Reduced daily temperature range influences the magnitude of soil microbial activity in a Creosotebush bajada in the Chihuahuan desert in Big Bend national park
Hyndman, Michael Lee
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Global climate change models for desert ecosystems not only predict an increase in mean temperatures but also an increase in the daily low temperatures resulting in a decrease in daily temperature range (DTR). While soil moisture and antecedent precipitation regimes are considered in current models for desert ecosystem functioning, these models are limited in understanding the role of temperature and seasonal variability, specifically the daily temperature range and the response of soil microbial communities to available soil moisture during different times of the year. This study seeks to utilize new automated soil gas monitoring technology and solar shading, altering the DTR by increasing nightly low temperatures and decreasing daily high temperatures, to measure soil microbial community respiration activity in response to reduced DTR compared with unaltered soil temperature dynamics in control plots. The research was conducted in a creosotebush bajada in the Chihuahuan Desert in Big Bend National Park. Over the course of a year of with observations beginning in November 2012 and ending in December 2013, the soil monitoring equipment recorded hourly temperatures and CO2 emissions from the top 15 cm of soil. Soil sampling over the study period was conducted to determine levels of microbial biomass of carbon (MBC), fungal substrate activity (FSA), fungal substrate richness (FSR), extractable soil ammonia (NH4-N) and nitrate (NO3-N), extractable phosphorus (P), available potassium (K), soil organic matter (SOM), and soil pH. For some of the experimental parameters there was no significant difference between treatments on individual sampling dates, but there was an observed difference between treatment groups for some parameters across the whole time period of the study. Reduced DTR plots experienced an increase 7.7% in CO2 flux, an increase of 28.9% in MBC, a 32% increase in FSA, and a 26% increase in FSR. These results indicate that in addition to the pattern of rain events and levels of soil moisture, soil microbial activity in arid and semi-arid regions is governed in some way by soil temperatures over the day. Reductions in DTRsoil appear to ultimately allow soil microbial communities to respond to rain events more robustly than previously predicated by moisture responses alone. Thus, DTRsoil changes the magnitude of the response but not the overall seasonal timing of the response. Even when soil moisture is optimal, microbial activity as measured by CO2 efflux was reduced when DTR soil was high. The incorporation of DTR response in microbial communities in desert soils will help to fully understand and predict the impact of further climate change in regions that are already sensitive to variable climate and precipitation.