15N and 13C patterns in savana vegetation: dependence on water availability and disturbance

Abstract

1. Plant leaves were collected from four sites in Kenya, East Africa and analysed for %N, 015Nand ol3e. Each site had different kinds and amounts of water supply and physical disturbance. There were no uniform relationships among 015N,ol3e, % leaf N and plant growth forms. These data indicate that many of the previously published relationships for these variables were peculiar to the ecosystems investigated and should not be assumed as general models. 2. At all sites, 015Nvariability was large and extreme values were clustered into definable categories, suggesting different N sources. Plant-available soil N could not be treated as a single source at any site and no model for estimation of in situ N2 fixation could be applied. 3. Putatively N2-fixing acacias could not be distinguished reliably from non-N2-fixing plants on the basis of leaf 015Nand/or % leaf N. 4. Leaf 015N values of non-N2-fixing plants were not related to distance from a putative N2-fixing tree, except where other data suggested that soil-water relations also changed with distance. 5. Leaf ol3e suggests that there is little scope for selecting more or less water-use efficient genotypes of the mature acacias examined, as they preferentially use ground-water and responded uniformly to drought when prevented access to ground-water. 6. The spatial relationship found between soil water supply and plant 015Nsuggested that site water relations may strongly influence the 015Nsignature of soils and plants as well as their heterogeneity at anyone site.