Diversity and function of decomposer fungi from a grassland soil

Despite the substantial interest to ecologists of the relationship between species diversity and ecosystem functioning, little is known about how the high species richness of decomposer (saprotrophic) fungi and their relative frequencies of occurrence influence the decomposition of organic matter. Three experiments were conducted to test the ability of culturable saprotrophic fungal isolates to utilise a range of artificial and more natural substrates that occur in organic matter, with the aims of (1) characterising the functional potential of ‘common’ and ‘occasional’ taxa in an upland grassland soil and (2) determining whether there was a high degree of apparent functional redundancy in these communities. ‘Function’ was defined as the ability of a fungal isolate to utilise broad categories of substrates (e.g. sugars, cellulose, lignin) that occur in organic matter and which change in proportion during decomposition. The terms ‘common/abundant/frequent’ and ‘occasional/infrequent’ usually referred here to the frequencies of occurrence of taxa estimated using Warcup soil plates. Accepting the difficulties of sampling fungi in soil, this appeared to be the most useful isolation method to produce a general picture of the microfungal community with an estimate of frequency of occurrence for every taxon obtained, and to provide cultures for use in function tests. The influence of this technique on the interpretation of the results is discussed.Forty-eight fungal isolates, obtained from an upland grassland in Roxburghshire, UK, were selected to cover the most ‘abundant’ taxa and a range of ‘occasionals’. Pure cultures of anamorphic fungi and members of the Zygomycota, Ascomycota and Basidiomycota were tested. Although there was apparently a high degree of functional redundancy (equivalence) in assemblages of culturable decomposer fungi, with ‘frequent’ and ‘infrequent’ taxa largely utilising the same substrates, the ‘infrequent’ taxa played important roles in decomposition. ‘Infrequent’ microfungi tested were potentially more active in decomposition than the ‘frequent’ taxa, i.e. several had a higher overall activity, were able to utilise a wider range of substrates and were more combative than the ‘abundant’ taxa. When ‘abundant’ and ‘occasional’ taxa from the same putative guild were inoculated together on grass litter, there was slight evidence of ‘positive’ indirect effects on decomposition and cellulose degradation. Some ‘negative’ effects on lignin degradation, probably as a result of combat, were observed.It is possible that the ‘occasional’ taxa increased the temporal resilience of the ecosystem process of decomposition, and were ‘waiting in the wings’ to replace the abundant taxa. Nevertheless, greater functional diversity could be associated with the uncultured taxa not studied here.