Foliar 15N natural abundance in Hawaiian rainforest: patterns and possible mechanisms.

Foliar samples were obtained from symbiotic nitrogen-fixers and control plants (non-fixers) along elevational and primary successional gradients in volcanic sites in Hawai'i. Most control plants had negative δ15N values (range-10.1 to +0.7‰), while most nitrogen-fixers were near 0‰. Foliar 15N in the native tree Metrosideros polymorpha did not vary with elevation (from sea level to tree-line), but it did increase substantially towards 0‰ on older soils. The soil in an 197-yr-old site had a δ15N value of approximately-2‰, while in a ∼67000-yr-old site it was +3.6‰. We suggest that inputs of 15N-depleted nitrogen from precipitation coupled with very low nitrogen outputs cause the strongly negative δ15N values in non-nitrogen-fixing plants on early successional sites.