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Abstract: Biological soil crusts (BSCs) abound in the Hallamish dune field in the western Negev Desert, Israel. While their abundance may imply high adaptability to environmental change, such as fluctuations between wet and dry conditions following winter rains, summer rains, although rare, may also occur in the Hallamish dune field. The aim of the present paper is to examine crust responses to winter and summer rains, focusing particularly on its biomass components, chlorophyll and carbohydrate. In addition, species composition, during summer and winter was examined. Analysis took place during an exceptionally wet winter (1994/95 with 172mm) and a summer rainstorm (12.5.93 with 9.7mm). The data showed a 2–3 fold increase in chlorophyll a and total carbohydrates and a much richer species composition following the heavy winter rains of 1994/95. Yet, the data also showed ~15–30% decrease in the chlorophyll content of the crust (with no concomitant significant decrease in total carbohydrates) following the summer rainstorm. Intense weathering by the summer rain coupled with cell mortality may explain the decrease in the chlorophyll content following the summer rainstorm, suggesting possible changes in the BSC following a potential change in the precipitation regime due to global climate change. [Copyright &y& Elsevier]

Abstract: Dictating the length of time during which water is available, evaporation plays a cardinal role in ecosystem ecology and structure. This is especially so in arid and semi arid zones, where precipitation is highly limited. Covering the surface with a distinct 1–10cm thick layer, biocrusts (called also biological soil crusts or microbiotic crusts) may affect the evaporation rate and hence the water loss of the underlying parent material. Owing to their wide occurrence in arid and semi arid regions, examination of their impact upon the evaporation rate of the underlying soil is of special importance. The evaporation of water from sand covered by four types of cyanobacterial crusts and one moss-dominated crust in the Negev Desert was examined along with bare sand. The crusts and sand were subjected to wetting by rain and sprinkling (in Petri dishes and within 25cm diameter plots), and the crust and subcrust temperatures and moisture content were measured. In addition the albedo of the crusts was also measured. Whereas thick and high-biomass crusts tended to retain higher amounts of water in the Petri dishes, they were found to increase subsurface temperatures (by up to 3.9°C at 5cm depth) under field conditions, subsequently decreasing the 0–10cm moisture content. Moisture content at the crusted plots was significantly lower than that of non-crusted plot, with crusted plots having an evaporation rate of 1.24–1.57 (with average of 1.38) times higher than the bare sand. These findings were supported by field measurements under natural rain precipitation. In both cases, the field data under sprinkling and rain precipitation showed that the non-crusted surfaces retained their available moisture at the upper 10cm of the soil profile for 5–7days longer than that of the crusted surfaces. The findings may explain contradictory data published in the literature regarding field and lab experiments, and may point to the negative effect that biocrusts may have upon the soil water budget and hence on water availability for higher plants. [Copyright &y& Elsevier]