How do warming and grazing affect bacterial diversity in the Mongolian steppe?

Background/Question/Methods

Soils are amongst the most biodiverse habitats on the planet. Bacteria constitute a significant portion of this diversity, yet neither the impact of bacterial diversity on an ecosystem, nor the drivers that shape this diversity are well understood. An especially pressing question is how soil bacterial communities may be shaped by changes in climate and land-use. In northern Mongolia, air temperatures have increased by 1.6 °C since 1960, and changes to grazing patterns are occurring due to urbanization and a shift to more sedentary pastoralism. To investigate the interactive effects of climate and land-use change on bacterial communities, soil samples were collected over three years (2009-2011) from an experiment in northern Mongolia, undertaken by the PIRE Mongolia Project (http://mongolia.bio.upenn.edu/). Climate was manipulated using passive open-top warming chambers (OTCs), and was crossed with grazing (ambient and ungrazed). DNA was extracted from each sample and 16S rRNA V4 PCR amplicons were sequenced using the Illumina platform through the Earth Microbiome Project (www.earthmicrobiome.org). The resultant community profiles were analyzed using multivariate, non-parametric statistical tests to explore changes in alpha (i.e. species richness within a sample) and beta (i.e. species overlap between samples) diversity resulting from shifts in grazing intensity and temperature.

Results/Conclusions

Alpha diversity was calculated for each sample using the Chao 1 metric and treatment effects of climate and grazing were tested using ANOVA. Beta diversity (measured with the Unifrac metric) was determined by comparing all possible pairs of samples to obtain a dissimilarity matrix of phylogenetic relatedness and a PERMANOVA was performed to determine significant treatment effects. Alpha diversity differed between years, and climate and grazing interacted to affect alpha diversity such that OTCs increased alpha diversity in ungrazed areas but lowered diversity in grazed areas. We hypothesize that this decrease in diversity could be the result of a decrease in soil moisture, which is caused by a combination of warming and grazing. Beta diversity differed between years, and was significantly affected by climate. There was also a marginally significant difference between grazing treatments and controls. These results suggest that both warming and grazing treatments affect bacterial community composition. However, year had the greatest influence on alpha and beta diversity, indicating considerable temporal variation in soil bacterial diversity, which may be the result of climatic variation between years.