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

据预测，随着全球变暖促进北极和北方地区的永久冻土融化，将释放数百亿吨的碳。这种碳的释放被认为是潜在的最重要的碳循环反馈，目前对本世纪地球变暖速度的预测没有考虑到这一点。碳的预期释放可能会使二氧化碳排放的社会成本增加10%至20%，并使避免气候变化最危险的后果变得更加困难。令人担忧的是，实地测量已经证明，在永久冻土融化的地方，以前冻结的土壤有机质(SOM)可以分解释放二氧化碳。此外，观测到的碳释放速率如此之高，以至于气候变暖导致的植物生长增加抵消了这些土壤碳损失的可能性微乎其微。然而，尽管植物生物量本身的变化可能太小，但更高的植物生产力可能会增加向土壤输入碳的速率，促进新的SOM的形成。虽然我们对控制冻土分解速率的知识在过去十年中有了很大的提高，但我们仍然对如何控制SOM的形成速度以及新的SOM是否能够在土壤基质中稳定和保护从而长期储存知之甚少。关键是，能够测量永久冻土融化后土壤碳储量变化的极少数研究表明，新的有机质形成是重要的，可能抵消相当大比例的分解损失。然而，由于我们对不同类型的高纬度土壤中SOM的形成和稳定是如何控制的缺乏了解，我们目前无法预测预期的永久冻土融化碳损失可以在多大程度上得到抵消。近年来，已经证明，通过对来自植物的新输入进行同位素标记，可以量化新的有机质形成和稳定的速率。这些研究开发了土壤有机质研究的新范式，包括土壤可能具有稳定和保护有机质的最大能力的证据，以及土壤接近饱和这一能力的程度可能决定碳是因全球变化而失去还是获得。最重要的是，冷冻扰动(冻结和融化引起的土壤剖面的垂直混合)等过程导致不同类型的永久冻土具有非常不同的剖面，即碳含量如何随深度变化，因此不同层位与其最大稳定能力的接近程度可能会有所不同。因此，在对比多年冻土中测试碳饱和度假设，对于发展对预测新的有机质形成和稳定化速率所需的理解具有巨大的潜力。在加拿大西北部，我们将收集永久冻土类型的样本，这些土壤类型储存了高纬度生态系统中存在的大部分碳，但从碳储量随深度变化的角度来看，这些土壤类型存在根本差异。我们将在同位素标记的大气下在这些土壤中种植具有代表性的高纬度植物物种。这将使我们第一次能够量化这些对比强烈的土壤中SOM形成和稳定的潜在速率，并将这些速率与先前存在的有机物的分解速率进行比较。对不同土壤类型的关注可以检验关键假设，并将所形成的理解扩大到区域和环极尺度，从而首次估计新的有机质生产在抵消永久冻土融化造成的碳损失方面可能发挥的作用。迫切需要这些信息来改进对永久冻土反馈程度的预测。

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.