Human activities weaken the positive effects of soil abiotic factors and biodiversity on ecosystem multifunctionality more than drought: A case study in China's West Liao River Basin.

Watershed-scale ecosystem biodiversity has been adversely affected by human disturbances and climate change for many years, leading to degradation of ecological functions (i.e., decreased ecosystem multifunctionality, EMF). However, the driving factors and their mechanisms are unclear. Here, we analyzed the effects of human activities, climate, biodiversity, and soil abiotic factors on EMF in China's West Liao River Basin along a natural drought intensity gradient. The beneficial effects of biodiversity on EMF were influenced by the drought intensity; biodiversity increased plant density in humid zones, plant diversity in semi-arid zones, and soil microbial diversity in arid zones, thereby improving watershed EMF and indicating that drought determines the direction and strength of the effects of biodiversity on EMF. The relative abundances of soil microbial keystone taxa such as Actinomycetes and Gemmatimonadetes were the most important predictors of EMF. These results indicate that any loss of plant community diversity or plant density, soil microbial diversity, and the abundance of keystone microbial taxa could reduce EMF. Human activities and drought directly decreased EMF, but also indirectly reduced EMF by reducing soil pH and soil water content (SWC), plant diversity. As human activity increases, EMF's sensitivity to drought increases, and this implies that in regions with high levels of human activity, the effects of climate warming on EMF may be greater than expected. Overall, human activities (including direct, indirect, and total effects) are primary drivers of changes of biodiversity and soil abiotic factors in watershed ecosystems, and regulate the watershed's EMF. The results provide new insights to improve predictions of the direction, magnitude, and regulation mechanisms of EMF and its responses to global climate change.