Controls on soil bacteria and fungi in the Southwestern US may provide insight into community shifts under future climate change

Background/Question/Methods   Climate models project extreme droughts and increased inter-annual precipitation variability in the Southwestern US over the next several decades. Increased precipitation variability will undoubtedly alter fundamental ecosystem properties and processes such as soil carbon storage and cycling. Although the activities of soil microbial communities are strongly linked to moisture and temperature, it is unclear how soil bacteria and fungi will respond to climate change predicted for the Southwestern US. Moreover, the interactions of soil mineralogy and potential biological response remain unclear. We collected soils at the extreme ends of elevation gradient (temperature and precipitation range of >10ºC and >50 cm, respectively) in the Santa Catalina Mountains, AZ and quantified bacterial and fungal markers and carbon utilization during pre- and post-monsoon precipitation conditions. Contrasting parent materials (schist and granite) were paired at each elevation. We expected climate to determine the relative abundance of soil fungal and bacterial markers and diversity of soil C utilization, and differences in parent material to modify these responses through controls on soil physical properties. We used Real Time Polymerase Chain Reaction (RT-PCR) and EcoPlate^TM C utilization assays to determine the relative abundance of soil bacterial and fungal populations and rate and diversity of carbon utilization. We analyzed for soil organic matter, particle size analysis, and moisture content to examine the soil physical and chemical controls on these responses.

Results/Conclusions   RT-PCR showed that mean total soil bacterial and fungal biomarker levels increased from 7.8 x 10^7 to 2.3 x 10^8 markers per gram soil from the pre- to the late-monsoon season, respectively, representing an increase of nearly 400% over all sites. As expected, observed increases were smallest in low elevation granite-derived soils, where bacterial biomarkers increased by only 50%, from 2.7 x 10^8 to 4.1 x 10^8 markers per gram soil in pre- and late-monsoon, respectively. Reduced bacterial response to precipitation in low elevation granite soils compared to schist soils could be explained by physical controls (coarser texture, reduced water holding capacity), rather than chemical controls (carbon quantity). However, rate and diversity of C utilization was lower at the low elevation granite sites compared to the high elevation granite sites. Our findings suggest that soil physical properties derived from parent material may strongly control microbial biomass under changing climate conditions, whereas both chemical (carbon availability) and physical soil properties impact microbial diversity of carbon use.