Biological processes influencing nutrient limitation in a Costa Rican lowland tropical forest

Background/Question/Methods

Nutrient limitation by nitrogen (N) and/or phosphorus (P) is pervasive in most ecosystems on Earth. In tropical forests, however, it is not well understood how biological processes interact with nutrient availability to influence carbon (C) accumulation and storage through net primary productivity (NPP). We conducted a fertilization experiment with the objective of directly testing if N, P or N and P availability limit NPP in a lowland tropical forest in Costa Rica and to explore how tree species composition and size, responded to added nutrients. Our experiment included four treatments (control, +N, +P, and +NP) and six replicates, arranged in a complete block design. We added 100 kg ha^-1 yr^-1 of N as ammonium nitrate and urea and 50 kg ha^-1 yr^-1 of P as super triple phosphate to 20 x 20 m plots, in two yearly applications. We conducted soil nutrient measurements (e.g., N mineralization, nitrification, available P, total elements), and NPP measurements (e.g., stem diameter increase, litterfall productivity, root productivity) during 2.5 years. We also measured foliar, litter, and root N and P concentrations.

Results/Conclusions

Overall, there was no significant effect of either N or P fertilization on tree diameter increase, or litterfall and root productivity at the ecosystem scale. However, there were interesting and contrasting responses among tree species and size classes. Trees from Pentaclethra macroloba, the most abundant tree species in this forest and a legume, did not grow more with N or P additions but increased mean foliar N concentration with added N, resulting in increased litterfall N concentrations. In contrast, individuals from Socratea exohrriza, the most abundant palm species in this forest, showed larger mean diameter increase and higher mean foliar P concentration after P additions, which suggest that this species is P limited. Results obtained in this study are consistent with an increasing body of experimental evidence showing that in these forests, although soils have high N relative to P, NPP is not necessarily limited by P, or at least does not respond to either N or P fertilization in the short term. However, observed species-specific changes in foliar nutrient concentrations could have important effects for nutrient cycling, which could feedback to influence NPP in a longer time scale. This study highlights the importance of considering biological factors, such as species composition and life history traits, before making generalizations regarding nutrient limitation in tropical forests.