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 N-15 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 N-15 abundance of ecosystem components. delta N-15 values (N-15 abundance relative to N-14) 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 parts per thousand delta N-15) of N-15 occurred at the litter-topsoil interface at all sites, and the magnitude of the enrichment correlated negatively with N-15 depletion in the foliage, irrespective of ectomycorrhizal abundance. Foliar delta N-15 values significantly positively correlated with their N concentrations. Foliar (and litter and root) delta N-15 values correlated positively with NO3 availability, and negatively with NH4 availability. The two positive correlations of foliar delta N-15 with foliar N and NO3 availability were inconsistent with the assumption that stronger nitrification (hence a greater nitrate availability) produced a more N-15-depleted active inorganic N pool. The isotopic fractionation during the passage of N through ectomycorrhizas to plants might explain the positive correlation of foliar delta N-15 and N concentration; however, this mechanism could not fully explain the correlation in our case because strong foliar N-15 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.