Can the formation of new soil organic matter offset decomposition losses from thawed permafrost soils?

It is predicted that 10s of billions of tonnes of carbon will be released as global warming promotes permafrost thaw in arctic and boreal regions. This release of carbon is considered to be potentially the most important carbon-cycle feedback that is not accounted for in current predictions of how rapidly the Earth will warm this century. The anticipated release of carbon could add 10 to 20% to the social costs of our carbon dioxide emissions, and make it even more challenging to avoid the most dangerous consequences of climate change.

Worryingly, measurements made in the field have already demonstrated that, where permafrost thaws, previously-frozen soil organic matter (SOM) can decompose to release carbon dioxide. Furthermore, observed rates of release are so high that there is little chance of warming-induced increases in plant growth offsetting these soil carbon losses. However, while plant biomass changes themselves may be too small, greater plant productivity may increase rates of carbon input into soils, promoting the formation of new SOM. While our knowledge of the controls over decomposition rates in permafrost soils has improved considerably in the last decade, we still know very little about how rates of SOM formation are controlled, and whether new SOM can become stabilised and protected in the soil matrix and, thus, be stored for a long time. Critically, the very few studies that have been able to measure changes in soil carbon storage following permafrost thaw have suggested that new SOM formation is important, potentially offsetting a substantial proportion of decomposition losses. However, due to our lack of understanding of how SOM formation and stabilisation are controlled in different types of high-latitude soils, we currently cannot predict the extent to which anticipated carbon losses from permafrost thaw could be offset.

In recent years, it has been demonstrated that by isotopically-labelling new inputs from plants, rates of new SOM formation and stabilisation can be quantified. These studies have developed new paradigms in SOM research, including evidence that soils may have a maximal capacity for stabilising and protecting organic matter, and how close a soil is to saturating this capacity may determine if carbon is lost or gained in response to global change. Critically, processes like cryoturbation, the vertical mixing of soil profiles due to freeze and thaw, result in different types of permafrost soils having very different profiles of how carbon contents vary with depth, so may differ in terms of how close different horizons are to their maximum stabilisation capacity. Thus, testing the carbon saturation hypotheses in contrasting permafrost soils has great potential for developing the understanding required to predict rates of new SOM formation and stabilisation.

In north-west Canada, we will collect samples of the permafrost soil types that store the majority of carbon present in high-latitude ecosystems, but which differ fundamentally in terms of how their carbon storage varies with depth. We will grow a representative high-latitude plant species in these soils under an isotopically-labelled atmosphere. This will allow us to, for the first time, quantify potential rates of SOM formation and stabilisation in these contrasting soils, and to compare these with rates of decomposition of pre-existing organic matter. The focus on different soil types allows key hypotheses to be tested and the understanding developed to be up-scaled to the regional and circumpolar scale, providing the first estimate of the role new SOM production could play in offsetting carbon losses from thawing permafrost. This information is urgently required for improving predictions of the magnitude of the permafrost feedback.

1. Who will be the main beneficiaries of this research?
Our ultimate aim is to improve understanding of whether the formation of new soil organic matter (SOM) can offset the anticipated losses of carbon associated with the decomposition of previously-frozen SOM following permafrost thaw. The permafrost feedback is potentially the most important carbon-cycle feedback not currently included in Earth system model projections of future rates of climate change, and the amount of carbon that is expected to be released could make limiting warming to 2oC by 2100 even more challenging. Therefore, improving predictions of the future magnitude of the permafrost feedback has high societal and policy relevance. Thus, our findings will be directly relevant to IPCC Working Group I who aim to assess the physical scientific basis of climate change including climate-carbon feedbacks.
Secondly, with recent media coverage, the impacts of global change in the Arctic and the Boreal are of great interest to the general public in the UK, Canada and globally. Furthermore, given the importance of climate change for future generations, a key group of beneficiaries is school children, and the topic of our research and our findings will be particularly relevant to pupils studying the soils, weathering and climate component of the UK geography curriculum.

2. How will they benefit?
The IPCC is placing a greater emphasis on Earth system modelling and in particular climate-carbon cycle feedbacks. In chapter 6 'Carbon and Other Biogeochemical Cycles' of the most recent IPCC report, the uncertainty regarding the potential magnitude of the permafrost feedback is strongly emphasised. By improving our understanding of the role of new SOM formation in offsetting soil carbon losses from thawing permafrost, our work will provide critical information needed for improving model predictions of how much carbon will be released as permafrost soils thaw. Therefore, to maximise the societal impact of our research it is essential that we work with relevant groups to ensure our findings are reflected in the IPCC-facing models used to predict future rates of climate change.
The general public will benefit from increased understanding of why changes taking place in remote high-latitude ecosystems matter to all of us in terms of influencing future rates of global warming and the impacts of climate change that we all experience. The First Nations and local communities in Canada will benefit from learning how thawing soils in their region are affecting the whole Earth system. On the other hand, changes taking place in remote areas of the Arctic may not seem of immediate concern to people living in the UK, so it is important that the effects of thawing permafrost are communicated to the general public and reflected in school curriculums. Our activities will help school pupils and other members of the public to gain from up-to-date understanding of how the biosphere is changing and what the implications could be for them in terms of the rates and impacts of climate change.

In summary, improved understanding of the potential magnitude of the permafrost feedback is of great importance for predicting how the climate will change this century. Therefore, the key beneficiaries of our project will be IPCC and policy-facing modellers who are actively incorporating the permafrost feedback into Earth systems models, as well as the general public who will benefit from a greater understanding of how changes taking place in arctic soils will affect the whole Earth system and the climate change that they will experience within their region.