Soil Crusts – Nature’s Soil Fixers

Soil crusts might sound might sound dry and uninspiring, but are quite the opposite when presented by David Eldridge with slides by Heino Lepp. It seems we have been largely overlooking a valuable and fascinating component of our dry land ecosystems.

Biological crusts play an important role in maintaining healthy soils in the semi arid grazing areas of NSW. The semi-arid zone makes up about 40% of the state, and much of it is leased to pastoralists for sheep grazing. The crusts also occur over larger parts of the eastern slopes and are even found in the in the Sydney basin, particularly the Cumberland Plain.

Crusts are formed by groups of small organisms, technically known as cryptogams, growing on or in the top layers of soil. Lichens, mosses and liverworts and cyanobacteria (blue-green algae), green algae and fungi are involved. The association with the soil is an intimate one: As David puts it: “you can’t take the crust away from the soil and you can’t take the soil away from the crust.”

David’s research interest dates from time he spent in drought affected areas while working for the Soil Conservation Service. He says: “there were hardly any plants around but crusts were everywhere….I thought they must have been pretty important.” He found that there was virtually no work being done on them from an ecological perspective.

David and colleagues have since carried out a systematic study involving 350 sites across western NSW. These brave little organisms (the soil crusts) have been experimentally deluged, blasted in wind tunnels and trampled with artificial sheep’s feet – and we now have a much better idea about what they do.

Crusts perform a number of services; most importantly stabilising the soil and preventing erosion, fixing nitrogen (the cyanobacteria do that), providing habitat and food for invertebrates, assisting with water infiltration, and providing niches for plants to germinate in.

Much of the experimental work was done in Yathong Nature Reserve and Mungo National Park, where the soils have been well managed and crusts are in excellent condition. Work with wind tunnels at Mungo showed that the presence of a soil crust can be crucial in preventing soil erosion. A patch of encrusted ground was able to withstand high wind speeds without significant erosion. However once the soil crust was subjected to stimulated stock trampling (this is where the artificial sheep’s foot comes in), the unprotected soil quickly began to blow away.

David pointed out that when soil crusts are removed, a large proportion of the silt and clay materials from near the soil surface are lost as well. It is these particles which carry most of the soil nutrients such as nitrogen and phosphorus.

Besides providing a physical shield to protect the soil, crusts provide a rough surface to trap moving sand grains, and the cyanobacteria exude a polysaccharide gel that binds soil particles.

The soil crusts role in water infiltration, becomes very important when soils becomes compacted by overgrazing. A healthy soil contains a network of holes- the larger ones (macro pores) are made by termites and invertebrates – the really small ones (matrix pores) are maintained by the cyanobacteria in the soil crusts. If the soil gets trampled, the macropores will quickly collapse. The only way water can now be absorbed into the soil is via the tiny matrix pores.

Soil crusts are vulnerable to fire, it is estimated that full recovery form a burn can take up to 16 years and fires less than 10 years apart can convert a lichen dominated crust to a cyanobacteria dominated – that has a lot of implications for soil stability.

The most serious threat however is overgrazing. The soil crust organisms are vulnerable to trampling by hard hooved animals, and they are unable to recolonise an area while the soil is still unstable

Once disturbance pressures are removed the crust will slowly re-establish. Firstly a physical crust will form on the soil surface. As David explained: “raindrops bash the soil, taking the silt and clay out and sealing surface, as soon as that happens cyanobacteria, algae, a few fungi come in – gradually the soil gets more stable, a few mosses then eventually lichens come back in.”

It follows that biological crusts are a good indicator of soil health, definitely in the western part of the state and probably further east as well. Within the biological crust assemblage there are particular species that have low disturbance thresholds, their disappearance can be used as early indicators of soil damage.

Biological crusts characteristically occur in areas where high temperatures and lack of water constrain plant growth. At Cobar in summer the air temperature might be 45? but the soil surface can be up to. 60? David discussed some the adaptations the soil crust organisms employ to deal with this, for example:

• short life cycle, and production of lots of large spores that survive in the soil after the plant has dies e.g. Goniomitrium – a typical dry area moss;
• specially adapted leaf structure in mosses allowing the plant to survive in a dry and shrivelled state then absorb up to 10 times its weight in water once it rains. David refers to mosses as classic resurrection plants. Museum specimens stored in herbaria for decades still retain this water absorbing capacity;
• chemical “sunscreens” such as in the bright yellow lichen Fulgensia;
• reflective surface structure, eg the crystalline looking lichen Psora (Eremastrella) crystallifera;
• powdery surface covering such as in the lichen Trapelia, thought to help prevent water loss;
• black pigmentation such as Collema – one of the most common lichens in the west of the state;
• protective scales e.g. the liverwort Riccia has black scales on its underside and as the plant dries out it folds over so the black surface protects the more sensitive green surface. When folded up these are hardly noticeable but they open up and increase their surface area 4 or 5 times – very useful for protecting the soil surface from erosion;
• habitat selection e.g. by choosing sites below rock overhangs where small amounts of available water will be channelled to them. This is important for liverworts as they require free water for sexual reproduction.

One of David’s favourites cryptogram is the vagrant lichen Chondropsis semiviridis. When there is moisture about it lies flat on the ground, but as it dries out it curls up into a ball about the size of a marble and is transported by the wind.

Most of us probably remember from high school biology that a lichen is a symbiotic relationship between a fungus and an alga, The fungus provides the structure and the alga photosynthesis the food (or can fix atmospheric nitrogen in the case of a blue green alga). Most lichens are very widespread and can be found in similar habitats across the globe – there are probably only 4 or 5 endemic to Australia.

But how does an organism like this reproduce and disperse? This isn’t fully understood. It has long been thought that the two partners met by chance to form a new lichen. Fungal spores are travellers – they are produced in large quantities and they are minute enough to float into the upper atmosphere and get carried by airstreams – but the odds of the right species of spore landing on the right species of alga must be pretty remote. The idea that they reproduce asexually is now starting to gain currency: little bits of lichen containing both the fungal and algal component get broken off and can regrow a new plant.

Want to know more?
David Eldridge and Merrin Tozer have produced a book called “A Practical guide to Soil Lichens and Bryophytes of Australia’s Dry Country”. Its a must for those into cryptograms but also recommended for admirers of the photographers art. The book is available from the Department of Land and Water Conservation’s bookship in Bridge Street Sydney for $14.95 (allow $1.50 for postage and handling).

We didn’t have much time to discuss re-establishment during David’s talk but the book provides some advice:

“Once soil crusts have been destroyed, natural regeneration is very slow. Artificial techniques to manually replace patches of crusts have been tested in Victoria and whilst being effective, are slow and costly. The National Parks Service in the United States has used both wet and dry slurries of salvaged scalped crust to regenerate areas where crust has been destroyed. This salvage method needs to take into account how the soil will be stored. As the organisms in the crust need to photosynthesise and are active when wet, they need to be either kept dry or spread thinly where they can get light”.