COS 102-10
Arbuscular mycorrhizal fungi influence long-term plant-soil feedbacks as much as soil pathogens in neotropical forests
Plant-soil feedback (PSF) can influence plant community composition and diversity, as well as soil carbon and nutrient cycling. Negative PSF dominates natural systems and are typically driven by detrimental, density-dependent effects of soil pathogens on aboveground plant communities. Here we present evidence that belowground symbiotic arbuscular mycorrhizal (AM) fungi and sterile soils alone can cause positive and negative PSF that is similar in magnitude to those of soil pathogens. We collected AM fungal communities and soils from 26-year-old, monodominant, replicated plantations of four neotropical forest trees: Hieronyma alchorneoides, Pentaclethra macroloba, Virola koschnyi, and Vochysia guatemalensis; control plots were placed in adjacent primary forest. To examine PSF, sterilized soils and AM fungal communities were crossed in a full factorial design and were planted with all plant species, except Vochysia. These were grown in the greenhouse at La Selva Biological Station, Costa Rica for six months immediately following seed production by each tree species. To detect PSF, we measured total plant biomass; to understand potential AM fungal mechanisms relating to observed PSF, we measured phosphatase activity and soil phosphorus concentrations.
Results/Conclusions
Plant-soil feedback was highly context-dependent. Pentaclethra generally had the smallest feedback to AM fungi, but grew slightly larger on AM fungi from the primary forest relative to both heterospecifics and the sterile control. Virola plants grew larger on AM fungi from Pentaclethra and Vochysia compared to their own, and were consistently bigger than plants in sterile controls. In contrast, Hieronyma plants had the largest biomass when grown with their own AM fungi compared to AM fungi from primary forest and Pentaclethra, but plants with AM fungi did not always grow better than sterile controls. Net pairwise feedbacks of plants to AM fungi indicated that PSF might mediate coexistence between Virola and both Pentaclethra and Hieronyma, but not between Pentaclethra and Hieronyma. Feedback was independent of soil origin, which had no effect on plant biomass. Minimal soil effects may reflect the observed lack of difference in available phosphorus or phosphatase activities in these soils. Our field manipulation represents the longest PSF conditioning phase to date and therefore may be illustrative of long-term plant-soil dynamics in mature forests. Indeed, mature individuals of Hieronyma are rare in the primary forest that we sampled, while Pentaclethra is abundant. Over the extended lifetime of individual forest trees, interactions between AM fungi and plants may not be as universally beneficial as indicated in other short-term PSF experiments.