Microbial metabolic responses to short-term soil warming in a temperate deciduous forest

Background/Question/Methods

Temperatures are predicted to rise by 5 °C in northern latitudes over the next century due to increased greenhouse gasses in the atmosphere.  Globally soils contain about 80% of the carbon in terrestrial ecosystems.  The mechanisms underlying the storage of soil carbon and how these mechanisms might be altered at elevated temperatures are not well understood.  Microorganisms drive soil carbon cycling by metabolizing organic compounds.  In the short term, if microbial metabolism is limited by temperature then elevated soil temperatures may increase metabolic rates of soil microorganisms.  However, in the long term, availability of labile carbon and other resources may become limiting to microbial metabolism.  When this occurs the rate of microbial metabolism will decrease, causing an apparent acclimation of metabolism to elevated temperature.  This study documents the changes in the metabolic function of the soil microbial community after a short-term warming (+4 °C) and moisture manipulation (+30% ambient precipitation) experiment in a sugar maple forest in Northern Michigan.  In the majority of soil warming studies water availability is lower in warmed versus unwarmed plots.  The fully factorial design of the soil warming and moisture manipulation experiment allows us to tease apart independent treatment effects and their interactions. 

Results/Conclusions

Enzyme assays were conducted on soils to determine the relative differences in the activity of enzymes capable of degrading a broad range of substrates from labile to recalcitrant compounds.  After one month of warming the enzyme assays showed no significant differences between treatments for the six enzymes tested at 20 °C.  Microbial biomass-specific respiration at 20 °C also showed no significant differences between any of the treatments.  Taken together these results suggest that no short-term acclimation of microbial metabolism has taken place.  According to typical enzyme kinetics the warmed plots would likely show higher metabolic rates if enzyme activity and microbial respiration were measured at field temperatures.  Soil respiration measurements performed in situ were significantly higher in warmed versus unwarmed plots, supporting this rationale.  This study was conducted after one month of soil warming and it is possible that more differences between treatments will emerge as experimental manipulation and investigation continues.