Alteration in microbial functionality due to reduced daily temperature variability in the Chihuahuan desert

Background/Question/Methods

Daily minimum temperatures are predicted to increase at a significantly higher rate than daily maximum temperatures resulting in a reduced daily temperature range (DTR). The effect of reduced DTR on soil biogeochemical processes and microbial community structure and function is poorly understood despite the vital role of soil microorganisms in decomposition and nutrient cycling. A study to address these concerns was initiated in the Chihuahuan Desert at Big Bend NP, Texas in 2006.

To reduce DTR_soil we used polyethylene shade cloth suspended above the soil surface. This reduced solar radiation but allowed normal precipitation infiltration and airflow. Our study included five shaded plots established in 2006, five shaded plots established in 2009, and five control plots. This design allowed us to evaluate the short and long-term consequences of changing DTR_soil. Soil fungal and bacterial community functional diversity was evaluated using Biolog microtiter plates. Substrate utilization of carbon and nitrogen sources was examined. Additionally, enzyme activities of important carbon and nitrogen cycling enzymes were evaluated.

Results/Conclusions

DTR_soil was reduced by 2-6°C in the shaded plots with increased daily minimum temperature and lower daily maximum temperature. The decrease in DTR_soil increased the functional diversity of fungi on carbon sources by 46-65%. Increased activity in the shaded plots was due to increased utilization of carbohydrates, amino acids, amides and polymers. Similarly, functional diversity of fungi on nitrogen sources was increased by 9-36% on the shaded plots when compared to the control plots. Overall, the utilization of carbon and nitrogen was higher in August than in March. Carbon substrate utilization by soil bacteria was higher in the shaded plots only with respect to polymer utilization. Likewise, β-glucosidase and β-glucosaminidase activity was 31% and 32% higher in the shaded plots, respectively. However, no noticeable change was observed for phenol oxidase activity among the treatments.

These results show that simulated reduction in DTR_soil increases the functional diversity of soil fungi to a greater extent than soil bacteria. If DTR_soil continues decreasing in these systems as predicted, microbial communities in arid ecosystems may shift towards fungal dominance.