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

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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.