Slope Orientation and Soil Particle Size Determine the Soil Microbial Community in Desert Ecosystem.

Desert ecosystems are determined by harsh abiotic conditions with high radiation and low annual unpredictable rainfall in time, frequency and dispersion. The soil in this ecosystem as every other soil is composed by particles of different sizes sand (0.05-2 mm), silt (0.002-0.05mm) and clay (< 0.002mm). Slope orientation e.g. North and South facing slopes and location on the slope had been found to be strongly affected by abiotic conditions. The aim of this study is to determine the effect of slope orientation, location on the slope and particle size on soil microbial diversity and community composition. Soil samples were collected in the Negev Desert, Israel from a North and South facing slopes, from three different locations e.g. top, middle and the bottom sites. As control we had used the meeting site between the two slopes. Soil samples were collected in wet (winter) and the driest (autumn) seasons.  The soil samples collected were fractionated by dry sieving to the mentioned particle size fractions. Soil abiotic parameters (soil moisture, organic carbon and electrical conductivity) and T-RFLP analysis with 16S rRNA genes as target gene for bacteria and the ribosomal ITS region for fungi were conducted, in order to assess the diversity and dissimilarity of the soil microbial community. In both seasons soil moisture was higher on the north facing slope. In the wet season soil moisture was found to be 2 fold higher on the top position than at  the bottom of the slope. Organic carbon was also found in both seasons to be higher in the slope facing north. No significant differences were observed in the electrical conductivity on spatial and temporal basis. T-RFLP analysis showed that the community composition in the clay fraction is differed from the community composition on the sand and silt fractions. Fungal community composition shows a significant difference between the two slopes and the different location along the two slopes.  Both seasonality and soil particle size had been found to have a major effect on both fungal and bacterial community composition. No dissimilarities were found in the effect of slope orientation and height level on bacterial community composition. Fungal community seemed to be more influenced by the topographic parameters slope orientation and height level along the slope than was the bacterial community. Both microbial communities were influenced by particle size fraction and seasonality. The results obtained elucidate the need of further studies that should determine the microbial community composition and function.