Human alteration of the nitrogen cycle: Implications for denitrification and nitrous oxide emission from forest soils
The magnitude of the human alteration of the global nitrogen (N) cycle due to agricultural intensification and industrial development is way bigger than any other elemental cycle on earth. The use of synthetic N fertilizer and fossil fuel results in the emission of reactive N (Nr) into air, which over time is re-deposited on land. As a result, soils under natural ecosystems including forests are now exposed to excessive Nr deposition. This additional Nr input supports forest growth, but its amount in soils in some regions is way above the forest growth demands. Thus too much of this good thing is indeed an environmental quality concern leading to higher greenhouse gas nitrous oxide (N2O) emission, for example.

Denitrification is the only dominant natural process in soils responsible for removing the Nr permanently by converting it back into the non-pollutant dinitrogen (N2) gas. However, during this process N2O gas is also produced when Nr availability is high. Simply too much of the Nr deposition locks the soils in a vicious cycle of swapping one pollutant for another thus negating the benefits of the enhanced biomass production. There is a need to better understand the controls of the denitrification processes in soils so that forests could be managed in way that N2O emission in to air is reduced. This PhD project would address two key challenges:

1) Methodological challenge: Among available methods, the 15N gas-flux method has the advantage of providing field measurements, but it may stimulate denitrification due to the added 15N tracer. Accounting for the stimulatory contribution of 15N tracer to denitrification if any, is thus imperative for robust measurements.

2) Mechanistic challenge: Depending on soil conditions, denitrification varies by an order of magnitude in space and time. A better understanding of the controlling factors such as the quality of soil organic carbon (SOC), soil moisture, temperature and nitrate content in forest soils is needed for modelling the response of denitrification and N2O emission from forest soils to Nr deposition.

Project aims: To evaluate the performance of in situ 15N Gas-Flux method in quantifying denitrification and to elucidate the implications of differences in SOC quality in influencing denitrification and N2O emission from soils under mature and restored forests exposed to chronic Nr deposition.
Soil cores will be collected from two restored and a mature forest site of the Birmingham Institute for Forest Research (BIFoR). The cores will be incubated using the He/O2 gas flow system at Rothamsted Research for performance evaluation of the 15N Gas flux method. Following laboratory incubation, in situ denitrification will be determined in the field together with high frequency measurement of soil moisture and temperature using a novel sensing technology called FO-DTS. The abundance of soil SOC and its potential sources using elemental C/N ratios, d13C and d15N values as well as lipid biomarkers (hydrocarbons, alcohols/sterols, fatty acids, lignin-derived phenols) will be determined to elucidate key controls of SOC on denitrification.

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 in research methods and core skills. Throughout the PhD, training will progress from core analytical skill sets to master classes specific to the student’s projects and themes.

This proposal entails training in operation of analytical instruments including chromatography, colourimetry, FO-DTS and stable isotope techniques. Training in the use of the only UK facility of the He/O2 flow systems at Rothamsted Research and SOC characterization at the Littoral University in France is part of the project and training.

Year 1:

Months 0 to 6: Literature review and training in analytical instruments at Birmingham.

Month 6 to 9: Placement at Rothamsted Research (5 days) and France (15 days) for training in the He/O2 gas flow system and SOC characterization.

Months 10 to 12: Research plan finalization.

Year 2:

Months 1 to 6: Incubation of soil cores using the 15N-Gas flux method at Birmingham and He/O2 gas flow system at Rothamsted Research (10 days).

Months 7 to 9: SOC and ancillary biogeochemical characterization of soils at Birmingham.

Months 10 to 12: Data analysis for method performance and identification of key controls.

Year 3:

Month 1-9: Field validation of 15N Gas flux method under changing soil conditions of moisture and temperature using the FO-DTS technology for measuring in situ rates.

Month 10 to 12: Data analysis completion of experimental work.

Year 4:

Months 0 to 6: Thesis write up, defence and publications.

Partners and collaboration (including CASE)
Dr Laura Cardenas at Rothamsted Research agreed to share the He/O2 flow facility for this research. Training of the student at the world’s oldest Agricultural Research station would have a significant added value in terms of learning cross-disciplinary skills. Dr S. Gontharet at Université du Littoral-Côte d’Opale in France will be part of the supervisory team to contribute to the training of the student in SOC characterization over a 30 day placement in France. The student will then utilize the learned techniques at Birmingham University using analytical facilities and expertise of Drs S. Ullah, S. Krause and N. Kettridge.

Further Details
For further information about the project or the CENTA PhD programme, please do not hesitate to contact the supervisory team by email ([email protected]).

  • Close Date:
  • Additional Dates:
  • Contact: Sami Ullah email