“…The slope of the soil water retention curve (WRC) was also found to be significantly influenced by ρ b (Dexter, 2004a). In addition, soil water-holding capacity and infiltration rate and capacity also exhibited complex responses to variations in ρ b caused by traffic compaction, tillage, topographic variability, or rainfall-induced surface sealing (Assouline, 2004;Mohammadshirazi et al, 2016;Pan et al, 2017;Strudley et al, 2008). It is necessary to relate soil hydraulic properties to ρ b for improved understanding and prediction of water, solute, and gas transport in soil.…”