Nitrogen isotopes in tropical ecosystems

Background/Question/Methods

Tropical forests play an important role in the global nitrogen (N) cycle because their vegetation cycle involves vast amounts of N by growth and litterfall processes; mineralization-nitrification rates are relatively high in tropical forest soils, contributing to high rates of N₂O losses, and exporting substantial amounts of N via riverine export.  Neotropical forests harbor a high abundance of members of the Fabaceae family. Some species of this family are able to fix nitrogen from the air (BNF) via symbiosis with Rhizobium bacteria. Several authors have hypothesized that the N abundance in tropical forests are mainly due to the presence of these N-fixing trees. However, many Fabaceae trees are apparently not fixing N from the air in undisturbed mature tropical forests. The stable nitrogen isotopes ratio (express as δ¹⁵N) has been extensively used in order to help elucidate the nitrogen dynamics in tropical forests. The rationale for using nitrogen isotopes is that N losses discriminate against ¹⁵N atoms, as a consequence the δ¹⁵N of N-rich systems tend to have higher δ¹⁵N. The aim of this presentation is to review what we know so far about the nitrogen cycle in tropical forests through the lens of nitrogen isotopes.

Results/Conclusions

Initial results have shown that the δ¹⁵N of tropical forests was higher than temperate forests, suggesting  that the nitrogen cycle in tropical forests were less conservative than in temperate forests with heavy N-losses.  Denitrification was later identified as the major process leading to a ¹⁵N enrichment of tropical soils and plants. Age chronosequence studies in tropical and sub-tropical regions further demonstrate that young successional forests growing after a natural disturbance (volcano eruption – thousands of years ago) or agricultural abandonment (less than a hundred years ago) exhibit conservative N-cycling properties similar to N-limited forests in younger soils in temperate latitudes. In these age chronosequences it was observed that as the successional forests mature, N accumulates in the system followed by an increase in N losses and increase in δ¹⁵N values of soil and vegetation. The results above exemplify how nitrogen natural abundance in tropical systems has been an important integrative tool for understanding the N cycle in these systems. However, it is important that we advance in this field by forming a pan-tropical consortium devoted to discussing and planning future studies on the topic.