Plant functional diversity shapes soil respiration response to soil moisture availability

Narrowing uncertainties associated with land-atmosphere carbon (C) fluxes is critical for projecting climate futures, but large uncertainties in modeling soil respiration (RS) hinder progress. Difficulties accounting for how biological communities will respond to altered precipitation contribute to those uncertainties, but remain underexplored in situ. In a rainfall and grassland diversity manipulation experiment altering both plant richness and community composition, we measured RS monthly for four growing seasons, along with multiple physical (soil moisture and temperature) and biological drivers (aboveground, root, and microbial biomass) of RS. Relationships between plant richness and RS were dependent on plant community composition and soil moisture conditions. Elevated RS was associated with grass diversity, likely governed by enhanced soil moisture at 12 cm. Microbial biomass was the strongest independent predictor of RS. Though soil moisture was a strong predictor of RS, covariance with precipitation treatments and microbial biomass suggests it operated through multiple indirect pathways. Even after accounting for several RS drivers, plant community composition and richness still accounted for a nontrivial amount of variation in RS. This suggests that unexplored pathways associated with biological complexity (for example, microbial community composition) influence RS. Finally, altered precipitation changed diversity-RS relationships over time, suggesting that soil microbes can respond relatively rapidly to altered precipitation, perhaps due to the diversity of specialist microbes in our initial common soils. Our work demonstrates how biological complexity can interact with physical drivers and changing climates to influence RS in ways currently unaccounted for in models.