Turbo-charging the mycorrhizosphere - Could more productive ecosystems threaten soil carbon stocks in boreal and sub-arctic zones of transition?

The Context of the Research - Many high-profile research papers and syntheses have equated increased vegetation productivity and shifting vegetation types in northern high latitudes with increased net carbon (C) sequestration from the atmosphere. Although logical and intuitive, this largely overlooks the potential fate of pre-existing soil organic carbon (SOC) in these regions. This is a problem because soils at high latitudes are notably C-rich (containing ~570 Pg C in boreal/taiga forest and tundra soils alone; note, 1 Pg (Peta-gram) = 1,000,000,000 tonnes) and this pool is dynamic, intrinsically interacting both with vegetation cover and with climate.

Although challenging to investigate, we cannot overlook below-ground processes if we are to understand net C budgets on timescales relevant to the Climate Emergency. Understanding the fundamental mechanisms controlling the accumulation, stability, and loss of soil organic matter (SOM) is as essential for predicting the Earth's future climate as understanding photosynthesis and plant productivity. However, our understanding of, and ability to model, SOM dynamics lags far behind that of primary productivity. Furthermore, rapid warming at high northern latitudes adds urgency to understanding controls on whole-ecosystem C cycling, net fluxes of CO2 between ecosystems and the atmosphere, and the vulnerability of SOM to changes in both climate and management (for example, tree planting for C-sequestration).

Aims and Objectives - In MYCONET we focus on the 'mycorrhizosphere' (the soil and organisms directly influenced by roots and their mycorrhizal fungi) of C-rich soils of northern high latitudes and its potential response both to increasing plant productivity and to shifts to woodier shrub and tree communities. We hypothesise that associated changes in the mycorrhizosphere could, paradoxically, result in net losses, rather than gains, of soil C over timescales (i.e. several decades) of relevance to the Climate Emergency. This would represent a 'positive feedback' on climate change (i.e. when the rates of CO2 emission to the atmosphere, due to SOM decomposition, exceed net rates of CO2 uptake via photosynthesis).

We will push the frontiers by applying ground-breaking techniques in the use - and innovative experimental deployment - of natural abundance (and depleted) radiocarbon (14C), together with metagenomics, soil and root-tip enzyme assays and SOM chemistry, to quantify and understand the processes and dynamics of the mycorrhizosphere and how these affect SOC stocks. We focus, in detail, on the process of 'priming' (which occurs when material added to soil affects the rate of decomposition of SOM, either positively or negatively), and the specific role of mycorrhizal fungi in this, and related, processes. We will measure these processes both in situ (in the Arctic and the UK uplands) and in controlled experiments (using specific combinations of tree, shrub and mycorrhizal symbionts), as part of an integrated package of mechanistic studies, soil profile analysis and dynamic SOM modelling, to quantify and understand how priming works, and the implications for SOM dynamics, ecosystem C fluxes, and nutrient cycling.

Potential applications and benefits - By applying ground-breaking techniques MYCONET will transform our understanding of plant-soil interactions and the role of mycorrhizal fungi in SOM dynamics. The fundamental new knowledge gained will significantly improve 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 ecosystems being managed for C-sequestration or 're-wilding'. This project is especially timely, given the major policy emphasis and public interest in tree planting for C sequestration.