Some of the costs and benefits of reactive nitrogen in the environment can be seen clearly in nitrogen-deficient habitats like the Bornean dwarf forests known as “kerangas.” Kerangas grow in soils that are acidic, sandy and podzolized. Essential elements enter the soil from decaying leaf litter, but most of these elements--magnesium, carbon, calcium and nitrogen, in particular--leach away very quickly, and are only available at meaningful concentrations in the top few inches. Phosphorus seems to leach away more slowly. Continual deposition of leaf litter is critical to the system, and disease, fire and logging or clearing for agriculture will convert kerangas to a barren habitat known as padang, dominated by grasses and sedges. The kerangas-adapted she-oak Gymnostoma nobile bears nitrogen-fixing root nodules. Orchids obtain carbon via mycorrhiza; to what degree nitrogen is transferred this way is the subject of some controversy, but among plant families in the kerangas, orchids show the greatest species diversity, possibly supporting suspicions that micorrhizal nitrogen transfer plays an important role in orchid nutrition. Pitcher plants (Nepenthes spp.) trap insects in modified water-bearing leaves. At least one Bornean species, N. rajah, secretes a nectar that attracts tree shrews whose droppings are captured in the pitcher to nourish the plant. In perennially wet padang habitat, Bladderworts (Utricularia spp.) and Sundews (Drosera spp.) also trap small arthropods. Epiphytic ant plants (Hydnophytum spp.) form symbiotic relationships with ants, by providing them shelter while receiving protection from their colony and nitrogen and other nutrients from their wastes.
Kerangas provide a good lens to see the many avenues of nitrogen transfer in nitrogen-deficient habitats and suggest the complexity of monitoring and managing human-caused changes of reactive nitrogen in the environment, in general.
|
