Patterns of natural 15N abundance in the leaf-to-soil continuum of tropical rain forests differing in N availability on Mount Kinabalu,Borneo

We investigated the natural abundance of ¹⁵N in sun leaves and other components of tropical rain forests on altitudinal sequences of eight sites that form a gradient of soil N availability with varying ectomycorrhizal abundances on Mt. Kinabalu, Borneo. We investigated how soil N availability and ectomycorrhizal abundance related to the ¹⁵N abundance of ecosystem components. ¹⁵N values (¹⁵N abundance relative to ¹⁴N) increased consistently in the following order at each site: sun leaves, leaf litter, fine roots and from shallower organic to deeper mineral soil horizons. Enrichment (3–6 ¹⁵N) of ¹⁵N occurred at the litter–topsoil interface at all sites, and the magnitude of the enrichment correlated negatively with ¹⁵N depletion in the foliage, irrespective of ectomycorrhizal abundance. Foliar ¹⁵N values significantly positively correlated with their N concentrations. Foliar (and litter and root) ¹⁵N values correlated positively with NO₃ availability, and negatively with NH₄ availability. The two positive correlations of foliar ¹⁵N with foliar N and NO₃ availability were inconsistent with the assumption that stronger nitrification (hence a greater nitrate availability) produced a more ¹⁵N-depleted active inorganic N pool. The isotopic fractionation during the passage of N through ectomycorrhizas to plants might explain the positive correlation of foliar ¹⁵N and N concentration; however, this mechanism could not fully explain the correlation in our case because strong foliar ¹⁵N depletions occurred at the sites that lacked ectomycorrhizas. Alternatively, the positive correlation across sites reflected the tightness of N cycling. Strong nitrification and associated isotopic fractionation might have occurred at N-richer sites and the subsequent removal of NO₃ from the system could decrease isotopically `lighter' N at these sites.