Impact of extreme soil moisture changes and decreased daily temperature fluctuations on microbial community structure

Background/Question/Methods

Reduction in daily temperature variability has been predicted as one of the side effects of climate change in many climate models. Different field studies have supported the predictions by observing the increase of daily minimum temperatures at a significantly higher rate than daily maximum temperatures. Although this aspect of climate change has been addressed scantily in plant physiology, how the reduction in daily temperature range (DTR) affects belowground processes is poorly understood. To study microbial community response to the decrease in DTR of soil (DTR_soil), a study was initiated in the Chihuahuan Desert at Big Bend NP, Texas in 2006 and is currently ongoing.

Polyethylene shade cloths were suspended above the soil surface for reducing DTR_soil on ten 4X3m plots, which were referred as shade plots. This design reduced solar radiation but allowed normal precipitation infiltration and airflow. Uncovered plots of the same dimension were considered control plots. Soil microbial community structure was evaluated on the basis of phospholipid fatty acid (PLFA) profiles. Additionally, activities of enzymes involved in carbon, nitrogen, and phosphorus acquisition 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 without a change in soil moisture status. The decrease in DTR_soil increased the concentrations of total PLFA, Gram positive and Gram negative bacteria, and AM fungi significantly (P < 0.05). Higher number and concentration of PLFA was observed when conditions were drier. At 4% soil moisture total number of PLFA indicators detected was reduced by about 40% compared to the average soil moisture status of 1.5% and total PLFA concentration was reduced by more than 50%. Activities of phosphodiesterase were strongly negatively correlated with total PLFA concentrations. The enzymes β-glucosidase, β-glucosaminidase, alkaline phosphatase and phosphodiesterase had significantly higher activities in the reduced DTR_soil plots (shade plots) than in the control plots (P< 0.05).

These results show that continuous reduction in DTR_soil may increase the abundance of bacteria and AM fungi. Aside from potential changes to microbial community structure under conditions of reduced DTR_soil, nutrient cycling rates could be increased due to higher enzyme activities in this arid system.