Lead Research Organisation: UNIVERSITY OF EXETER
Department Name: Geography


Plants are currently reducing the rate of 21st Century climate change by absorbing a substantial amount of the carbon dioxide that Humankind releases to the atmosphere through the burning of fossil fuels. However, the rate of carbon dioxide production by soils as plant material decomposes (known as soil respiration) increases at higher temperatures. Therefore, as global temperatures rise, it is feared that ecosystems which are currently absorbing carbon dioxide may begin to release it, with models predicting that this could increase the rate of climate change by 40 %. This prediction is based largely on knowledge of how soil respiration responds to short-term changes in temperature. However, in long-term warming experiments, following the initial stimulation of activity, rates of respiration tend to decline back towards pre-warming levels. This has led to the suggestion that the micro-organisms responsible for breaking down organic matter may be acclimating to compensate for the warmer temperatures, and that this phenomenon may preserve carbon stocks in the world's soils. There is an alternative explanation for the patterns observed in long-term warming experiments. The initial stimulation of activity may result in the depletion of soil carbon stores, leaving microbes with less to break down, and so reducing rates of respiration. While acclimation could preserve stocks, the carbon depletion explanation implies that the reduction in respiration rates is simply a consequence of the continuing loss of carbon from soils to the atmosphere. Therefore, it is critical to distinguish between these two possible explanations. Previously, methodological limitations have prevented us from determining which explanation is correct. The problem was that when soils are warmed up, acclimation and carbon loss are both expected to reduce respiration rates, making it impossible to distinguish between them. We have shown that this problem can be overcome by using soil cooling. When soils are cooled, initially activity will decline but if acclimation occurs to compensate for the lowering of temperature, rates of respiration should subsequently increase. On the other hand, as carbon losses continue at the lower temperature, albeit at a reduced rate, they cannot be implicated in any recovery of respiration rates. So carbon loss and thermal acclimation are now working in opposite directions, allowing us to distinguish between them. This logic was applied to determine whether microbial activity in soils taken from arctic Sweden acclimates to changes in temperature. After cooling, respiration rates showed no signs of recovery. Rather, many days after temperatures were reduced, respiration rates in the cooled soils continued to decline steeply, with no such response being observed in soils maintained at a warmer temperature. So the effect of cooling was amplified over time. It appears that the soil microbes were responding to the colder temperatures by further reducing activity. Looking at this in reverse, a more active microbial community survived at higher temperatures; so microbial community responses enhanced the effect of temperature on decomposition rates. This phenomenon has not been observed before, and we do not know how prevalent it might be. By extending our work to soils sampled from different ecosystems and at sites ranging from the high Arctic to the Mediterranean, our grant proposal aims to investigate how important soil microbial community responses to temperature are in controlling decomposition rates in European soils. We will determine whether acclimation occurs or whether microbial community responses generally enhance respiratory responses to temperature. We will also investigate how the overall response is controlled. Our project will improve understanding of how global warming will affect decomposition rates in soils, and allow more accurate predictions of rates of 21st century climate change to be made.


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Description Soil samples have been collected from multiple excosystem types across a broad temperature gradient from the Arctic to tropical rainforest.

By determining how soil microbial respiration responds to temperature in these contrasting soils we have been able to determine that compensatory thermal adaptation (a change in activity that compensates for the direct effects of temperature) is rare, occuring in less than 10% of the soils that we looked at. On the other hand, we observed evidence for enhancing thermal adaptation (a change in activity that enhances the direct effects of temperature), in a number of locations. Rather than reducing the potential for C release from soils in a warming world, our data suggest that the effects of temperature changes on the functioning of soil microbial communities may be greater than previously predicted, especially at high latitudes. These findings fundamentally alter our understanding of how soil respiration will respond to global change.

Our results also demonstrate that the initial composition of the soil micorbial community was the best predictor of the magnitude and sign of the adaptation response. However, at a more mechanistic level, we did not observe consistent changes in biomass, mass-specific activity or potential enzyme activity, indicating that current theory is inadequate in explaining microbial community responses to temperature. Further research is needed to determine what aspects of the microbial community composition controls the type of adaptation response observed.
Exploitation Route We have been granted access to field sites by a number of organisations including the RSPB and Forest Reserach. We are preparing short reports which will explain what we found and how their site fits into the overall picture. The experimental has now finished and the papers are being written up. These are likely to be high impact, and the plan is to submit to Nature. In collaboration with project partner Goran Agren we are working on developing models to reflect our findings. Once these developments are complete we will use our strong linkages in Exeter to earth systems modellers to ensure our results have the greatest possible impact in terms of influencing predicted rates of climate change.
Sectors Environment

Description Findings have been discussed with Earth systems modellers to see how they can be reflected in models used for predicting rates of climate change.
First Year Of Impact 2014
Sector Environment
Impact Types Policy & public services

Description NERC Discovery Science
Amount £794,307 (GBP)
Funding ID NE/P002722/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 12/2016 
End 05/2020
Description Research visit by Luis Lopez Sangil 
Organisation University of Barcelona
Country Spain 
Sector Academic/University 
PI Contribution As a direct results of the soil sampling in Catalonia, a PhD student from the University of Barcelona, visited Exeter for one month, to collaborate with Kristiina Karhu and Iain Hartley on a publication.
Start Year 2013
Description Invited Lecture to the Royal Meteorological Society, Edinburgh 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Professional Practitioners
Results and Impact This was an evening talk to members of the Royal Meteorological Society, on Tuesday 15 December 2015, entitled "Biogenic feedbacks and the 'Arctic Amplification' of climate change". There was a lively debate of the issues addressed following the talk, and an increased awareness that climate is linked to ecosystem dynamics in this area of the world where change is occurring so quickly.
Year(s) Of Engagement Activity 2015
Description Media coverage of Karhu 2014 Nature paper 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact Kristiina Karhu was interviewed by multiple media organisation which resulted in 19 articles internationally about our work. This included coverage on the BBC News website.

The research has been taken up by blogs and google accounts
Year(s) Of Engagement Activity 2014
Description Public ('inaugural') lecture in connection with professorial appointment to the University of Stirling. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Professorial inaugural lecture in connection with appointment, entitled "Biospheric feedbacks and the 'Arctic Amplification' of climate change."

The purpose of this is to 'profess' the discipline, and provide an accessible overview of a personal career contribution. The talk was delivered as one of the opening contributions to the University of Stirling's 2018 Research Week.
Year(s) Of Engagement Activity 2018
Description Public Lecture, University of Calgary 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact An invited public lecture as part of the Arctic Institute of North America's 2017 - 2018 Arctic Speaker Series. Entitled "Biospheric feedbacks and the 'Arctic Amplification' of climate change". Good discussions subsequent to presentation.
Year(s) Of Engagement Activity 2018