Nitrogen (N) and phosphorus (P), either individually or in combination, limit primary productivity in most terrestrial ecosystems. Plant adaptations to limitation by these nutrients feed back strongly to control ecosystem rates of nutrient cycling. To explore how nutrient availability influences plant-soil-microbial-feedbacks and thus nutrient limitation in a diverse, wet-tropical forest, we performed a fertilization experiment at EARTH University Forest Reserve in Costa Rica. Since May 2007, we have fertilized 24 30x30 m plots with N, P and both N and P (NP) in a full factorial block design by adding 100 kg/ha/yr of N, 50 kg/ha/yr of P or both. We measured soil nutrient parameters including Melich-extractable P, extractable ammonium and nitrate, net N mineralization, total carbon, N, P, and other micronutrients prior to fertilization, and at one and two years after fertilization. To calculate aboveground net primary productivity, we recorded basal area increase of trees, litterfall and coarse litter production.  Additionally, we measured foliar and litter nutrient concentrations. Finally, to evaluate the effect of increased nutrient availability on the decomposer community, we performed a common-substrate decomposition experiment.

Results/Conclusions

Two years after fertilization there was a significant increase in Melich-extractable soil P and total soil P in the 0-10cm soil profile. Mean Melich-extractable soil P was 1.27 (± 0.77) ug/g in the control plots versus 3.07 (± 3.19) and 3.42 (±1.62) ug/g in the P and NP treatments respectively. After two years, P fertilization resulted in an 8% increase in total P. However, several control plots still had higher total P concentrations than fertilized plots. This increase in extractable and total P in the surficial soil did not result in an increase in foliar P of the trees with diameters larger than 10 cm or in an overall increase in basal area in the fertilized plots. However, certain species, such as Pentaclethra macroloba grew more in the plots fertilized with N and P. There were no differences in net N mineralization, net N nitrification or dissolved inorganic N. We are currently analyzing data for dissolved organic N, total soil N and foliar N. Preliminary results from our experiment suggest that both N and P co-limit productivity in our study site but this limitation is framed by individual species and their interactions. Our study highlights the key role of species composition and life history traits in nutrient cycling and limitation in diverse tropical forests.