COS 29-1 - Plant phylogeny and functional traits shape soil biogeochemistry in an experimental tallgrass prairie

Abstract: Ecosystem functioning increases with plant species diversity, in part because more speciose communities contain more functional and phylogenetic groups than lower diversity communities, thereby occupying more niche space. However, the relative roles of species richness, phylogenetic diversity, and functional diversity have proven difficult to separate. Plant phylogeny and functional traits are frequently entangled as closely related species often share traits, resulting in phylogenetic and functional group classifications that are one and the same. Therefore, in 2016, we established a tallgrass prairie experiment in which we explicitly manipulated plant functional and phylogenetic diversity in 15 species polycultures, using a diverse suite of 127 species also grown in monocultures. In 2021, we surveyed the plant communities and collected soil samples to evaluate the effects of phylogenetic and functional diversity on soil carbon (C), nitrogen (N), and phosphorus pools and fluxes. We also evaluated the extent to which the relative abundance of traditional functional groups – graminoids, forbs, small shrubs, N-fixers, and monocots – could predict variability in soil biogeochemistry. Finally, though Asteraceae were no more than three of the 15 polycultures species, they have thrived in this experiment, so we also evaluated the effect of this dominant phylogenetic group on biogeochemistry.

Phylogenetic identity had a larger effect on soil biogeochemistry than diversity or functional groups. Neither phylogenetic nor functional diversity altered soil biogeochemistry in the polycultures (P≥0.15). Moreover, the relative abundance of traditional functional groups had no effects on soil biogeochemistry (P≥0.08), though increased shrub abundance was associated with increased peroxidase activity (P=0.006). Instead, the relative abundance of Asteraceae was associated with larger C pools and smaller nitrate pools in polycultures (P≤0.05). Asteraceae-dominated plots also had lower pH soils (P=0.02) and tended to be richer in organic matter (P=0.07), which may be driving these patterns. This “Asteraceae effect” also manifested in the monocultures: Asteraceae monocultures contained 21% more organic C and 35% less nitrate than plots occupied by species from other families (P≤0.02). However, the penultimate drivers of the Asteraceae effect are unclear. The ability to predict soil ecosystem functions from plant communities is of critical importance for understanding plant-soil dynamics and restoring degraded ecosystems. Our results broaden the definition of “phylogenetic functional groups” that are often limited to comparisons of monocots and dicots or grasses, forbs, and legumes. Instead, sub-groups within these categories – such as the Asteraceae forbs – may be important manipulators of soil biogeochemistry in species-rich tallgrass prairies.