IAP-24-052

Understanding the potential soil carbon benefits of agroforestry

Globally, forest and woodland creation is being used to meet net zero greenhouse gas emission targets. To increase the terrestrial carbon sink in the UK, the government is aiming to increase forest and woodland cover from 13% to 17-20% by 2050 with Scotland aiming for 21% forest/woodland cover by 2032. However, forest and woodland creation can come into conflict with other land-uses, such as food production. Integrating trees into agricultural landscapes, including arable crop and pasture systems, offers the opportunity to expand tree cover across a wider land area while maintaining food production and associated rural livelihoods. Agroforestry is defined as the integration of trees on land either cultivated for crops or grazed by domestic herbivores (Li, Niu & Luo 2012; De Stefano & Jacobson 2018). Meta-analyses across studies have shown that agroforestry and afforestation typically lead to a net increase in aboveground carbon density, but the evidence for soil impacts is less consistent. Some studies suggest an increase in soil carbon stocks (De Stefano & Jacobson 2018) while others show the reverse (Li, Niu & Luo 2012). Critically, there is a paucity of soil data for agroforestry systems in the UK. Greater understanding of the processes shaping soil carbon dynamics in agroforestry is hence required to determine factors associated with the highest potential to increase soil carbon to guide land management decision-making. Key knowledge gaps in our understanding of impact of agroforestry on soil carbon include:
1. Does agroforestry increase soil carbon storage with increasing time since tree planting?
2. How does the amount of soil carbon stored by agroforestry compare to that of grasslands and forests?
3. How important is soil type, particularly organic versus brown soils, in determining the soil carbon in agroforestry?
4. What is the relative importance of tree species and their symbiotic fungal in determining positive or negative effects of soil carbon under agroforestry?
5. How does tree planting design influence the spatial heterogeneity of soil carbon storage?
6. Based on understanding of soil type, tree species and planting design can we predict likely impacts of agroforestry on soil carbon storage?

A strong focus of the studentship will be on field-based measurements, supported by lab-based process studies and advanced analytical techniques. A core part of the studentship will be to address these knowledge gaps that are timely and relevant to farmers, policy makers and researchers alike with important applied findings to the potential value of agroforestry for future land-use carbon sequestration strategies and NetZero ambitions.

Fieldwork for this project will capitalise on a pre-existing network of 30 agroforestry sites in the NERC Treescape funded Farm Tree project and additional James Hutton Institute network of sites, all distributed across Scotland. Agroforestry sites represent a range of time since tree planting from 1960 to 2023, thus presenting an ideal experimental set-up for a space-for-time substitution design of planted vs. unplanted pasture. This design is necessary to answer questions of the effect of long-lived trees on soil carbon stocks and use of nearby pasture and forest to assess respective soil carbon storage. As part of the Farm Tree project there has been no empirical measurement of soil carbon storage, instead reliance on national soil inventory maps and modelling.

As part of the studentship, soil carbon surveys will capture effects of trees on surface and deep soil carbon, with soil surveying to 60 to 80 cm. The sites also include planting of different tree species with different fungal symbionts, with tree species replicated across sites. This would allow for the investigation of the role of tree species, their fungal symbionts and soil microbial diversity influencing soil carbon storage and linkages between plant and soil biodiversity and carbon storage.

In addition to these spatial surveys of soil and aboveground carbon stocks, we envisage a determination 13C isotope enrichment down the soil profile. Previous research has shown that as bacteria and fungi decompose organic matter, the lighter 12C isotope is disproportionately lost as CO2, while the heavier 13C isotope accumulates in the remaining material (Thornton et al. 2015). Additional investigation of soil organic matter stability under contrasting vegetation cover based on soil fraction and organic matter quality analysis can be used to supplement the isotopic approach. Soil drainage and organic matter accumulation impacts on organic matter decomposition, which can be detected in ¹³C signatures, and investigating ¹³C abundances throughout the soil profile can provide critical and novel information on the role of tree planting and soil type or drainage on the decomposition process.

Lastly, surveys can be designed to reflect difference in tree planting designs on farms and likely spatial heterogeneous impact on trees on soil carbon storage. This would inform optimal planting designs to maximising soil carbon storage in agroforestry, but also allow the scalability of soil carbon measurements when accounting for the distribution of trees that can be monitored using remote sensing or drones. Combining space-for-time substitution, 13C enrichment and spatial explicit surveying, the studentship has the potential to advance the state of science towards predictive modelling of potential areas where agroforestry may be positive and negative for soil carbon storage over a given timescale.

Whilst some background in plant and soil surveying and analysis is an advantage, there is no expectation that applicants have expertise in these areas. There is significant scope for the successful applicant to design experiments according to their own interests and strengths, supported by the supervisory team. Specific training, for example for isotopic approaches and soil organic matter analyses will form part of the training during the PhD, leveraging the breadth of expertise amongst the supervisors.

Please contact Dr Jens-Arne Subke with any queries regarding this project: jens-arne.subke@stir.ac.uk

Year 1

Literature or systematic review on agroforestry and soil carbon storage. Identification of specific field sites and detailed planning of sampling strategy; survey method development for soil sampling; skills training (soil survey methods, elemental and isotope analysis, soil microbial community, fungal symbionts); start of site sampling and establishment of any experiments with soil microbial community of fungal symbionts.

Year 2

Main sampling campaign; soil profiles measuring soil carbon stocks, soil carbon isotope composition, other soil properties across agroforestry sites; continued monitoring of soil microbial community experiments.

Year 3

Finishing lab and field experiments; data evaluation, and publication of results.

Year 3.5

Final stages of publications and thesis writing.

1. Field work methods, including survey design and experiments, soil surveying techniques, such as soil respiration, soil pits, coring and processing.
2. Methods in fungal symbiont monitoring i.e., staining
3. General laboratory skills, including carbon and carbon isotope determinations, FTIR spectroscopy and soil fractionation methods.
4. Experience working with farmers and other stakeholders
5. Numeracy, data analysis, ecological modelling & informatics. These skills will be gained through targeted training courses within the IAPETUS consortium, with further courses available at Stirling.
6. Complementary training in transferable skills and core scientific skills (data management, analysis, presentations, paper writing).