Synergistic effects of canopy chemistry and autogenic soil biota on a global invader

Soil biota have strong effects on plants, but we have a poor understanding of how plant chemistry might modify these effects. We examined the effect of soil biota associated with an exotic invasive tree, Prosopis juliflora, versus that associated with native species, from seven sites across India on conspecifics and two other plant species. We then measured changes in species-specific soil biota effects (identified as plant–soil feedbacks, PSFs) when leaf leachate from P. juliflora or from native plant species was added to soil containing respective live and sterile soil inoculum. We quantified the amino acid L-tryptophan from leaf leachate of P. juliflora, Leucaena leucocephala (another invader), and two native species. We also tested effects of P. juliflora or native species soil inoculum amendment of tryptophan on P. juliflora, P. cineraria and L. leucocephala across seven sites. We then quantified the microbially metabolized derivatives of tryptophan, phytohormone indole-3-acetic acid (IAA) and intermediates after adding tryptophan into P. juliflora and native soils. Soil biota associated with P. juliflora generated positive effects on conspecifics and L. leucocephala, but negative effects on the native congener P. cineraria. When P. juliflora leaf leachate was added to soil with live P. juliflora inoculum, PSFs became more positive for P. juliflora and other species, compared to leaf leachate amended with sterile soil inoculum. Native leaf leachate interacted weakly with soil biota to impact biomass of conspecifics and heterospecifics. There was roughly 10× more tryptophan in the leaf leachate of P. juliflora than in the leaf leachate of other species. Tryptophan generally increased positive PSFs associated with P. juliflora relative to soil biota associated with other plant species. When tryptophan was added to live P. juliflora soil, IAA and its intermediates were produced at five of seven sites, and at four of these sites soil biota from P. juliflora had positive PSFs. Synthesis. These results provide the first experimental evidence that a chemical leached from the leaves of an invader regulates PSFs. Our results indicate that canopy effects and PSFs, which are usually studied independently, can interact in ways that strongly affect conspecifics and neighbouring species.