Shedding light on the 'dark ecology' of grasslands

The fire–ecosystem functioning relationship in temperate native grasslands of southern Australia has been well-described. Recurrent fire promotes native grass productivity, benefits inter-tussock native flora and can reduce the potential invasion of exotic plants. As such, it is recommended that fire be used as a management tool to manage these endangered ecosystems. Much of the recent research has been about the timing and frequency of fire to achieve these aims.

However, the ecological consequences of frequent, prescribed fires on below-ground microbial community composition and, as a consequence, the long-term impact of repeated burning on soil health, have not been deciphered. This is not unexpected - it's hard to study the diversity of an ecosystem that remains hidden from view, belowground in the soil. The 'dark ecology' of ecosystems remains virtually unexplored, but new tools are changing this. Our new paper - led by La Trobe Research Fellow Dr Eleonora Egidi - has just been published (early online) in FEMS Microbiology Letters, and sheds some light on the impacts of fire on soil biota.

To understand the relationship between fire and the soil mycobiome, we conducted the first analysis of the soil fungal communities in native Themeda triandra-dominated grasslands undergoing regular burning in temperate Australia. The mycobiome was characterized in relation to Fire-Frequency (1, 2, >3 yr long-term fire intervals) and Time-Since-Last-Fire. The impact of these fire parameters on fungal community composition, richness and diversity was quantified by MiSeq Illumina sequencing of PCR-amplified nuclear rRNA ITS1 fragments to assess indirect and direct effects of prescribed fires.

Our first impressive finding was that we found 503 OTUs in grassland soils. There are approx. 10X more entities in the soil than the number of species observed in the standing vegetation. While we could not put names on the entities in the soil, our sequencing allowed us to discriminate between taxonomic units and hence, give an indication of the diversity of life occurring under the vegetation.  Ascomycota were the most abundant OTUs. Members of the Ascomycota are commonly known as the sac fungi or ascomycetes and they are the largest phylum of Fungi. Members of the Glomeromycota were also common - 167 of the 503 OTUs. This group is important as they form arbuscular mycorrhizas (AMs) with the roots of land plants.

We found that the fungal community composition was influenced by fire regime (frequency) moreso than time-since-last fire. Overall, the change in composition was related to small changes in relative abundance of common taxa and to more dramatic changes in frequency of lower abundance taxa, implying a general resilience of the fungal community to fire disturbance. Several OTUs present in low fire frequency samples (>3 yr burning) were not found in the frequently (annual) burnt samples, and vice versa, suggesting that they may represent fire-sensitive and fire-resistant taxa respectively.

The Fire-Frequency related shift in composition was particularly significant for fungal taxa assigned to the phylum Glomeromycota. Members of this phylum are mutualistic, obligate symbionts able to form arbuscular mycorrhizal (AM) associations, suggesting that the differences observed between >3yr and annually burnt grasslands may be driven by the difference in the plant community composition that results from different fire regimes. However, this conclusion remains very speculative and worthy of further investigation.

Soil microbes represent the majority of biodiversity in terrestrial ecosystems and are clearly a diverse component of temperate native grasslands. Because they are intimately involved in ecosystem functions, we suggest that changes in fungal community composition should be taken into account when assessing the impact of land management strategies in temperate grasslands, and we now have (relatively cheap) sequencing technology to do this. It goes without saying, but there is still much to be learnt.