The objectives of this study were to examine the temperature sensitivity of microbial growth efficiency in soils subjected to chronic warming and to evaluate the potential impact of altered efficiencies for soil C storage using the DAYCENT soil organic matter model. Soil samples were collected from the Soil Warming × Nitrogen Addition Study at Harvard Forest in Petersham, MA, USA. Microbial efficiency was determined in control and heated soils by amending samples with 13C-labeled glucose or phenol, incubating at three temperatures (5, 15 and 25°C) and tracing the 13C into microbial biomass, DOC, and respired CO2.
Results/Conclusions
Microbial efficiency of glucose-amended soil was temperature insensitive. In contrast, the efficiency of phenol utilization exhibited a strong temperature response. Phenol-amended soils had much lower efficiencies compared to glucose (12-32%) and showed nearly a 60% drop in efficiency when incubated at 25 versus 5°C. The effect of temperature on efficiency was not altered by chronic soil warming. Reducing the efficiency parameter in DAYCENT by a comparable degree as observed in our experiment (for phenol) resulted in a loss of about 400 g C m-2 over a six year period. The decrease in efficiency led to a decreased microbial biomass due to more C lost as CO2 during organic matter decomposition. When soil warming was applied in the model, more soil C was lost for a comparable decrease in efficiency. Our results suggest that the degree of compound lability determines the temperature sensitivity of microbial efficiency. Regardless of compound lability, soil warming did not directly affect the temperature sensitivity of microbial efficiency. However, long-term warming may alter compound lability in the soil and thus determine the direction of warming effects on both microbial communities and soil C storage.