Roles of soil microbial community in driving soil multifunctionality in Chinese red pine (Pinus tabuliformis) forests across a soil pH gradient

Soil pH has long recognized as a key driver of microbial community assembly and individual soil functions, but its roles in linking microbial features to overall functioning of soils remain largely unknown. We collected soil samples from plots inside and outside 16 natural forests dominated by Chinese pine ( Pinus tabuliformis ) along a 2000 km transect that covers a broad soil pH gradient. We aimed to examine how the relationships between microbial community characteristics and soil multifunctionality (SMF) vary with increasing soil pH. The SMF overall showed a bell-shaped curve along increasing soil pH, exhibiting the highest values at pH = 7.76 for inside-forest soil and at pH = 7.66 for outside-forest soil, respectively. Such threshold impacts of soil pH on SMF were also observed on soil microbial diversity, but directions of the impacts were opposite for outside-forest soil and inside-forest soil. The SMF was positively correlated with microbial α-diversity but negatively with microbial β-diversity in outside-forest soil, and these correlations were particularly pronounced in the soil with pH higher than the corresponding pH threshold. SMF increased with network complexity of microbial community in both inside- and outside-forest soils, but only when the soils had pH higher than the thresholds. Inside forests, soil pH explained 32% and 60% of SMF variation below and above the threshold, respectively, while outside forests these values were 72% and 75%. Soil pH contributed to SMF of inside-forest soil in a direct manner, while in outside-forest soil the contribution of soil pH occurred indirectly by regulating the assembly processes of soil microbial community. Conclusively, our study reveals threshold effects of soil pH on soil microbial community and multifunctionality as well as their relationships, and the difference in these effects between outside- vs. inside-forest soils highlights the important roles of natural pine forests in influencing overall soil functioning.