Tree species effects on belowground biogeochemistry in diverse tropical rain forests

Background/Question/Methods

Tropical forests display remarkable species diversity from local to regional scales and it follows that the accompanying aboveground biogeochemical heterogeneity could contribute to complex patterns of soil biogeochemistry. Indeed, results from temperate ecosystems show that tree species significantly affect the soil beneath them, with species-specific ‘footprints’ observed for both soil pools and fluxes. However, the limited data that exist for diverse tropical rain forests are notably more equivocal, with some data suggesting the same linkages between above- and belowground biogeochemistry as observed in higher latitude ecosystems and other data suggesting an absence of such relationships. Understanding the potential for such species effects in tropical forests, as well as the factors that interact to accentuate or diminish the relationships, has global implications for predictions of how environmental change will affect tropical forest structure and function. Here, we assessed whether tree species identity was related to vertical patterns in carbon, nitrogen, and phosphorus cycling, as reflected in live canopy leaves, recently-senesced leaves, bulk leaf litter, and soil for six common tree species. We then explored the literature to contextualize our results within previous observations to assess larger relationships between tree species and belowground biogeochemistry in the diverse tropical forest biome. 

Results/Conclusions

Soil biogeochemical pools and fluxes varied significantly among tree species for all measured forest components. For instance, N inputs via free-living N₂ fixation varied significantly by species and, for each layer of the profile, variations in phosphorus drove much of the observed species-specific variation in N₂ fixation. Further, we found strong links between canopy and forest floor nutrient concentrations: canopy phosphorus was correlated with bulk leaf litter phosphorus for individual trees and, in turn, soil phosphorus was significantly, negatively correlated to foliar phosphorus resorption. But why are species relationships absent in some tropical forests? Beyond differences in factors such as litter and soil mixing by fauna, the detection of species differences could depend upon the measurements made. Multiple studies focus on bulk soil chemical indices alone, but our data indicate that processes (e.g., N₂ fixation, resorption, phosphatase activity) are more sensitive to the unique chemical properties of a given tree species. Taken as a whole, the data suggest (1) interspecies variation aboveground does drive significant variation in belowground biogeochemical cycling; (2) mechanisms behind species-specific effects are often themselves biogeochemical in nature; (3) distilling the complexity of rain forests into tractable relationships could prove essential in predicting changes to whole-ecosystem function.