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

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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.'