Do Tree Species in a Bornean Mixed Dipterocarp Forest Display Preferential Uptake of Different Nitrogen Forms?

Lambir Hills National Park in Malaysian Borneo is composed of dipterocarp tree species which specialize on different soil types. Although the mechanisms contributing to this distribution of tree species are unknown, one hypothesis is that tree species at Lambir differ in their preferences for different nitrogen forms. In this study we investigated (1) whether clay soil and sandy loam soil types at Lambir differ in the availability of ammonium and nitrate and (2) whether tree species specializing on either clay soil or sandy loam soil at Lambir have differing preferences or capacities to take up ammonium, nitrate, or glycine (organic nitrogen). We expected sandy loam specialists to show a greater capacity to absorb ammonium, relative to clay specialists, and clay specialists to exhibit greater capacity to absorb nitrate, relative to sandy loam specialists. Generalist species were expected to have no preference for different nitrogen forms. We had no apriori expectations with regard to the expected uptake of glycine. An anion and cation exchange resin experiment revealed that as expected, clay soil consisted of significantly more nitrate than ammonium. However, contrary to predictions, sandy loam soil did not have significantly more ammonium than nitrate. We conducted a 15N tracer experiment by injecting 15N labeled solutions of nitrate, ammonium, or glycine in the soil of potted seedlings with different soil type specialization. 15N and 13C analysis of leaf tissue from treated seedlings did not support our hypotheses that sandy loam specialists would have significantly greater uptake of ammonium over nitrate, and that clay specialists would have significantly greater uptake of nitrate over ammonium. For a majority of the species, uptake of ammonium, nitrate, and glycine was similar. This indicates that these species have the ability to take up organic nitrogen in the form of the simple amino acid, glycine. It appears that nitrogen form preference in not likely to be a mechanism in the distribution of tree species at Lambir