Multiple dimensions of phylogenetic diversity are needed to explain the complex aboveground-belowground diversity relationships

The complex relationship between aboveground and belowground diversity and whether they act as surrogates for one another remains unresolved. Increasing evidence suggests that investigating phylogenetic diversity could provide valuable insights into the interplay between plants and soil microbes, but the proliferation of phylogenetic diversity metrics has hindered comparative studies and the identification of general patterns. To overcome this challenge, we implemented a multi-dimensional framework that classifies phylogenetic diversity metrics into three dimensions: richness, divergence, and regularity, each of which captures different ecological aspects of species differences. Then we applied this framework to investigate the relationship between above and belowground diversity in a subtropical forest in Eastern China. We found that phylogenetic diversity of plant and soil microbes, including bacteria and fungi, were more strongly correlated at the richness and regularity dimension compared with divergence dimension. Further analyses revealed that these observed correlation patterns can be attributed to the influence of soil total phosphorus content, which is the limiting factor of plant and microbial phylogenetic diversity at richness and regularity dimensions. Together, our study demonstrated the necessity of using a multi-dimensional approach to advance our understanding of the complex relationships between plant and soil microbial biodiversity.

Methods
This study was conducted in a subtropical forest located in Ningbo City, Zhejiang Province, Eastern China in 2017, with a subtropical monsoon climate characterized by an average annual temperature of 16.2°C and an average annual precipitation of 1700 mm. Dominant tree species in the area include Pinus massoniana, Schima superba, and Cunninghamia lanceolata. Our study comprised 16 plots situated in three different locations, dominated by various tree species ( 40 – 60-year-old stand), including both unmanaged secondary forests and forests that underwent close-to-nature silviculture treatment. In total, we obtained 16 plots, representing a clear diversity gradient and different composition structure across sites. For each plot, five bulk-soil cores (3.8 cm in diameter, 0-10 cm depth), excluding litter and organic horizons, were collected using a five-point sampling method and placed in separate sterile plastic bags. We obtained a total of 80 soil samples from the 16 plots in October 2019. The plant composition of all plots was investigated in August 2019. Every tree stem with a diameter at breast height (DBH) ≥ 5 cm in the plot was sampled. We found 117 plant species in total across all the study plots, both gymnosperms and angiosperms were included.