Date of Award
Doctor of Philosophy (PhD)
Brenda B. Casper
Microorganisms are the most diverse life forms on Earth and are the foundation of any ecosystem. As estimates of microbial diversity rapidly increase with advances in sequencing technologies, so does the need to identify the drivers of such overwhelming diversity. This is particularly true in soil—the most biodiverse habitat on the planet and the key component of terrestrial ecosystems, which are being altered by changes in climate and land use. In order to understand the potential consequences of these changes, we conducted a multi-year experiment to test the effects of global change on soil bacterial communities in northern Mongolia, a region where air temperatures have increased by 1.7 °C since 1960, and traditional land-use patterns are shifting with socio-economic changes. Set in the semi-arid steppe, our global change experiment allowed as to evaluate responses to multiple stressors at once over a range of spatial and temporal scales. Over the course of three years, we investigated soil bacterial diversity at two positions (upper and lower) along a south-facing slope and documented the response of these communities to three experimental treatments: a Watering experiment (upper slope only), a Grazing experiment (lower slope only) and a Climate Manipulation experiment (both slopes). We measured diversity using both the number and abundance of distinct bacterial taxa in a soil sample and then correlated these findings with corresponding measurements of biotic and abiotic factors, which included plant richness and biomass, as well as plant available N, pH, soil moisture and soil temperature. We found that temporal and spatial factors explained much of the variation in the bacterial communities. After accounting for temporal and spatial variation, soil moisture content was the primary driver structuring bacterial diversity across the landscape and within experimental treatments. In particular, the effects of climate change on these semi-arid grasslands may act primarily through soil moisture content. Concomitant shifts in key members of the bacterial community may ultimately be bioindicators of a drier future for Mongolia.
Macrae-Crerar, Aurora Alexandra, "Space, Time and Change: Investigations of Soil Bacterial Diversity and its Drivers in the Mongolian Steppe" (2016). Publicly Accessible Penn Dissertations. 1868.