Belowground drivers of aboveground cation and phosphorus cycling in fast-growing tropical trees

Background/Question/Methods

Secondary forests and plantations play an important role in the atmospheric CO₂ balance because they sequester atmospheric CO₂faster than do other terrestrial ecosystems. Given that nutrients are required to sustain high growth rates, the question becomes: What mechanisms enhance nutrient uptake in fast-growing trees? We explored the role of fine-root growth, expecting that allocation of carbon to fine-root growth would be a major mechanism by which plants increase uptake of all nutrients – both cations and anions – because fine roots increase the capacity to scavenge nutrients already on exchange sites within the soil environment. In contrast, other mechanisms would increase uptake of only phosphorus (P) and nitrogen (N), but not cations, where other mechanisms include transfer of photosynthate directly to arbuscular mycorrhizal fungi (AMF) or production of exudates that support rhizosphere heterotrophs and thus stimulate decomposition. By this rationale, we predicted that if allocation to fine-root growth were an important mechanism of nutrient uptake, P and cation uptake would be correlated directly with fine-root growth. We tested this hypothesis at La Selva Biological Station, Costa Rica, in replicated mono-dominant plantations of four species, where we measured fine-root ingrowth, and accumulated P and cations in tree biomass and soil.

Results/Conclusions

The four tropical tree species in this experiment, Hieronyma alchorneoides, Pentaclethra macroloba, Virola koschnyi, and Vochysia guatemalensis differed significantly in net cation uptake over the first 17 years of growth (P = 0.013, Ca; P >0.0001, Mg, Mn, K, Al, Fe, and Sr). For all cations, aboveground tree biomass was highly correlated with fine-root ingrowth length, with P values >0.0001 for all cations except Ca (P = 0.013). In contrast, although trends in P uptake among species were similar to cation trends, differences among species were only marginally significant (P = 0.062). Similarly, P in aboveground tree biomass was not significantly correlated with fine-root ingrowth (P = 0.068). Neither cation nor P uptake was correlated with measures of available P and cations, organic or total P in surface soil. For P, the less significant correlation with fine-root growth suggests that some other mechanism, such as symbioses with AMF, also play a role in P nutrition in this tropical Oxisol. Together, these results indicate that allocation to fine-root growth resulted in increased scavenging capacity and thereby served as the major mechanism whereby cation uptake, and to a lesser extent P uptake, kept pace with carbon cycling in fast-growing trees.