Large losses of inorganic nitrogen from tropical rainforests suggest a lack of nitrogen limitation

Background/Question/Methods

Dissolved inorganic nitrogen (DIN) losses from unpolluted mature tropical forests are over an order of magnitude higher than from analogous temperate forests.  We asked whether high DIN losses from tropical forests are consistent with N-limitation but elevated by enhanced plant-detrital cycling under tropical climate.  We evaluated this question with a simple analytical framework of terrestrial N limitation and cycling using global litterfall N and rooting depth data and compared our predictions to data of DIN in stream waters of mature temperate and tropical rainforests. 

Results/Conclusions

We find that the observed higher DIN losses from tropical forests is only consistent with N limitation if N uptake efficiency in plants is 2-3 times lower in tropical than temperate forests.  Our results contrast with the idea that a tropical climate promotes and sustains an up-regulated and leaky—but N-limited—N cycle.  Instead they are consistent with the notion that many tropical rainforests worldwide exist in a state of N saturation.