Microbial transport and soil integrity in drylands

Dryland soils are particularly vulnerable to erosion because of low plant cover and low organic matter which is linked with water availability and biological constraints. It is increasingly recognised that dryland soils can be managed to minimise erosion, which has benefits such as increasing soil fertility, carbon storage, and prevention of dust storms. Management may include monitoring, reduction of trampling, and planting. Interventions can help maintain the integrity of biological soil crusts (biocrusts), which are ubiquitous microbial communities living in the top few millimetres of dryland soils. Biocrusts perform numerous functions, including adhesion of particles which protects the soil from erosion and is widely regarded as beneficial. Biocrusts also fix carbon and nitrogen from the air, enriching the soil and resulting in production of biomass. This living biomass is subject to natural selection pressures, and the soil-surface microbial community is expected to be adapted for life in this complex environment rather than being transient or opportunistic. In fact, biocrusts probably colonised the land long before plants, so we suggest that their survival and dispersal strategies are exquisitely adapted, and that they have had an ever-present influence on geomorphological processes for billions of years.

Whilst it is recognised that biocrusts stabilise soils and that certain cyanobacteria contribute significantly to this, there has been little work to practically verify identities and ecological strategies of stabilising organisms. Biological adaptations for dispersal by wind, water, or vegetative growth are expected, and such adaptations are considered likely to differently influence geomorphic processes and ecosystem function. Adhesive adaptations may contribute to landscape stability, whilst adaptations for wind dispersal may provide a mechanism for nutrient transportation and biogenic dust production.

To investigate biological dispersal and related geomorphic processes associated with biocrust organisms, we used DNA sequencing to characterise microbial communities of various biocrusts and sediments in Diamantina National Park of western Queensland, Australia (23°36’44.8”S; 143°17’46.9”E). Climate in the region is semi-arid characterised by a summer-dominant rainfall pattern with a mean annual precipitation of 270 mm a-1 and high inter-annual variation. Central to the study site is 25 Km2 of erosion active claypan, bordered by sand dunes and a river. Aeolian activity constantly moves sediment from the dunes and claypan, whilst periodic flooding (interval approx. 3 years) brings fresh river sediment and biological inoculum to the claypan.

We characterised the microbial community and physical properties of biocrusts situated in the claypan, on nebkha within the claypan, and on the sand dunes. River sediments were analysed in the same way, and a wind tunnel was operated on the sand dune to provide a controlled erosive treatment. Wind-eroded particles were analysed in comparison to adjacent untreated soil to determine whether particular organisms are preferentially mobilised or tend to stay adhered to the soil under windy conditions. These analyses yielded 18 million DNA sequences representing many thousands of distinct microbial taxa. The differential erosion of taxa from sand dunes, and their relative abundance at different sites will be discussed in relation to microbial ecology, geomorphology, and land management.