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

Abstract

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. δ15N 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‰ δ15N) 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 δ15N values significantly positively correlated with their N concentrations. Foliar (and litter and root) δ15N values correlated positively with NO3 availability, and negatively with NH4 availability. The two positive correlations of foliar δ15N 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 δ15N 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.