The variability of fungal 6"N values within one sample population can be large and unpredictable. The observation that fungi are often "N-enriched relative to nearby plant foliage may reflect a mycorrhizal relationship or it may be fortuitous and chiefly arise from other causes. In a cultivated system (decorative flower bed), the 15N-enrichment of fungal caps of fruit bodies relative to stipes suggested that much of the observed variation was caused by internal cycling of fungal N. Controlled experiments are needed in order to identify the sources of observed isotopic discrimination. Of the systems sampled, the two natural ones had larger ranges of 6''N among all samples than the cultivated one.
Vascular plants often form associations with fungi (mycorhim) which may alter their nitrogen acquisition and and, hence, potentially change' both the internal partitioning of plant 6I5N as well as whole-plant 8'N relative to external nitrogen sources. Handley et a1.4
found that non-mycorrhizal Ricinus commuis was ca. 2%0 15N-enriched relative to infected plants. Pate et aLs found a similar amount of "N-enrichment in non-mycorrhizal trees relative to other nearby plants known to have root symbionts h a field survey in semi-arid Western Australia.
A field survey in a semi-arid area of South Africa6 revealed even larger "N-enrichment (4.7%0) in non-mycorrhizal proteas, relative to woody species having root symbionts.