Nitrous oxide (N2O) is a powerful greenhouse gas. It is important, therefore, to understand the factors controlling its emission – particularly from agricultural systems. Urine patches are believed to represent important N2O emission “hot spots” and “hot moments” in pastures grazed by cattle and sheep. Whilst much is known about emissions from sheep-grazed lowland pastures, emissions from extensive upland systems are uncertain. Emission estimates currently employ assumptions based on measurements made in lowland systems which may not be valid in sheep grazed upland environments. Limited reports of emission rates from upland soils suggest that they are variable and may be controlled by interactions between fluid movement and the properties of near surface soil horizons – particularly if thick litter and organic layers are present (where nitrification may be limited). In this project, interactions between fluid movement, urea hydrolysis, nitrification and denitrification in upland soils will be investigated via a combination of field measurements, laboratory experiments and numerical modelling. This work will link closely with an ongoing NERC project which aims to quantify the spatial and temporal dynamics of N2O emissions from sheep-grazed upland pastures.
Laboratory incubation assays will be conducted to determine potential rates of urea hydrolysis, nitrification and denitrification in material sampled from upland soil litter and organic horizons. Experiments will also be conducted to quantify rates of N transformation in materials receiving artificial and real sheep urine in realistic ratios. Nitrification may be limited by pH and dissolved oxygen concentrations in the pore water which will, in turn, limit denitrification (due to a shortage of nitrate). There is also some uncertainty about the composition and functional competence of the microbial communities in upland soils which may be adapted to using alternative electron acceptors to nitrate under anaerobic conditions. The dynamics of solute transport and transformation in different materials in upland soils will be explored using state-of-the-art solute transport models such as HYDRUS-2D.
Training and Skills
Students will be awarded CENTA2 Training Credits (CTCs) for participation in CENTA2-provided and ‘free choice’ external training. One CTC equates to 1⁄2 day session and students must accrue 100 CTCs across the three years of their PhD.
You will become proficient in the design and execution of manipulative experiments looking at solute – soil interactions. You will master techniques for the measurement of fluid flow in soils, sampling pore water for solute analysis and the monitoring of N2O emission from the soil surface. You will also become an expert in the application of state of the art modelling of flow, solute transport and reaction in soil (e.g. Hydrus 2D).
You will join a thriving community of environmental scientists studying gas emissions from land to the atmosphere and have access to a range of cutting-edge analytical and field equipment.
Year 1: Analyse existing data from field and laboratory experiments (Rothamsted and Bangor) and characterise typical upland soil profiles for these existing experiments (e.g. in terms of texture, organic matter content, pH, REDOX potential, microbial biomass). Familiarisation with model framework and perform initial simulations to represent solute transport and reaction scenarios for existing data on N2O emission from upland soils.
Year 2: Plan and conduct manipulative experiments on solute infiltration, advection, diffusion and reaction in different profiles with a specific focus on the role of changing organic matter content and nature in litter and organic horizons (simulating the behaviour of urine patches). Characterise nitrification and denitrification potential in different soil horizons.
Year 3: Apply the 2D model of transport and reaction as a framework for integrating and interpreting the experimental findings. Publication of papers and conference presentations, with a target of presenting at international meetings. Complete analysis and write thesis.
Partners and collaboration (including CASE)
Dr Mick Whelan and Dr Joerg Kaduk will supervise this project. The group have interests in understanding the fate and transport of environmental contaminants and trace gas emissions from terrestrial systems, including applications of the biosphere exchange model JULES. The Leicester team will be supported by Dr Laura Cardenas from Rothamsted Research (North Wyke) and Prof. Dave Chadwick from the University of Bangor. They have a wealth of experience in understanding soil nitrogen dynamics and regularly contribute to the refinement of emission factors for national and international Greenhouse gas inventories.
Contact details: Dr Mick Whelan (firstname.lastname@example.org), in the School of Geography, Geology & the Environment, University of Leicester (https://www2.le.ac.uk/departments/geoggeolenv)