Contrasting surface soil hydrology regulated by biological and physical soil crusts for patchy grass in the high-altitude alpine steppe ecosystem

Abstract

How the presence of biological soil crusts (BSCs) and physical soil crusts (PSCs) regulates surface soil hydrological processes in the high-altitude alpine ecosystems is still a pressing scientific issue owing to a lack of convincing field observation data. This study highlights the interesting phenomenon whereby BSCs and PSCs are jointly distributed in opposite orientations under a canopy of patchy Achnatherum splendens grass on the alpine steppe of the Qinghai Lake watershed (Qinghai-Tibet Plateau, northwestern China). The vegetation characteristics (plant species, cover, and biomass) and soil properties (volumetric soil water content, hydraulic conductivity, temperature, salinity, and macroporosity) were investigated for BSCs zones, PSCs zones, and interpatch zones (IPZs) in the A. splendens patches (ASPs) system. The experimental results show that BSCs increased infiltration during wet periods by increasing significantly greater hydraulic conductivity and macroporosity (p < 0.01), and reduced soil evaporation by maintaining a lower soil temperature than PSCs and IPZs during dry periods. BSCs had an important role in improving water retention in ASPs and thereby provided highly favorable conditions for plant recruitment and survival. However, PSCs reduced the infiltration capacity of surface soils and were suspected of generating runoff after intense rainfall events, which might promote connectivity of the horizontal water flux for ASPs. PSCs had a negative effect on the vegetation community by creating a saline environment (e.g. 28-fold higher salinity than for BSC zones and IPZs at 5 cm depth) for other herbaceous plants. The predominant wind direction, canopy effects of A. splendens on local microclimate, and salt accumulation were identified as the main factors determining the opposite distribution of BSCs and PSCs. The results suggest that BSCs and PSCs play contrasting roles in soil hydrology in ASPs, and provide valuable insights for understanding the spatial distribution of soil crusts and vascular plants as well as their interactions in high-altitude alpine steppe ecosystems.