Thermal Acclimation of Soil Microbial Respiration

Consequences for global warming-induced carbon losses

Terrestrial ecosystems play a key role in regulating atmospheric chemistry and therefore our climate. Currently, soils store more carbon than is present in plant biomass and the atmosphere put together. Worryingly, as global temperatures rise, there is the potential for decomposition rates in soils to increase. A large release of carbon dioxide from soils has been predicted, with the potential to accelerate the rate of climate change.

However, the effects of changes in temperature on the communities of micro-organisms involved in breaking down the organic matter present on soils are poorly understood.

The overall aim of this research project is to investigate whether these microbes may respond in such a way as to compensate for the effects of rising temperatures (thermal acclimation). Determining how the microbes respond has fundamental implications for predictions of how much soil C could be lost, and therefore for the rate of 21st century climate change.

Background and Aims

In the short term, the production of CO2 by the respiration of the soil microbes involved in decomposition has been shown to increase strongly as temperature rise. This has led to modelling studies concluding that increased CO2 release from soils due to global warming could accelerate 21st century climate change by up to 40%.

However, in the longer term, ecologists are increasingly recognising the ability of organisms to alter their metabolic rates to compensate for changes in temperature (this is known as thermal acclimation).

For example, although warming initially increases rates of plant respiration, over time, acclimation results in respiration rates subsequently declining substantially (i.e. the long-term effect of the warming is much less than the short-term effect). There is also increasing evidence that the response of soil microbial respiration to warming declines over time. If this were due to down-regulation of microbial activity in response to the increased temperatures (thermal acclimation), then this would greatly reduce the potential for soil C losses under global warming. When a soil is warmed, however, the initial stimulation of activity at the higher temperature results in the loss of some of the most readily-decomposable organic matter. Modelling studies have shown that this loss of rapidly decomposing C could also explain the decrease in respiration rates in long-term experiments. It is essential to determine whether acclimation results in respiration rates declining before substantial amounts of CO2 are released, or whether reductions in respiration are simply a symptom of the on-going loss of carbon from soils.

This project aims at answering the following key questions:

Does soil microbial respiration acclimate to changes in temperature?
Does the magnitude of acclimation vary between ecosystems and climatic zones?
What are the mechanisms that underlie any acclimation response?

Our Approach

Incubation experiment

To detect thermal acclimation, substrate availability must be controlled to ensure that the results observed are not caused by the loss readily decomposable carbon. The first step in achieving this is to remove soils from the field, thus eliminating C inputs from plants. The second key step is to use a novel soil cooling approach, developed by the research team [Hartley et al. (2008) Ecology Letters 11, 1092-1100]. During the pre-incubation period we allow the most labile soil C to decompose and the soils to recover from the initial disturbance. After the respiration rates have stabilised, substrate availability declines much more slowly, and at this point part of the soils are cooled. Thermal acclimation can be studied by cooling the soils. Acclimation must be a reversible process and thus if acclimation to higher temperature results in a down-regulation of respiration, similarly acclimation to colder temperature should lead to a subsequent up-regulation of respiration after cooling. Critically, this approach avoids the issue of higher temperatures increasing the rate that readily-decomposable organic matter is lost. Thus, if soil respiration rates increase and recover after cooling, this would provide clear evidence of thermal acclimation; such a result could not be caused by an increase in the availability of organic matter in the soils as there are no carbon inputs in our incubation experiment. If there is slight decrease in respiration rates, this will be most likely due to a slow decrease in organic matter availability that still happens also in the cooled soils. If respiration decreases faster in the cooled soils, than soils that stayed at the higher control temperature, this will indicate enhancement. In contrast to acclimation, “enhancement” means that microbial community responses can enhance the effect of a temperature change on respiration rates.

Soil sampling

In order to be able to generalize our results to wide geographical areas, and to say where acclimation or enhancement is likely to occur, we will sample soils from a climatic gradient from the Mediterranean to the high Arctic.

‌This is possible through cooperation with numerous collaborators and institutes. We will collect samples from five ecosystem types: arable, coniferous evergreen forest, deciduous broadleaf forest, grassland and ericaceous heath. During the first year of the project, samples from the Scottish and English sites have been collected and are being incubated.

Characterizing microbial communities

To study the possible mechanisms behind any acclimation response we will characterise changes in the structure and function of the microbial community after cooling. We are using both physiological and biochemical analyses, as well as DNA-based techniques to understand the changes in the microbial communities.

Soil carbon modelling

At the end of the experiment we will also look at the implications of our findings by developing new models of soil organic matter decomposition. We will compare how the mechanisms identified in this study change the outcome of modelled soil carbon losses in response to global warming.

Research Team