From juniper woodlands to mountain meadows: Impacts of rainfall intensity on soil bacteria and CO2 pulses

Background/Question/Methods

As global climates warm, rainfall regimes are being altered with future climate scenarios forecasting extended periods of drought and increased frequency of extreme rainfall events. Rainfall change will influence soil moisture dynamics; alter patterns of resource availability and physiological stress on soil bacteria; and impact microbial-mediated ecosystem functions, such as soil CO₂ efflux. Despite these rainfall effects, our understanding of how intra-annual rainfall variablity influences specific soil bacterial taxa remains limited. We addressed this knowledge gap by conducitng long-term rainfall manipulations in three ecosystems (i.e., a juniper woodland=JW, aspen-conifer forest=CF, and mountain meadow=MM) along a mean annual rainfall gradient across the Wasatch Plateau (Utah, USA). Rainfall treatments consisted of two frequencies (i.e., a rainfall event once a week versus once every three weeks) of all naturally occurring rainfall during each time period to produce a low and high intensity rainfall regime. After three years of manipulating rainfall, we: created a single event; measured soil respiration rates; and tracked the activity of bacteria phyla through reverse transcriptase quantitative PCR of 16S ribosomal RNA. The ecosystems collectively captured a spectrum of rainfall amount, soil moisture, and C resource availability that may influence how soil bacteria respond to rainfall.  

Results/Conclusions

We found that rainfall effects on CO₂ pulses and shifts in bacterial phyla activity were influenced by both ecosystems’ position across the rainfall gradient and rainfall intensity.  The rain event induced pulses of CO₂ that lasted at least two days in all three systems; however, JW and CF respiration rates were at least 30% higher under high intensity rainfall.  Thus, the drier soil conditions created between more extreme rain events induced larger pulses. An increase in Gammaproteobacteria and Betaproteobacteria activity coincided with higher respiration rates in CF under low intensity rainfall (slope > 0.22, R² > 0.84) and in WB under high intensity rainfall (slope > 0.08, R² > 0.16).  Conversely, a decrease in Gammaproteobacteria and Betaproteobacteria activity was correlated with higher respiration rates in JW and CF soils under high rainfall intensity. Therefore, these phyla may contribute substantially to CO₂ pulses in soils with less variable moisture conditions, but may be overshadowed by other taxa responding to extreme shifts in soil moisture. Our results add to the growing evidence contributing to the ecological classification of soil bacteria and have important implications for predicting soil CO₂ dynamics as the intensity of rainfall events continues to increase due to climate change.