Responses of a soil microbial community to reduced daily soil temperature variability: A field study in the Chihuahuan Desert

Background/Question/Methods

Recent climate models predict that night-time warming will increase significantly in arid systems leading to decreases in daily temperature range (DTR). As desert ecosystems are traditionally regulated by extremes in temperature and low moisture availability, the effects of a reduction in DTR_soil is likely to be more pronounced in such systems by altering ecosystem functions through effects on microbial activity, biomass production and biodiversity. Furthermore, understanding how decreasing DTR_soil may alter microbial respiration dynamics will be critical to predicting changes to the carbon budget under conditions of reduced DTR_soil with climate change.  To investigate the role of DTR_soil in regulating microbial community structure and function, we set up a DTR_soil manipulation study in the Chihuahuan Desert, at Big Bend National Park.

The study was established in 2006 within a creosotebush bajada. Initially, erosion control blankets were used to reduce DTR_soil from 2006-2008.  We altered the design in 2009 by suspending polyethylene shade cloth on a PVC frame 13 cm above the soil surface, which minimized alterations to the boundary layer while successfully reducing DTR_soil.  In 2009, we established five additional plots with the new design along with the five converted plots, and five unshaded plots continued to serve as controls.

Results/Conclusions

Shading resulted in lower or similar maximum daytime temperatures and higher minimum nighttime temperature depending on season, reducing the daily temperature variability. Soil temperature variation was reduced by 6.6°C on the surface and by 2.9°C at 15cm depth of shaded plots compared to that of controls. The decrease in DTR_soil significantly increased microbial biomass carbon and nitrogen (up to 90% and up to 70% respectively) while reducing soil free nitrogen (NO3-N by 24%) during late summer of 2010. Similarly, soil CO₂ flux increased by 38-61%, and FAME analysis showed significantly increased saprophytic fungal composition (up 50%) in the reduced DTR_soil plots compared to controls.  Gram negative bacteria and actinomycetes abundances based upon FAME analyses were higher in controls compared with reduced DTR_soil plots.  Soil moisture was slightly higher in the reduced DTR_soil plots as a consequence of reducing soil temperature maximum during the summer, but not enough to account for observed changes as differences occurred irrespective of rainfall events.

Current DTR_soil may account for the lower microbial activity associated with desert soils.  Moreover, as DTR_soil decreases in these systems with climate change, the contribution of deserts to global carbon flux may actually increase from present proportions.