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Sink or source - how CO2 levels affect the soil C cycle and the implications for climate change


   Applications accepted all year round  Funded PhD Project (Students Worldwide)

About the Project

PROJECT TITLE: Sink or Source - How CO2 levels affect the soil C cycle and the implications for

climate change

DTP Research Theme(s): Dynamic Earth, Living World, Changing Planet

Lead Institution: University of Bristol

Lead Supervisor: Dr Timothy Knowles, School of Chemistry, University of Bristol

Co-Supervisor: Professor Iain Hartley, Geography Department, University of Exeter

Co-Supervisor: Dr Ian Bull, School of Chemistry, University of Bristol

Project Enquiries:

Project keywords: Carbon cycle, climate, soil, radiocarbon, isotope

Growth chamber with gas flux monitoring The Bristol MICADAS accelerator mass spectrometer

Project Background: Soils store around 75% of terrestrial carbon, and soil organic carbon (SOC)

constitutes more C than both the atmosphere and total plant biomass combined. Soils store C

from the atmosphere, stabilised as SOC, but the potential for soils to become net carbon sources

to the atmosphere under a changing climate represents a major tipping-point risk. Understanding

the effect of increasing levels of atmospheric CO2 on the rates and fluxes of C uptake, storage, and

release into, and from, soil organic matter at the molecular level is critical to enabling predictions

of how such systems will respond to future climate scenarios. Laboratory incubation experiments

and free air CO2 enrichment (FACE) experiments, combined with cutting-edge radiocarbon and

stable isotope techniques, provide an ideal opportunity to study these processes, as we can

manipulate CO2 levels, and apply stable isotope (

13C) and radiocarbon 14C tracer approaches to

follow atmospheric CO2 carbon through the entire soil carbon cycle.

Project Aims and Methods: This PhD project will use the latest radiocarbon accelerator, and

isotope ratio mass spectrometric instrumentation and cutting-edge complimentary techniques to

investigate the effect of elevated CO2 on the ability of forest and grassland soils to act as a carbon

sink. The project aims to quantify the routes, rates and fluxes of C from atmospheric CO2 into the

soil as a multitude of carbon forms, and its subsequent stabilisation. Incubation experiments will

be carried out using soil/plant mesocosms, and samples taken over a time-course for analysis to

elucidate the fate of atmospheric CO2. Radiocarbon and stable isotope analyses at the bulk- and

compound-specific level targeting root exudate compounds and phospholipids will help determine

the mechanisms by which carbon sequestration/loss occurs. Moreover, the effect of elevated CO2

on the nature of stored C, at the molecular level and how this affects carbon sequestration, shall

be determined. The candidate will also conduct 14C measurements of soil-respired CO2 to

determine the balance between these soils as sources or sinks under elevated CO2. This PhD

project will involve aspects of experimental design, building incubation apparatus and modifying

analytical procedures to enable these analyses. This initial work will feed into larger scale

experiments with soils from FACE sites. Within the overarching goals of the proposed research, it

is expected that the direction of the work will be predominantly student led with supervisors

providing the necessary support to ensure decisions are well-informed.

NERC GW4+ DTP Projects 2023

Candidate requirements: A profound interest in biogeochemistry, climate change and a

commitment to positive science-led change is fundamental to this project. A background in

analytical chemistry is essential and a background in Earth/soil sciences highly desirable. Also key

are good laboratory and data analytical skills coupled with a willingness to learn new techniques

and develop existing ones. An aptitude for statistical analysis and a knowledge of R would be

desirable We welcome and encourage student applications from under-represented groups. We

value a diverse research environment.

Project partners: The Organic Geochemistry Unit at the University of Bristol has a long history at

the forefront of developing applying compound-specific stable isotope techniques to

biogeochemical systems. In parallel, the Bristol Radiocarbon Accelerator Mass Spectrometry

(BRAMS) Facility is equipped with the latest generation of radiocarbon accelerator and works at

the cutting edge of compound-specific radiocarbon analyses. The University of Exeter is a UK

centre of expertise for carbon cycle and climate science and have developed novel laboratory and

field techniques for studying soil C dynamics, including new approaches for tracing 14C into

different soil fractions, as well as CO2 and CH4 emissions.

Training: Training in biogeochemistry, radiocarbon science and analytical techniques will be

provided in both BRAMS and NERC-NEIF/UoB OGU at the University of Bristol. Full one-to-one

training will be provided for all instruments and techniques relevant to the project, including GC-CIRMS, EA-IRMS, GC-HRMS, GC-PFC and AMS. The student will have the opportunity to attend a

wide variety of internal UoB courses (via the Bristol Doctoral College) and lectures, including

training in statistical analysis in addition to personal development courses throughout the project.

External training courses will be encouraged where appropriate. Fieldwork and sampling trips will

also be an integral part of this project. Extensive training around soil incubation and labelling using

experimental mesocosms as well as additional techniques for soil characterisation will be provided

at the University of Exeter.

Background reading and references: Stewart CE et al. (2007) Biogeochemistry 86, 19, Carney, K. M. et al. (2007)

PNAS, 104(12), 4990–4995, Dietzen, C. A. et al. (2019). Global Change Biology, 25(9), 2970–2977

Useful links

NERC GW4+ DTP Website:

For more information about the NERC GW4+ Doctoral Training Partnership please visit

Bristol NERC GW4+ DTP Prospectus:

How to apply to the University of Bristol:

The application deadline is Monday 9 January 2023 at 2359 GMT.

Interviews will take place during the period 22 February – 8 March 2023.

Please note: If you wish to apply for more than one project please contact the Bristol NERC GW4+ DTP

Administrator to find out the process for doing this.

General Enquiries:

Bristol NERC GW4+ DTP Administrator


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