Managing soil carbon in the uplands – impact of mycorrhizal shifts following afforestation of former pastureland

Current policy aims to increase forestry throughout Scotland, with a target of planting 10,000 hectares (~25 Million trees) per annum for over 10 years. Forests have a high potential for carbon sequestration, particularly in young rapidly growing forest stands. However to understand the full benefit of tree planting for long-term sequestration of CO2 from the atmosphere, we need to also account for changes in the underlying soil carbon (C) stock. Soils under grassland vegetation can store substantial amounts of carbon in labile and humic forms, and the impact of transitions from pasture to forestry is ambiguous in the scientific literature; both decreases in soil C stores (Guo and Gifford, 2002), and increased overall C stock (Mu et al., 2014) have been reported. Concurrent with the significant change in vegetation is an important shift in soil microbial communities. Whilst roots of grasses have associations with specific fungi to form arbuscular mycorrhizal (AM), temperate trees (and coniferous trees in particular) have specific associations with different groups of fungi to form ectomycorrhizal (ECM) symbioses. The fungi involved in the two contrasting mycorrhizal forms have fundamentally different growth forms and biochemical capabilities; AM commonly show a more limited spatial reach into the soil, and produce comparatively few forms of enzymes for breaking up soil organic matter, whereas ECM often support extensive fungal networks throughout soil layers below the litter layer, and are capable of a wider variety of decomposing enzymes (Read and Perez-Moreno, 2003).


Aim
The aim of this studentship is to investigate the development of soil C stocks following a transition from pasture to forestry. Specifically, the studentship will address:
1. What are long-term impacts on C stocks of afforestation on grassland?
2. Are changes in C turnover and/or pool sizes related to plant species used in afforestations?
3. Are mycorrhizal associations (ECM or AM) critical for the stabilisation and/or decomposition of soil organic matter?


Methods
Through the collaboration with Forest Research, the student has access to forest sites of different age since plantation establishment, and with recorded land use and management history. By selecting plantations on land previously used for pasture, the aim is to establish a data set of chronosequences (including grasskand control sites) for which soil C stock can be established.

The role of ECM fungi in decomposing soil C stocks will be addressed by transplanting soil cores between pasture and forest sites. Nylon netting of different mesh size will be used to selectively allow or block access to transplanted soils by roots and/or mycorrhiza (e.g. Subke et al., 2011). Measurement of CO2 flux from these transplanted soils will identify increased or decreased decomposition, whilst addition of common substrates can be used to focus on specific forms of organic matter affected by root and mycorrhizal priming. A novel aspect of the methodology is the measurement of natural abundance radiocarbon (14C) in soil organic matter pools and respired CO2 to determine the changes in turnover and age of carbon emissions following a transition between vegetation types.

Field experiments will be supplemented by lab studies using mesocosms of different plant/mycorrhizal assemblages in soils of contrasting management origins. The student will develop his/her abilities to culture plant/fungal assemblages and set up novel experiments to investigate mechanisms of organic matter formation as well as decomposition. The student would also have access to Prof. Johnson’s state-of-the-art mobile laboratory to monitor gas flux from soil cores at field sites, enabling novel short- and long term responses to manipulations in aboveground and belowground C dynamics. The student will closely collaborate with François-Xavier Joly (University of Stirling) who works on complementary questions to that of the studentship.


Training
1) General field work methods, including gas flux measurements and soil transplant/mycorrhizal in-growth experiments as well as Mycorrhizal identification and ecophysiological techniques; critical laboratory skills, including elemental analysis and application of 14C in experiments and stable isotope methods (13C and 15N).

2) Numeracy, data analysis, ecological modelling & informatics. These skills will be gained through targeted training courses within the IAPETUS consortium and available at Stirling (e.g. Programming and Analysis of Environmental Data in R, GIS & Remote Sensing for Environmental Managers).

3) Complementary training in transferable skills and core scientific skills (data management, analysis, presentations, paper writing).