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

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.'

This is a 'Discovery Science' proposal, addressing a fundamental aspect of the terrestrial C cycle. As such, it has important implications in global change science, for ecosystem management, and for broader society.

The specific beneficiaries of the research include:
- The global change biology and climate sciences research communities (refer to 'Academic Beneficiaries' for specific information on 'how'), and the national and international environmental policy communities. Through tackling the secondary 'cascading' effects of land-use, or climate driven shifts in plant community composition on ecosystem processes and properties, the project aims to quantify and understand a neglected aspect of global change science. The project will link with international research programmes/research networks such as IGBP iLEAPS (Integrated Land Ecosystem-Atmosphere Processes Study) and AIMES (Analysis, Integration and Modeling of the Earth System), and ITEX (the International Tundra Experiment);

- Statutory agencies concerned with C management, provision of ecosystem services, rural land-use, conservation and landscape, catchment hydrology and flood risk. Although, for robust scientific reasons (see 'Case for Support'), we have chosen to base our field research in northern Scandinavia, the processes being addressed have both pan-Arctic as well as UK relevance. In Sweden we will make contact with Naturvårdsverket (The Swedish Environment Protection Agency) to discuss our work and its implications. In the UK, upland management, in particular, has the potential to benefit from the work (e.g. Scottish Environment Protection Agency & Scottish Natural Heritage, and the equivalent devolved agencies in England, Wales and Northern Ireland; Joint Nature Conservation Committee; Forestry Commission (FC); Deer Commission for Scotland; National Park authorities). Our group has research links with the FC and Forest Research and we will ensure that we establish contact with the Scottish Government/Forestry Commission Scotland's Woodland Expansion Advisory Group (WEAG; http://www.forestry.gov.uk/weag);

- NGOs and charities concerned with landscape, conservation, tourism and access (e.g. John Muir Trust; National Trust for Scotland (NTS); Moorland Forum; Scottish Native Woods; Woodland Trust; Scottish Wildlife Trust; RSPB; Game and Wildlife Conservation Trust; Ramblers Scotland). Through several of these organisations there are strong links with the general public;

- Reindeer herding communities, and the commercial private sector: Within the specific context of northern Fennoscandia, and more broadly across northern Russia, our work has relevance for reindeer herding communities. Both in Scandinavia and the UK there are also potential links to the power generators (renewables sector; wind/hydro) and water companies, as well as commercial forestry;

- Broader society: There is huge public interest in the wildlands of the North, including the UK, while those living and working in these areas are aware of the on-going and changing land-use demands, potential conflicts, and the challenges of earning a viable living. It is important that society has a robust science base upon which to develop rational policies for upland land-use, and that the ecosystem services provided in these areas can be sustainably managed. Furthermore, through their effects on biodiversity and landscape quality, 're-wilding' and forest regeneration will have significant implications for quality of life, health and aesthetic value. This is relevant both for the local population and businesses reliant upon the uplands, as well as for visitors (from the UK and overseas) and recreational users of these areas.

The 'Pathways to Impact' describes what will be done during and after the project to increase the likelihood of the research reaching the identified beneficiaries and maximise the likelihood of the identified benefits being achieved.