HYDROLOGICAL PROCESSES Hydrol. Process. 16, 2989–2994 (2002) Published online 19 June 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1082 Cyanobacterial crust and soil particle detachment: A rain-chamber experiment R. D. Hill, 1 Sanjay Nagarkar 1 and A. W. Jayawardena 2 * 1 Department of Ecology and Biodiversity, The University of Hong Kong, Hong Kong, People’s Republic of China 2 Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China Abstract: Cyanobacteria are known to form a crust on soil surfaces holding soil particles together and thereby offering resistance to erosion. A controlled experiment was carried out to throw light on this issue. The experiment consisted of subjecting erosion cups filled with soil to artificial rainfall in the laboratory. Three sets of erosion cups, each set consisting of six, were used. One set consisted of soil with inoculated cyanobacteria and the second set consisted of soil with naturally colonized cyanobacteria, both over a period of about 8 months. The third set consisted of soil with no bacterial growth. The results indicate that the soil erosion cups with the inoculated cyanobacterial crust had at least one order of magnitude less erosion for coarse soils, and about two orders of magnitude less erosion for fine soils, compared with erosion cups with no bacterial crust. Copyright  2002 John Wiley & Sons, Ltd. KEY WORDS cyanobacteria; soil erosion; erosion cups; artificial rain INTRODUCTION Cyanobacteria, also known as blue–green algae, are widely distributed on soil surfaces around the world where they form a crust, which consists of water-stable soil aggregates held together by a variety of cyanobacterial species. Cyanobacterial crusts play an ecologically important role in soil, influencing soil erosion, fertility, infiltration, nutrient uptake and vascular plant germination (Johansen, 1993; Perez, 1997). Cyanobacterial crust can reduce both wind and water erosion because of the adhesive property of the cyanobacterial mucilaginous sheath binding clay and silt materials of the top-soil (Avinoam and Gerson, 1983). The filamentous cyanobacteria form an intricate webbing of fibres on the soil surface, which also penetrates the soil a few centimetres (Nagarkar, personal observation). Consequently, cyanobacterial growth provides a skeleton and cementing to the upper layers of soil that lead to soil aggregate stabilization (Perez, 1997). Although cyanobacteria have long been known to form erosion-resistant crusts in the terrestrial environments of arid and semi-arid regions, the existence of crusts in humid tropical areas has not been reported until recently (Watson and Stojanavic, 1965; Gayel and Shtina, 1974; South and Whittick, 1987; West, 1990; Zimmerman, 1993; Van Elsas et al., 1997; Hill et al., 2000). At a site in Hong Kong where a long-term study of soil erosion is being carried out using the erosion-pin method, rapid initial rates of soil loss were noticed on two 6 m ð 20 m plots on which the growth of vascular plants was prevented by the application of a herbicide (Hill and Peart, unpublished data). On a steeper plot (average slope 27 ° ) the average annual soil loss between the beginning of 1994 and the end of 1997 was close to 13 mm year 1 . In 1998 and 1999 this fell to about 3 mm year 1 and well-developed cyanobacterial crust was noted [a species list is given in Hill et al. (2000)]. By the end of the year 2000, no erosion was detected using the method employed. On a gentler slope (mean slope 16 ° ) initial rates of soil loss averaged * Correspondence to: A. W. Jayawardena, Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China. E-mail: hrecjaw@hkucc.hku.hk Received 18 June 2001 Copyright  2002 John Wiley & Sons, Ltd. Accepted 10 December 2001