Will more productive Arctic ecosystems sequester less soil carbon? A key role for priming in the rhizosphere ('PRIME-TIME')

高纬度地区变暖的速度比其他地区更快，因此成为气候变化研究的关键领域。此外，它们在地球系统的运行中发挥着至关重要的作用，将大量的碳(C)储存在土壤有机质(SOM)中。这些碳库对气候和土地利用变化的脆弱性存在重大不确定性。以前的研究主要集中在气候变暖对植物生长和/或土壤有机质动态的直接影响上，但越来越多的证据表明，植物-土壤相互作用使这些关系变得复杂。植物不仅控制凋落物的输入(质量和数量)，而且如果分配给根际(定义为植物根部附近的土壤，其化学和微生物受到其生长、呼吸和养分交换影响的土壤区域)的碳量与微生物活动和较老的顽固有机物的分解呈正相关，则还可能影响分解速度。这一过程被称为根际“启动”，尽管有人认为这可能是决定生态系统碳储量的关键，但人们对它的了解仍然非常少，特别是在自然和半自然生态系统中。由于气候变化和景观管理，特定群落的分布已经发生了变化，这可能会对碳储存产生意想不到的影响。我们的研究团队在瑞典亚北极开展的工作表明，从贫瘠的荒原生态系统转变为更高产的落叶林，可能会导致土壤中碳的净损失，这与人们的直觉相反。没有考虑根际植物-土壤相互作用的C循环模型不能很好地模拟这种响应。相反，大多数模拟预测，如果高纬度地区的生产率提高，碳储量将大幅增加。这一建议将确定植物(和相关的菌根)功能组成的变化对土壤有机质动态的影响。具体地说，我们将调查瑞典北极从苔原荒原向高大灌木群落和落叶林地转变的后果。我们将在田间和实验室使用新的实验方法相结合，来量化和了解根际引发效应(RPE)对SOM动态的作用。在瑞典拉普兰的田野里，我们将通过实验(环剥；去除树皮，包括韧皮部组织)来操纵穿过山区白桦林-苔原石南交错带的根际，以减少有机碳向根部的韧皮部运输。我们将结合使用“生长”核心(它选择性地阻止细根和/或真菌菌丝(丝)进入)来操纵根际过程，以确定根、菌根真菌和异养(土壤分解者)代谢的贡献。我们将部署最先进的微生物分子分析来评估处理对真菌群落结构的影响(特别是针对关键的菌根真菌组)，以及新的碳同位素方法来量化RPE。该项目的成果有可能显著改善气候-生物地球化学相互作用的区域和全球模拟，特别是关注植物群落变化的间接影响。该项目不仅关系到泛北极地区的灌木化，也关系到英国高地生态系统的重新开垦。

High latitudes are warming faster than other regions, and thus serve as critical arenas for climate change studies. Furthermore, they play a pivotally important role in the functioning of the Earth system, storing significant amounts of carbon (C) in soil organic matter (SOM). There is major uncertainty over the vulnerability of these C stores to both climate and land-use change. Previous research has focussed on the direct effects of warming on plant growth and/or SOM dynamics in isolation, but there is increasing evidence that plant-soil interactions complicate these relationships dramatically. Plants not only control litter inputs (both quality and quantity), but may also influence rates of decomposition if the amount of C allocated to the 'rhizosphere' (defined as the area of soil in the vicinity of plant roots in which the chemistry and microbiology is influenced by their growth, respiration, and nutrient exchange) is positively related to microbial activity and the breakdown of older more recalcitrant organic matter. This process is referred to as rhizosphere 'priming', and despite suggestions that it may be critical in determining ecosystem C storage, it remains extremely poorly understood, especially in natural and semi-natural ecosystems. Changes in the distribution of particular communities, which are already taking place due to climate change and landscape management, may have unexpected impacts on C storage. Work carried out by our research team in the Swedish sub-Arctic suggests that transformations from unproductive heathland ecosystems into more productive deciduous forests could result in counterintuitive net LOSSES of C from soils. Such responses are inadequately simulated by C-cycle models which do not take into account plant-soil interactions in the rhizosphere. Rather, most simulations predict C storage will increase substantially if productivity increases at high-latitudes. This proposal will determine the impacts of shifts in plant (and associated mycorrhizal) functional composition on the dynamics of SOM. Specifically we will investigate the consequences of a shift from tundra heath to tall shrub communities and deciduous woodland in the Swedish Arctic. We will use a combination of novel experimental approaches, in both the field and the lab, to quantify and understand the role of rhizosphere priming effects (RPEs) for SOM dynamics. In the field in Swedish Lapland we will manipulate the rhizosphere across the mountain birch forest-tundra heath ecotone using experiments ('girdling'; removal of bark, including phloem tissues) to reduce phloem transport of organic C to the roots. We will combine this with manipulating rhizosphere processes using 'in-growth' cores (which selectively prevent fine root and/or fungal hyphal (filament) access) to determine the contributions of roots, mycorrhizal fungi and heterotrophic (soil decomposer) metabolism. We will deploy state of the art microbial molecular analyses to assess the impact of the treatments on fungal community structure (specifically targeting key mycorrhizal fungal groups), and novel C-isotope approaches to quantify RPEs.The project outputs have the potential to improve significantly regional and global modelling of climate-biogeochemical interactions, with a particular focus on the indirect effects of shifting plant communities. The project has relevance for the pan-Arctic 'shrubification', as well as for UK upland ecosystems being managed for 're-wilding.'