Effects of canopy tree species diversity on belowground ecosystem processes in a wet tropical rain forest

Background/Question/Methods

Tropical rain forests are known for their tremendous biological diversity, but the effects of plant diversity on important ecosystem processes remain unclear. Interspecies differences in both the demand for nutrients and in foliar nutrient concentrations could drive differences in litter chemistry that affect both pools and fluxes of belowground resources. Yet, our understanding of the effects of aboveground biogeochemical heterogeneity on belowground ecosystems is poor. To investigate the effects of tree species diversity on belowground biogeochemical processes, we examined how carbon (C), nitrogen (N), and phosphorus (P) cycles vary under canopy tree species – including legume and non-legume species – that vary in foliar leaf nutrient concentrations in a wet tropical forest in southwestern Costa Rica. We hypothesized that aboveground tree species diversity creates spatial heterogeneity in belowground N and P pools and fluxes. In addition, we predicted that legumes (putative N-fixers with relatively high foliar N content) stimulate larger belowground N pools and fluxes compared to non-legume canopy tree species, and that higher N availability drives higher P availability through important N/P interactions. 

Results/Conclusions

We found significant differences in belowground C, N and P cycling under different canopy tree species. For example, litter, soil and microbial C:N ratios, microbial P and free-living N fixation rates in litter varied between species. In addition, litter C:N was positively correlated to soil C:N and negatively correlated to soil inorganic N, implying that plants impart a biogeochemical imprint on the soil in this diverse tropical forest. We also saw differences between functional groups. For example, litter C:N was significantly lower under legumes than non-legumes, while soil acid phosphatase activity was significantly higher under legumes compared to non-legumes. Our results suggest a tight coupling between N and P cycles, such that increased N inputs from N-rich legume litter stimulate production of N-rich phosphatase enzymes. Overall, our results suggest that aboveground plant diversity creates belowground biogeochemical heterogeneity, driving important element interactions that may play a key role in regulating C, N and P cycles in tropical forests.