Effects of temperature, fertilization, and carbon supply on mass-specific respiration of soil microbes
Mersmann, Calley Aileen
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Heterotrophic microorganisms decompose stores of soil organic carbon and as a consequence release carbon dioxide to the atmosphere. Although in the short-term microbial respiration rates increase in response to increasing temperatures, the effect of long-term temperature increases on respiration rates remains uncertain. Due to mechanisms such as evolutionary trade-offs in enzyme function and shifting community structure, it is expected that mass specific respiration (Rmass) rates will decrease as soil microbe communities adapt to higher temperature regimes. In order to test this potential, we used a laboratory microcosm approach to impose two thermal regimes (constant 12C or 28C) on twelve soil samples treated with nitrogen and/or phosphorus for 84 days. Additional carbon in the form of glucose was added weekly to one replicate of each soil to account for the possibility of substrate limitation masking the difference in Rmass rates between the two incubation temperatures. To determine Rmass rates of the soil microbes, we measured the amount of carbon dioxide produced by each soil sample using assay methods similar to those used in animal, plant, and microbial thermal adaptation studies. At intermediate assay temperatures, Rmass rates were expected to be greatest for the 12C experimentally incubated soils and lowest for the 28C soils, indicating thermal adaptation of microbial respiration. Additions of nitrogen and phosphorus were not expected to significantly affect respiration rates. Interestingly, although Rmass rates for soils with water-only additions followed predictions for microbial adaptation, those for glucose-treated soils did not, suggesting that substrate limitation and differences in biomass contributed to decreased potential respiration rates of warmer soils. Nitrogen and phosphorus additions had significant effects on soil respiration, indicating that nutrient levels are important in determining how soil microbes will respond to various temperature regimes. This research is necessary to not only more fully understand microbial respiration responses to changing temperatures and varying nutrient additions but also to more accurately predict possible feedbacks between microbial respiration and climate change.