Improved understanding of nitrous oxide “hot-spot” dynamics in upland soils

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 nitrous oxide 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.

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. 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 numerical modelling. It is envisioned that this will mainly involve the application of state-of-the-art solute transport tools such as HYDRUS-2D but will also require some bespoke code development (particularly to represent spatial and temporal variations in solute reactions). Fluid movement in litter and peaty soil is believed to differ 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.

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