The availability of different nutrients such as nitrogen (N) and phosphorous (P) in soil affects the plant community differentially depending on the plant's ontogenic stage, species, or functional type. In this sense, the nutrient limitation can play an essential role in the composition observed in a tropical forest. For example, P addition can increase root nodule production in nitrogen (N)-fixing legumes adult trees in the tropical dry forest. While legumes are the dominant family in tropical dry forests, we know little about their establishment and early growth and whether nutrient limitation plays a role. To understand if N-fixing legumes are limited by nutrient availability in their early stages, we planted seedlings of N fixing legumes and non-legumes into the understory of an existing, large-scale, fully factorial N and P fertilization experiment in Guanacaste, Costa Rica. The experiment consists of four different fertilization levels: (1) control (no nutrient addition), (2) N addition (150 kg N ha-1 yr-1, as urea), (3) P addition (45 kg P ha-1 yr-1, as phosphoric acid), (4) N:P (at the rates mentioned above). We grew seedlings from ten native tree species (five N-fixing legumes and five non-legumes). When the seedlings were about seven weeks old, we transplanted them into the experiment. Of each species, four individuals were planted per plot, for 40 individuals per plot. Censuses of survivorship, growth, and leaf number were carried out approximately every three months. Additionally, soil samples were taken to confirm the effect of the treatments on P. The soil samples confirmed that P addition increased soil P availability. In general, the plots with P addition have higher seedling growth rates compared to the control and the other treatments. N-fixer legumes have a higher growths rate than non-legumes in all treatments, except for N addition. Plants in the N and N:P treatments had fewer leaves than the control and the addition of P. Interestingly, the non-legumes had, on average, a higher number of leaves than the N-fixer legumes in all treatments. The mortality rate was higher in treatments with nutrient addition than in control plots. So far, our results support the hypothesis that, in general, the addition of P favors plant growth in its early stages, with only slight differences between legumes and non-legumes. Additional censuses are needed to determine if these patterns persist and whether there is differential survivorship over the dry season.
fertilization, nitrogen, nutrient limitation, phosphorus, seedlings, tropical dry forest, Legumes