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

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
The project aims to improve our understanding of the factors that control the spatial and temporal variability in GHG fluxes from agricultural soils (arable, grassland and outdoor pigs). In particular, according to your particular research interests, the studentship could address a combination of the following objectives:
i. Quantify soil organic carbon and nitrogen stocks within agricultural soils
ii. Determine inputs of carbon and nitrogen to soil from crop residues and application of fertiliser and organic amendments
iii. Quantify Land-Air fluxes of water, carbon and nitrogen using Eddy covariance flux towers and static chambers. The flux towers will also allow quantification of carbon dioxide (CO2) fluxes as greenhouse gases (GHGs). A series of collars at the land surface will be installed across the fields to allow direct chamber measurement of N2O and methane fluxes. This will provide information on how GHG fluxes vary between land uses and also through time during the growing season.
iv Model GHG fluxes under a range of arable rotations and future climates.