Multiscale Impacts of Cyanobacterial Crusts on Landscape Stability

Most soils are a mixture of inorganic (mineral) and organic (e.g. plant) material. In deserts, often there is not enough rain for large plants to grow, but organisms such as algae and lichens can survive. Our research focuses on cyanobacteria which live on or near the soil surface and produce sugars as they grow, called polysaccharides, which can stick small particles (e.g. sand grains), together. This binds the soil forming a 'cyanobacterial soil crust' that is helpful in the landscape because it makes it harder for soil erosion to take place. Cyanobacterial soil crusts occur naturally, but can also be made artificially as part of a land management plan. In deserts often soil erosion is caused by wind, however sometimes it rains causing water erosion, and the amount and intensity of rainfall affects crusts. Light rain causes cyanobacterial growth and helps to thicken and strengthen the crusts, but heavy rain can break up crusts, making them less able to protect soil from erosion. We know little about the relationship between different rainfall intensities and the ability of crusts to protect soil from wind and water erosion but it would be useful to do so because we could better plan activities such as where cattle graze (hooves break up weak crusts) and when to leave fields bare.

This project is exciting because it studies the impact of rainfall, runoff (surface flow) and wind erosion on cyanobacterial crusts at different scales. To begin, we control conditions by growing crusts on an artificial soil bed under a rainfall simulator that lets us choose how much rainfall occurs, and how long it lasts. We also choose whether the soil bed is flat or sloping, and can control runoff rate. After the simulated rainfall, we will use a wind tunnel on top of the soil bed to simulate wind erosion - again we choose the wind speed and can measure how much soil is blown away. By doing this, we can test the response of cyanobacterial soil crust to different rainfall events (does the crust get thicker? is it broken up and washed away?) and we can measure how good the crust is at preventing wind erosion. Our approach is unusual because it looks at how one set of processes (rainfall and runoff) affects a second process (wind erosion). From this we will develop a model to explain and predict the impact of rainfall/runoff on soil crust growth and susceptibility to wind erosion. The model will then be tested in the field using natural soils and cyanobacterial crusts. To guarantee a range of rainfall and wind events, the field tests will be partially controlled using a portable field rainfall simulator and wind tunnel.

Finally, the controlled experiments are conducted at a small scale but we will broaden the spatial and temporal scope of the project to the regional scale which is more applicable to understanding landscape stability. We will do this using remote sensing because aspects of cyanobacterial crust growth and development can be detected using satellite data. We will use these data to examine cyanobacterial crust response to rainfall and runoff (both also detectable from space) at the regional scale and monitor the time-lag between these hydrological inputs and dust storms to further test the model at larger spatial and temporal scales.

Cyanobacteria occur in many environments, e.g. the protective crusts they form are important in temperate climates where they protect soil between crops from water erosion, and they form on coastal dunes, where they reduce sand transport by wind. Having developed and calibrated a model for predicting the impact of water on the protective role of crusts in drylands, we can test the model on other soils and under alternative rainfall patterns (e.g. temperate or tropical). Scientists have predicted that the amount and intensity of rainfall in many areas will change in the future; it will also be possible to use our model to try and predict how this will affect cyanobacterial crusts.

Biological soil crusts occur worldwide and are recognised as important for protecting inter-crop areas from erosion in Europe, for stabilising UK coastal dunes and steep tropical slopes in Hong Kong and for facilitating recovery of soils following disturbance by agricultural activities such as grazing and ploughing and also natural processes such as deglaciation. The development and testing of a model for predicting and explaining the interaction of water and wind erosion is therefore potentially valuable for assessing the seasonal impact of activities that damage surface crusts (e.g. tillage) and for timing management intervention, e.g. inoculating soils with cyanobacteria to initiate stabilisation.

This research project is designed to improve understanding of the response of cyanobacterial crusts to hydrological inputs - whether from natural rainfall, high magnitude, low frequency flood events, or via controlled irrigation. The controlled laboratory and field experiments will be used to develop, test and calibrate a model for quantifying hydrological impacts on the resistance of cyanobacterial crusts to wind erosion. In central Australia, where the model will be tested, there is concern and awareness of the impact of land management strategies (such as stocking rates) on soil condition and the ability to assess the cyanobacterial component of soil condition and how it changes in response to hydrological events will be very valuable, particularly to:

- land managers seeking to optimise the timing of activities that damage crusts (such as grazing or ploughing) to natural seasonal climate patterns (for example seasonal rainfall); and
- those deploying artificial or relocated biological soil crusts as part of landscape restoration and/or stabilisation strategies, again with a particular focus on timing intervention procedures to maximise surface protection, but also for managing the use of additional resources (such as irrigation) to aid crust establishment.

The Pathways to Impact document outlines our current involvement in community science and an initial strategy for engaging land owners and managers within this project. In promoting this, we will use the existing DustWatch website to inform interested parties of our findings. We will also present aspects of the work through the Australian Rangeland Society. This is an independent, non-political organisation for people interested in the management and sustainability of natural and semi-natural rangelands. Its membership is primarily non-academic and is dominated by government organisations, natural resource managers and community groups. The Society runs a discussion forum, a journal (to which we will submit articles) and an annual conference that at least one member of the research team will attend each year.