Variation in leaf and soil δ15N in diverse tree species in a lowland dipterocarp rainforest, Malaysia

Lowland tropical rainforests in Southeast Asia are characterized by high species diversity despite limited soil nutrient conditions. The plant nitrogen isotope ratio (delta N-15) reflects plant uptake of soil nitrogen. We analyzed delta N-15 values and nitrogen content (N %) in leaves and roots of 108 woody species with different types of symbiotic microorganisms, of different life forms (emergent, canopy, sub-canopy, understory, and canopy gap species), and from different families in a Bornean lowland dipterocarp forest to gain more insight into the diversity of nitrogen uptake strategy in the rhizosphere. Leaf delta N-15 values in the species studied varied largely from -7.2 to 5.0 aEuro degrees, which is comparable to the values of known Asian trees including temperate, sub-tropical, and tropical mountain forests. Leaf delta N-15 also varied significantly among both life forms and families, though the phylogenetically independent contrast (PIC) relationships were not statistically significant among life form, family, and symbiotic types. Some families showed specific leaf delta N-15 values; Dipterocarpaceae, the dominant family in the canopy layer with symbiotic ectomycorrhiza in Southeast Asia, had small intraspecific variation and higher leaf delta N-15 values (0.03 aEuro degrees) compared with species exhibiting arbuscular mycorrhiza, whereas several families such as Burseraceae, Euphorbiaceae, and Myrtaceae showed large interspecific variation in leaf delta N-15 (e.g., from -7.2 to 5.0 aEuro degrees in Euphorbiaceae). These variations suggest that tropical species may have family- or species-specific strategy, such as root symbiotic microorganisms, for nitrogen uptake under low-nutrient conditions in tropical rainforests in Southeast Asia.