The effect of tree planting on soil carbon balance in upland ecosystems

Summary

There is uncertainty over soil carbon fate in tree-established upland ecosystems. This project will quantify system dynamics to understand biogeochemical and microbial mechanisms, improving understanding of net climate mitigation benefit of afforestation. 

Project background

Afforestation has been proposed as a robust low-cost solution to sequester carbon and mitigate climate change. Tree planting captures atmospheric carbon in tree biomass and there is evidence to suggest that it can also increase soil organic carbon (SOC) belowground (Zhai et al., 2024). The increasing focus on tree-planting delivering towards UK net zero ambitions is also tied to corporate carbon offset initiatives. However, most tree planting in the UK is happening in upland ecosystems where soils are carbon rich. There is uncertainty over the fate of SOC in response to tree-planting in these soils with potential large losses of SOC over relatively short (decadal) time periods (Friggens et al., 2020) reducing the total ecosystem carbon storage. Therefore, there is an urgent need to quantify the effect of tree planting in organic-rich soils on the SOC balance and to understand the mechanisms underpinning potential changes in soil carbon. 

Soil carbon balance is influenced by multiple biotic and abiotic factors. The amount of plant carbon inputs into soil as litter and the chemical composition of those inputs is the first significant factor (Schimel and Schaeffer, 2012). After soil microbes get access to and decompose this particulate organic matter, how microbes allocate that carbon, and their life history strategies (Malik et al., 2020) determine how much of the carbon taken up stays in soil against that lost as respired CO2 (quantified as the microbial carbon use efficiency). When microbes die, residues often referred to as necromass associate with the mineral matrix to become part of the stable and persistent mineral-associated organic matter pool (Sokol et al., 2022). The balance of the processes of decomposition and mineral stabilisation is likely to shift with a change in vegetation from grasses and shrubs to trees thereby impacting the persistence of SOC. This highlights the need to explore the biogeochemical mechanisms of SOC storage so that we can better predict and manage soil carbon in response to large scale aboveground changes. 

Research questions

What is the long-term change in whole soil column SOC and dynamics of greenhouse gas fluxes following tree planting in upland soils?

Does tree planting in upland soils affect the particulate and mineral-associated organic matter differently? Are these fractions chemically distinct and does that influence their long-term persistence?

What is the change in microbial community composition and traits such as carbon use efficiency in response to tree planting and what are the consequences of this shift on soil carbon stocks and greenhouse gas fluxes?

Methodology

The student will first sample 12 sites each with paired locally adjacent moorland versus tree-planted soils (facilitated by CASE partner Forest Research) to study the effect of tree planting on long-term changes in SOC across the soil column. Soil sampled in close proximity will allow investigation of the role of tree planting on soil carbon while minimising the influence of climate, geology and soil parent material. The student will also measure soil parameters and simpler microbial traits to understand the patterns across different sites with paired contrasts. On a few selected sites, the student will perform detailed analysis to identify biogeochemical and microbial mechanisms of SOC change. Measurements will include detailed chemical composition of the particulate and mineral-associated organic matter, microbial taxonomic and functional analysis using shot gun metagenomics and multiple time point in situ greenhouse gas flux measurements. 

Year 1: Student to review the literature on the topic, gain technical skills and hands-on experience in using the relevant analytical systems, and make field visits supported by Forest Research across Scotland and northern England. Some initial analytical trials will be performed to assess the suitability and success of the measurements planned. The student will present their project objectives with some preliminary results at a relevant national conference. During this period, the student will also prepare for their PhD confirmation panel interview.

Year 2: Once the planned methods and approaches are deemed appropriate through initial trials and assessments, the student can proceed to perform the full suite of analysis on the entire sample set in the second year. The large amount of data that will be generated through various analyses will then be processed through a rigorous statistical framework. The student will also have the opportunity to spend some time hosted at the CASE partner institution to develop complimentary skills. During this period, the student will present results at a national conference. 

Year 3: Data analysis will be completed, and the student will finalise manuscript(s) for publication. This will also help the student prepare their thesis. The plan is to submit at least one high profile manuscript to a journal before thesis submission. The student will present final results at an international conference. 

Training

A comprehensive training programme will be provided comprising both specialist scientific training and generic transferable and professional skills. Training will be available in technical skills linked to state-of-the-art geochemical analysis, metagenomics and bioinformatics, stable isotope tracing to measure microbial carbon use efficiency and ecological statistics. Student will be trained in using and applying these cutting-edge tools and to integrate the microbiome data with soil parameters and make data visualisations. Student will be encouraged to attend university-based and external courses on acquiring various technical skills, especially bioinformatics.

Requirements

We seek an enthusiastic PhD student with a degree with ecology, environment science, soil science or microbial ecology background. Statistics experience and enthusiasm for field work as well as interests in soil carbon and microbial ecology are desirable. 

CASE partner: Forest Research