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Ben-Gurion University of the Negev (2013)

Effect of heavy rainfall on desert soil bacterial community composition and dynamics

Azatyan, Ani

Titre : Effect of heavy rainfall on desert soil bacterial community composition and dynamics

Auteur : Azatyan, Ani

Université de soutenance : Ben-Gurion University of the Negev

Grade : Master of Science (MS) 2013

The dogma in arid regions is that water strongly limits primary productivity and therefore, arid ecosystems exhibit a pulse-dynamic response to rainfall, in which rain events offer brief pulses of resource availability that can exert a strong influence on productivity and function of plant and animal communities. However, the abundance, diversity and structure of soil bacterial communities in arid ecosystems were seldom studied. It was shown that soil bacteria in general are strongly influenced by soil temperature, moisture, salinity, pH, or combinations of these parameters. In addition, seasonal variations of the edaphic parameters often result in changes in the bacterial community structure, especially in dry ecosystems. Investigations of temporal patterns of soil bacterial diversity in arid environments, demonstrated that heavy rainfall is an important determinant of bacterial activity variation, but overall changes in abundance and diversity were not elucidated. It is not clear whether microorganisms in arid environments respond to rainfall events similarly to macroorganisms. To answer this question the diversity and abundance of active bacterial communities were studied in hot desert soil microcosms by closely following drought-raindrought cycles. The aim was to elucidate the bacterial response to the amount of rain (50 mm and 10 mm), the incubation temperature (25°C and 36°C), and diurnal cycles (36/10°C) assuming that these parameters coalesce with the desiccation rate that will in turn affect the bacterial community. To that end, barren desert soil (directly below the crust) was collected and packed into columns designed and constructed for the purpose of these experiments. Triplicate soil columns were equipped with rain and drain simulators and were operated for one month during each experiment. At constant intervals the soil was sampled from the columns for physicochemical (including pH, salinity, water content, nitrite, nitrate and total carbon) and bacterial analyses. The abundance, diversity and community composition of the bacterial community were explored, with special focus on the dominant Actinobacteria phylum. With an exception of water content, no major differences were observed in the physicochemical parameters of the soil incubated under different rain regimes, temperature or diurnal cycles. Interestingly, the biomass and diversity of the bacterial communities including the Actinobacteria phylum, were unchanged under the various treatments. However, the community composition was significantly altered within and between experiments : the rain and temperature alterations yielded significantly different soil bacterial communities, yet, diurnal cycles inflicted insignificant changes in the soil column communities. The obtained results suggest that unlike macroorganisms soil bacterial diversity and abundance in hot desert environments are unaltered by hydration-desiccation cycles. However, the community composition is markedly changed on a temporal scale following rainfall and desiccation. Moreover, the community changes seem to be due to the amount of rain and the desiccation temperature but not the diurnal cycles.

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