Nitrogen Fixation by Tropical Trees and Shrubs

Abstract

Biobgicalnitrogen fixation (BNF) has contributed most of the N in natural forest systems. Nodulated legume trees and Casuarina spp are being increasingly grown in industrialplantations and on-farm, particularly in Asia. BNF occurs in forests in a variety of ways with the major contribution usually from nodulated trees and understorey plants. The increase in soil N of some 39 Kg N⁻¹ha⁻¹yr⁻¹ since 1890 by the Broadbalk Wilderness, where legumes have been absent most of this century, suggests other systems can be important. Associative systems, especially cyanobacteria with lichens, mosses, cycad, coralloid roots or “nodules”, bacteria in litter contribute amounts of BNFdifficult to quantify. Dry deposition of (NH₄)₂SO₄ is an increasing source of N in forests in industrialised countries leading to soil acidification and increases in soilnitrate supply that can inhibit BNF.