Biocrusts intensify grassland evapotranspiration through increasing evaporation and reducing transpiration in a semi-arid ecosystem

Abstract

Biological soil crusts (biocrusts) are living ground covers that result from intimate associations between soil particles and organisms, which can modify soil properties and support fundamental ecosystem functions in drylands. Despite numerous studies concerning effects of biocrusts on soil hydrological processes, their impacts on grassland evapotranspiration have remained unanswered. By using micro-lysimeters, we conducted in-situ monitoring of evapotranspiration of bare soil vs. cyanobacterial-moss mixed biocrusts, gramineous grass vs. gramineous grass with biocrusts, and leguminous grass vs. leguminous grass with biocrusts over two months in a semi-arid ecosystem. Results suggested that compared to the bare soil, the biocrusts significantly increased soil evaporation rate by 20.9 %. Underneath gramineous and leguminous grasses, the biocrusts increased the average evapotranspiration rate by 9.4 % and 13.4 %. The evaporation of biocrusts contributed 8.1–10.6 % to the average evapotranspiration rate and 8.2–12.6 % to the cumulative evapotranspiration amount. Additionally, the biocrusts increased soil moisture by 12.5 % and decreased temperature by 3.4 % at 10 cm depth compared to the bare soil. However, the biocrusts beneath grasses led to a reduction in soil moisture by 10.0 % and 20.0 %, while causing an increase in temperature by 9.9 % and 2.9 %, respectively. Enhanced evaporation of the biocrusts could be mainly attributed to their higher water retention ability in comparison to the bare soil. Meanwhile, beneath grasses, the biocrusts decreased soil water content in deeper layers, potentially limited grass transpiration by reducing water availability for grass roots. Based on these results, we concluded that the biocrusts intensify grassland evapotranspiration through increasing soil evaporation and reducing grass transpiration. Our findings highlight that biocrusts play a crucial role in modulating surface water fluxes, potentially exert remarkable effects on dryland vascular plants development and ecosystem stability.