The observation that vegetation and soil properties show parallel changes across elevational and global latitudinal gradients has been fundamental to ecosystem science. Such convergence in ecological pattern implies that nutrient cycling and limitation also change predictably in a similar manner. For example, numerous studies over the last several decades have indicated that lowland tropical forests tend to be N replete while montane tropical forests tend to be N limited, analogous to the generalization that N limitation increases with latitude. However, the degree to which the temperature sensitivity of N cycling converges across global elevational and latitudinal gradients has not been rigorously examined. Here we combine new measures of soil chemistry and δ15N distributions collected across altitudinal transects in tropical mountain ranges with a global meta-analysis of soil chemistry and stable isotopes to examine the temperature sensitivity of plant-soil N cycling in tropical and temperate forests.
Results/Conclusions
Our analysis integrates soil chemistry and N isotope samples collected along seven independent elevational transects and published data from 106 individual sites distributed across the world’s major tropical forest biomes with a global soil data set spanning tropical to temperate forests. We find globally consistent evidence for increasing soil N content and δ15N depletion with increasing elevation in tropical forests that follows a general temperature sensitivity function. In contrast, N content and δ15N in temperate forests exhibit no trend across elevational gradients. When controlling for temperature across biomes we find suggestive evidence that the temperature sensitives of isotopic enrichment differ, however the mean isotopic signatures appear similar. Similarly, while lowland tropical forests show the highest isotopic enrichment globally, montane tropical forests have similar isotopic signatures as temperate forests. We discuss the degree to which patterns of N cycling converge and diverge across elevational and latitudinal gradients and the underlying mechanisms that may drive these patterns. These findings have implications for furthering our understanding around the biogeography of N cycling in tropical forests and may reveal how montane tropical forests will be impacted in the future under projected global change.