Overview

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).  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.  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.  The findings from the laboratory experiments and numerical modelling in this project will be used to help interpret data from larger field and lab experiments (including solute concentrations at different depths beneath sheep urine patches and N2O emissions) which are currently being collected.

FSchematic illustration of N2O emissions from sheep urine patches and how these might be affected by upland soil properties. U : Urine.

Methodology

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. The dynamics of solute transport and transformation in different materials in upland soils will be explored using numerical modelling using state-of-the-art solute transport tools such as HYDRUS-2D with some bespoke code development (particularly to represent spatial and temporal variations in solute reactions).  Fluid movement in litter and peaty soil differs from that in mineral soils, so some empirical investigations into the hydraulic properties of these materials will also be needed. Solute dispersion and transformation will depend on the volume of the urine deposited and the soil moisture content.

Training and Skills

CENTA students are required to complete 45 days training throughout their PhD including a 10 day placement. In the first year, students will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to CENTA research  themes.

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 soil fluid flow, sampling pore water for solute analysis and the monitoring of N2O emission from the soil surface. You will also become expert in the application of models of fluid flow, solute transport and reaction.  This combination of techniques will provide you with a unique set of skills that will be attractive to employers in industry, government and academia. You will join a thriving community of environmental scientists studying trace gas emissions from land to the atmosphere at Leicester and have access to a range of cutting-edge analytical and field equipment.

 

Timeline

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 will be the primary supervisor.  He has over 25 years of experience in environmental science with particular interests in understanding the fate and transport of environmental contaminants. Dr Joerg Kaduk is an internationally known for understanding trace gas emissions from lowland peats and 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.  Laura and Dave have a wealth of experience in understanding soil nitrogen dynamics and regularly contribute to the development and refinement of emission factors for international Greenhouse gas inventories.

Further Details

Contact Mick Whelan, University of Leicester, mjw72@le.ac.uk