Generally, phosphorus (P) is considered less abundant than nitrogen (N) relative to plant demand. However, N:P co-limitation is considered widespread and N additions have been shown to increase reproductive litter fall and to ameliorate declining relative growth rates when combined with potassium in lowland rainforest of Panama. In addition, N-fixation appears to occur throughout the maturation of tropical forests suggesting episodic localization of N-limitation may be common. Our objective in this study was to understand how the response of four tropical forest tree species to factorial N and P fertilization was reflected in ratios of heavy-to-light stable N isotopes (δ15N) – presumed integrators of multiple N-cycling processes. Through measurement of δ15N values from canopy leaves, senesced litter, and labile soil N pools (NO3-, NH4+, dissolved organic N) we sought to detect how flexible N and P uptake and retention was among co-occurring tropical tree species and, through 15N tracing of soil N sources, we also analyzed changes in preferred forms of soil N. Soil N and P availability was measured from soils extracted the same day as sampling or from in situ resin bag incubations in 16 lowland rainforest plots. Soil N pool δ15N values were measured using persulfate oxidation coupled to the denitrifier method.
Results/Conclusions
Soil δ15N values were relatively unaffected across treatments with the exception of an enriched residual pool of NO3- in the N and N+P treatments following high denitrification losses. However, only one species became significantly 15N-enriched in the N treatment relative to the control, a pattern corresponding to greater leaf N content. Litter fall C:N content was relatively unaffected by fertilizer additions whereas litter fall C:P was greatly elevated by P addition. Foliar δ15N values were positively correlated with litter fall δ15N but generally un-modified by changes in N resorption dynamics. After accounting for 15N fractionation by arbuscular mycorrhizae, foliar δ15N values could be explained by altered fertility influencing species-specific changes in N source uptake. Mass balance mixing models indicated that the urea fertilizer was in some instances (34%) rapidly taken up following fertilization followed by a period where the tree species switched to more abundant and mobile NO3- relative to control and P fertilized treatments. Combined with precipitation and fertilization induced spikes in nutrient availability, a nuanced picture emerges where tropical trees appear to switch N forms throughout the season. In conclusion, tropical N cycles remain poorly understood and integrative δ15N measurements offer unique insight.