Precipitation regulates shrub impacts on soil multifunctionality across soil depths at α- and β-scales in eastern Tibetan alpine grasslands

Global climate change is driving shrub expansion in alpine grassland ecosystems. It is well known that shrubs play a crucial role in alleviating environmental stress and maintaining multiple ecosystem functions, especially under extreme conditions. However, the impacts of shrubs on soil multifunctionality (hereafter “EMF”) at the α- and β-scales, along with the patterns and underlying mechanisms across soil depth and precipitation gradients, remain unclear. Here, we investigated the effects of dominant shrubs on EMF (at α- and β-scales) along a precipitation gradient (five sites ranging from 502 to 739 mm) and across three soil depths (0–15 cm, 15–30 cm, and 30–45 cm), and further identified the driving factors and regulatory mechanisms. We found that both α- and β-EMF significantly decreased with increasing soil depth, and exhibited a hump-shaped distribution along the precipitation. The positive effect of shrubs on α-EMF was only evident under extreme precipitation conditions (either too low or too high), regardless of soil depth. Meanwhile, in surface soil (0–15 cm), shrubs significantly reduced β-EMF at the site with 544 mm of precipitation, whereas in deeper soil layers (30–45 cm), shrubs also led to a significant reduction in β-EMF at sites with 502 mm and 739 mm of precipitation. Shrubs, soil depth, and precipitation affected α-EMF primarily through abiotic pathways (soil moisture and pH), whereas their effects on β-EMF were mediated through both abiotic pathways (soil moisture and pH heterogeneity) and biotic pathways (bacterial β-diversity). Our study provides a comprehensive understanding of shrub impacts on ecosystem functions at different scales across various soil depths in alpine ecosystems, and emphasizes the regulatory role of precipitation, enhancing our understanding of the potential impacts of shrub expansion.