The effect of moisture distribution on bacterial numbers and microbial activities along a hillslope, northern Negev, Israel

Long-term studies at the Sede Boker site show that great variability in the soil moisture regime exists over distances as short as a few meters. Sites sampled were located at the flat hilltop, 50 m down the slope at the contact zone between the upper rocky and the lower colluvial section, and 70 m further down at the base of the colluvial section. Soil samples for the determination of soil moisture and microbial activity were taken shortly after each storm and during the following drying periods. Microbial parameters determined were viable bacterial counts, dehydrogenase activity, CO₂ evolution, and proteolytic activity. A close correlation was found between moisture content of the soil and microbiological parameters. Upon the first significant wetting of the soil, bacterial numbers and activities increased two to threefold. At runoff recipient sites the onset of microbial activity occurred more than two months earlier than at sites receiving direct rainfall only. The first significant wetting period generated a strong burst of activity at all sites. A second prominent peak in microbial activity was observed in late spring at the lower colluvium only, which coincided with a growth burst of annual plants. Data obtained on the spatial and temporal variability in microbial activity indicate that under the arid conditions prevailing in the area, this activity is controlled by two limiting factors; water availability and availability of nutrients. -from Authors