Effects of Bromelia pinguin (Bromeliaceae) on soil ecosystem function in primary forests of Costa Rica

Background/Question/Methods

The high biodiversity of flora present in tropical regions results in knowledge gaps of the impact many plant species have on microbial community structure and nutrient dynamics.   Bromelia pinguin is a terrestrial bromeliad found under forest stands throughout the Neotropics.  The bromeliad’s fruit pulp possesses antifungal activity, raising questions on the effect of this plant on soil fungi and associated processes.  In our study, we evaluated the impact of B. pinguin on rates of C and N biomass development, laccase activity, and fungal diversity on primary forest soils in a lowland forest of the Northern Zone of Costa Rica.  We hypothesized there would be lower rates of biomass development and laccase activity (indicating lignin degradation), and less fungal diversity in bromeliad compared to the primary forest soils.  Rates of biomass development were determined using standard extraction methods in chloroform-fumigated and unfumigated soil samples.  Laccase activity was measured using the phenol oxidase assay.  Fungal community structure was assessed using terminal restriction fragment length polymorphism (T-RFLP) and DNA sequence analysis of fungal ITS (primer: ITS1f and ITS4) amplicons. 

Results/Conclusions

There was a lower rate of C (60.70 vs. 89.79 µg/g/day) and N biomass development (20.54 vs. 29.04 µg/g/day), less laccase activity (0.00238 vs. 0.00429 DOD/min), negative correlations between these metrics and bromeliad density (r = -0.375 to -0.479), and lower T-RFLP-based fungal diversity (2.39 vs. 1.90) and richness (19.0 vs. 8.8) in bromeliad than primary forest soils. Fungal DNA sequence analysis showed greater diversity (3.73 vs. 3.26) and richness (74.5 vs. 29.3) in primary forest soils. The most dominant Orders in bromeliad soils were Tremellales (27.5%), Agricales (22.5%), and Endogonales (12.5%), and in primary forests, Helotiales (44.4%) and Thelephorales (25%) were dominant.  The differences in biomass development and laccase activity were associated with differences in fungal community structure.  Confirmation of these diversity patterns by metagenomic analysis of soil DNA using 454 pyrosequencing is now occurring. Different fungal groups are known to specialize in utilization of different organic substrates, such that changes in these communities can drastically impact C and N biomass dynamics. These bromeliad soils provide natural conditions that can be used to control for certain fungal groups, thus, providing an opportunity to study the role they may play as drivers of nutrient cycles in tropical soils.