Differences in nitrogen cycling between tropical dry forests with contrasting precipitation revealed by stable isotopes of nitrogen in plants and soils

Abstract

AbstractDespite 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.