Most studies of plant-soil feedbacks have focused on the surface soil, but few have studied feedbacks of deeper soils, despite frequent native vs. exotic plant differences in rooting depth. In Northern California grasslands, we established experimental plantings in outdoor mesocosms, consisting of: monocultures of the most prevalent native perennial grass (Stipa pulchra), a mix of the two most common non-native exotic grasses (Avena barbata, Bromus hordeaceus), and a native + exotic mix. After ten years, differences in exotic vs. native soils included: 1) exotic soils had higher nitrification rates and nitrifier populations, 2) native and exotic soils differed in the arbuscular mycorrhizal community composition, and 3) soil carbon and water holding capacity were higher at deeper depths for natives vs. exotics. We assessed how growth and reproduction of native, exotic, and native+exotic vegetation were affected by prior soil treatments in a greenhouse feedback experiment. Control soils from each of the past vegetation treatments were added to pots with their intact depth profile. The control soils were compared to three soil alterations: 1) soil sterilization and inoculation with native vs. exotic soil, 2) addition of a nitrification inhibitor, and 3) mixing the soil depths to provide an even carbon depth distribution.
Stipa growth was lowest on native soil, but was negatively affected by the soil microbial community of all soils. In monoculture, aboveground biomass of Stipa more than doubled in the sterilized vs. control treatment, across all three soil origins. Inoculation of sterilized soil with either native or exotic soil decreased Stipa biomass to the same extent as control soils. The negative effect of the soil community was strongest when Stipa was grown in mixture with the exotic grasses-- Stipa only consistently established in the sterilized soil treatment. Exotic grasses had the greatest aboveground and belowground biomass when grown in exotic soil. While aboveground biomass of exotics was intermediate when grown in native soil, native soils increased exotic plant deep root biomass, and more than doubled exotic grass seed production. Similarly, mixing the soil depths of exotic soil had no impact on aboveground biomass, but increased the seed biomass and deep root biomass of both the native and exotic species. Mixing depths of native soil did not alter plant performance. Changes in soil carbon depth distribution is a key driver of plant performance in this Mediterranean climate, with deep soil carbon increasing deep root biomass, water holding capacity, and seed production.