We sought to understand how bacterial diversity is structured within the landscape and how it may respond to global change. We investigated soil bacterial communities in a climate change experiment set within the steppe of northern Mongolia, where air temperatures have increased by 1.6 °C since 1960 and land-use patterns are shifting with socio-economic changes.
In June and again in July 2012, we collected soil samples from our climate change experiment at two south-facing slope locations (lower versus upper). We used open-top chambers (OTCs) to create warmer and drier conditions, which were crossed with grazing and watering treatments on the lower and upper slope, respectively. We measured plant-available nitrogen, soil temperature and moisture were for each treatment plot. We recorded the plant species associated with each sample to determine how the plant community may affect the bacterial community, while accounting for abiotic factors. We extracted DNA and sequenced the 16S rRNA V4 PCR amplicons using the Illumina MiSeq platform.
We used structural equation models to determine how our experimental treatments affected bacterial diversity through either abiotic factors or plant community biomass or plant species richness. We also used indicator analysis to identify bacterial “indicator phyla” associated with our treatments.
Results/Conclusions
Soil moisture, temperature and available N, but not plant richness, affected bacterial diversity, suggesting that abiotic, but not biotic, factors structure soil bacterial communities in the Northern Mongolian steppe. Throughout the season and across the landscape, soil moisture stands out as the most responsive factor to our OTC, watering and grazing treatments, often with consequential changes in the bacterial community. Watering on the drier upper slope may push the bacterial community toward that of the lower slope while warmer and drier conditions may push the lower slope community toward that the upper slope. This statement is supported by predictable shifts in two bacterial indicator phyla, Elusimicrobia and Verricomicrobia, in response to watering and OTC treatments on the upper and lower slope, respectively.
Our results suggest that global change in the Mongolian steppe will act on bacterial communities by its affect on environmental variables and not through its affect on the surrounding plant community. 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 indicators of a drier future for Mongolia.