Asymmetric negative effects of precipitation changes on soil respiration of cryptogamic biocrusts in semiarid ecosystems

Global climate change is projected to cause a dramatic increase in precipitation uncertainty (increased or decreased precipitation) in the future, particularly in semiarid ecosystems. Biocrusts are a critical surface cover in semiarid regions, occupying 12% of the global land surface. They perform various ecological functions by influencing soil properties, such as regulating soil water and nutrient cycles, carbon and nitrogen sequestration, biodiversity, and vegetation recovery—effectively impacting nearly all surface ecological processes. Notably, they influence soil carbon emissions through respiration, thereby regulating the carbon balance in drylands. However, the patterns and mechanisms by which biocrust soil respiration responds to precipitation changes under semiarid climates remain unclear. Our precipitation manipulation experiment (–50%, –30%, –10%, +10%, +30%, and +50% of CK) conducted on the Chinese Loess Plateau revealed that increased precipitation (+10% to +50%) suppressed biocrust formation, while moderate precipitation (–10% and –30%) reduction promoted biocrust development. Compared to natural precipitation, increased precipitation (+10% to +50%) reduced biocrust respiration rates by 8.9%–22.1%. Conversely, moderate precipitation reductions (–10% and –30%) enhanced biocrust respiration rates, whereas extreme drought stress (–50%) suppressed these rates. Therefore, the response of biocrust soil respiration to precipitation changes exhibits a negative asymmetry effect. Our structural equation model further indicates that soil temperature and biocrust traits are the primary factors influencing the response of biocrust soil respiration to precipitation variations. These findings suggest that intensified precipitation variability driven by future global climate change may positively impact the stability of soil carbon stocks contributed by biocrusts in semiarid regions, thereby reducing dryland soil carbon emissions.