Despite the known links between climate and biogeochemical cycling of N in tropical forests, fundamental knowledge of N cycling is still far from complete. Our objective was to ascertain differences in the N cycle of two tropical dry forests under contrasting precipitation regime (1240 or 642 mm of mean annual rainfall). To do so, we examined a short-term metric of N cycling (N concentration) and a more integrated metric of N cycling (natural abundance 15N) in plants and soils at both sites. At both the relatively wet and dry sites, N cycling associated with two non-N2-fixing species was compared to N cycling associated with two potential N2-fixing species; all four tree species considered were dominant at both sites. The 15N abundance in plants was highest in the site with low rainfall, showing that N losses from the system may be large. By contrast, short-term N metrics did not vary with rainfall. Although there was a trend for leaf N concentration to be elevated in trees that have potential associations with N2-fixers, only 15N in the forest floor was significantly greater under trees with high canopy N (N2-fixing species) than those with low canopy N (non-N2-fixing species). Within each site, the influence of N2-fixing species on N cycling increased with a reduction in rainfall. Overall, our results demonstrate the role of climate as a driver of N cycling in the region, such that the projected decrease in precipitation in this region may lead to larger N losses in these forests. This study also shows how changes in tree species with and without N2-fixing associations may impact N cycling in tropical dryland forests in the future.