Measuring and modelling greenhouse gas fluxes between agricultural soils and the atmosphere

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Project Summary:

Soil is a major component in the global carbon cycle, containing about 1500 Pg (1 Pg = 1 Gt = 1015 g) of organic carbon (Batjes, 1996), which is about three times the amount in vegetation and twice the amount in the atmosphere. Through photosynthesis, plants convert carbon dioxide (CO2) into organic forms of carbon and return some to the atmosphere through respiration. The carbon that remains in plant tissue is added to the soil through their roots and as litter when plants die and decompose. This carbon is then stored in the soil as soil organic matter. Carbon can remain stored in the soil for millennia, or be quickly released back into the atmosphere as CO2. Climate, vegetation type, soil texture and drainage all influence the amount and length of time carbon is stored in the soil. Therefore, soils play a major role in maintaining a balanced global carbon cycle. However, the carbon content of soil is smaller today than a few hundred years ago owing to the intensification and mechanization of agriculture. Agricultural practices have depleted soil organic carbon pools by two main routes:

  1. Reducing the amount of carbon returned to the soil in litter by harvesting and removing the crop.
  2. Excessive use of tillage practices which breaks up the soil, increasing the decomposition rate of soil organic matter which leads to an increase in the release of CO2 from the soil.

In June 2019, the Government legally committed the UK to reaching ‘net-zero’ greenhouse gas (GHG) emissions by 2050. The agriculture sector accounts for approximately 10% of the UK’s GHG emissions. Therefore, achieving net-zero will pose significant challenges for farming and the farming communities. However, soils can also help mitigate climate change by absorbing or ‘sequestering’ carbon from the atmosphere. This can be achieved through changes in management practices, such as reduction in tillage, reducing fallow periods, improving efficiency of animal manure use and crop residue use, and planting cover crops between main cash crops. Additional gains can come from land use change, such as planting trees and hedges, and reducing nitrous oxide (N2O) emissions by modifying fertiliser application rates and methods. However, improvements in measuring, monitoring and verifying changes in carbon, nitrogen and GHG fluxes between the soil and atmosphere are needed for quantitative economic and policy analysis. Currently, data on soil carbon, land use and climate is combined to create models that estimate the change in GHG fluxes related to changes in farm management practices. However, uncertainty persists on the absolute mitigation potentials offered by many efficiency based GHG mitigation measures. Therefore, there is a requirement to increase and refine GHG measurements from a range of arable rotations for greater accuracy.

Aim and Objectives:

This is an exciting and innovative project with the potential to make a big contribution to the agricultural sector and their ability to achieve ‘net-zero’ greenhouse gas (GHG) emissions by 2050. The project aims to improve our understanding of the factors that control the spatial and temporal variability in GHG fluxes from agricultural soils used for arable crops, permanent pasture and outdoor pigs. In particular, according to your particular research interests, the studentship could address a combination of the following objectives:

  1. Quantify soil organic carbon and nitrogen stocks within agricultural soils
  2. Determine inputs of carbon and nitrogen to soil from crop residues and application of fertiliser and organic amendments.
  3. Quantify Land-Air fluxes of energy (sensible and latent heat) water, carbon dioxide (CO2) and nitrogen using eddy covariance flux towers (Figure 1) and static chambers. The flux towers will allow quantification of gaseous CO2 fluxes. A series of collars at the land surface will be installed across the fields to allow direct chamber measurement of fluxes of the non-CO2 greenhouse gases (GHGs) – nitrous oxide (N2O) and methane (CH4). This will provide information on how GHG fluxes vary between land uses and also through time during the growing season.
  4. Model GHG fluxes under a range of arable rotations and future climates.

Project Outputs:

The project will enable significant, timely advancements in our understanding of the factors that control the spatial and temporal variability in GHG fluxes from agricultural crops and management systems. Thus the project has the potential to make a big contribution to the agricultural sector and their ability to achieve ‘net-zero’ greenhouse gas (GHG) emissions by 2050 and informing policies on how this can best be achieved. The project will produce several outputs, including (i) 3–4 academic publications, at least one of which we anticipate being suitable for submission to a high-impact journal and (ii) policy briefing notes to inform Defra and other agencies how different agricultural systems impact upon GHG fluxes.

Figure 1. Eddy covariance flux tower in grassland field

Training:

The student will work under the supervision of Professor Pippa Chapman, Dr Richard Grayson and Dr Marcelo Galdos within the Faculty of Environment, University of Leeds, plus Dr Ross Morrison at the Center for Ecology and Hydrology (CEH), who have expertise in all aspects of the project. Fieldwork will be conducted in the UK, and the findings will be of international relevance. The student will have access to excellent training and field and laboratory resources at the University of Leeds and CEH, such as a network of Eddy Covariance (EC) flux towers focused on observing land-atmosphere fluxes of carbon dioxide (CO2) and water vapour (evapotranspiration), with some measuring other trace GHG gas fluxes such as methane.

The successful candidate will develop a range of research skills, including experimental design, field sampling, statistical analysis and data interpretation, modelling, academic writing skills and giving presentations. Training will be provided in field/laboratory health and safety procedures and the use of field and analytical equipment.

The student will be supported throughout the studentship by a comprehensive PGR skills training programme that follows the VITAE Research Development Framework and focuses on knowledge and intellectual abilities; personal effectiveness; research governance and organisation; and engagement, influence and impact. Training needs will be assessed at the beginning of the project and at key stages throughout the project and the student will be encouraged to participate in the numerous training and development course that are run within the NERC DTP and the University of Leeds to support PGR students, including statistics training (e.g. R, SPSS), academic writing skills, grant writing etc. Supervision will involve regular meetings between all supervisors and further support of a research support group. The student will also be part of water@leeds – a major interdisciplinary water research centre at the University of Leeds – where there are over 180 PhD students studying water-related topics. That network will broaden the student experience and enhance the network of contacts.

Student profile:

The student should have a keen interest in soil processes and environmental issues with a strong background in one or more of physical geography, meteorology, earth sciences, soil science, environmental sciences or related discipline. Strong analytical/statistical/fieldwork/programming skills are desirable but not essential, as full training will be provided during the PhD. The project is open to applicants with at least a 2i degree in Geography, Environmental Science, Chemistry, Biology or a closely-related subject and interests in biogeochemistry. Interested students are encouraged to contact the first supervisor to discuss the project and for further information.