- Broad-based training in cutting-edge techniques for linking N cycling to C under global change in forests (based at BIFoR, Birmingham, http://www.birmingham.ac.uk/research/activity/bifor/index.aspx) .
- International supervisory team for an integrated approach for training and research in global biogeochemistry.
- Innovation to validate the most robust methodology for denitrification to address global N pollution issues.
Introduction: Soil denitrification is considered the most unconstrained process in the global nitrogen (N) cycle due to uncertain in situ di-nitrogen (N2) flux measurements1. Denitrification reduces NO3 to NO, N2O and N2 gases, and constitutes the most important mechanism for the removal of excess/pollutant reactive nitrogen (Nr) in terrestrial soils2. Reliable measurements of denitrification and an understanding of its primary controls including the quality (functional moieties) of soil organic carbon (SOC) in varied age classes of forest ecosystems are imperative for robust quantification and in gauging the response of forest restoration and management to global change issues 3.
- Methodological challenge: The acetylene inhibition techniques1,4,5. Acetylene technique is becoming increasingly unpopular due its several limitations1. The He/O2 gas-flow method allows direct and precise measurement of N2 flux from soil cores; however, it requires long equilibration of cores in the laboratory under He/O2 headspace. The 15N gas-flux method has the advantage of providing in situ field measurements. A criticism of 15N gas-flux method has been the artificial stimulation of denitrification by the added trace. Thus to further improve and compare the validity of the 15N-flux method1, a need exists to relate its performance with the laboratory-based He/O2 gas-flow soil core method. This innovation would provide a much needed methodology for replication globally to improve our understanding of the global N cycle.Project aims: To evaluate the performance of in situ 15N Gas-Flux and the He/O2 flow soil core techniques in quantifying denitrification and to elucidate the implications of differences in SOC quality in influencing denitrification and greenhouse gas emission from soils under mature and restored forests (Figure 1). This knowledge will create a framework for ecological restoration and management of natural ecosystems for ensuring sustainability.
- b) Mechanistic challenge: The high variability of denitrification is commonly attributed to variations in the biogeochemical regulators of the process including SOC availability. However, our mechanistic understanding of the impacts of varied SOC types (e.g. phenols, fatty acids) in forest soils on controlling denitrification, N2O emission and N mineralization is very limited. Therefore, an evaluation of the difference in the quality of the SOC in influencing denitrification, N2O emission and N mineralization is critical for understanding the role of forests in removing Nr and in quantifying functional SOC, denitrification and mineralization restoration trajectories in restored forest soils.
- Methods for measuring soil denitrification (N2O and N2 fluxes) are mainly limited to three key techniques: the 15N-Gas flux, helium/oxygen (He/O2) gas-flow and
Intact soil cores will be collected from restored and mature forest sites managed by the Birmingham Institute for Forest Research. The cores will used for the determination of denitrification using the 15N gas flux and the He/O2 gas flow soil core techniques. In the UK, the He/O2 gas flow soil core system at Rothamsted Research, UK will be used for performance evaluation of the 15N Gas flux method. In situ denitrification will also be determined to validate field application. The 15N analysis in solids will be undertaken at Birmingham University, while the 15N gas samples will be analysed at the NERC’s LSMSF at CEH-Lancaster.
In parallel, soil cores will be also collected in order to characterize the abundance and the chemical composition of SOC in two density fractions. Vegetation samples (herbs, shrubs, roots, fragments of wood and bark of trees) will also collected and analysed to define the potential carbon sources. The abundance of SOC and its potential sources using elemental C/N ratios, 13C and 15N values7 as well as biomarkers (hydrocarbons, alcohols/sterols, fatty acids, lignin-derived phenols) in two density fractions of soils (light (labile) and heavy (stable) fractions8 will be measured using IRMS and GC mass spectrometry, and NMR spectroscopy at Birmingham.
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 the student's projects and themes.
This proposal entails extensive training in field experimental design and operation of environmental analytical instruments including chromatography, colourimetry, in situ sensing and application of stable isotope techniques for addressing issues of regional to global concerns. Training in the use of the only facility of the UK (He/O2 core systems) at Rothamsted Research and its coupling to the 15N-gas flux method will help explore unknown frontiers in biogeochemistry. Application of the advanced SOC characterization techniques at Littoral University, France to un-ravel novel relationships between elemental cycles are planned as well.
Months 0 to 6: Literature review and introductory training in analytical instrument at Birmingham.
Month 6 to 09: Placement visits at Rothamsted Research (10 days) and France (30 days) for training in the He/O2 gas flow soil system and SOC characterization with mock experiments.
Months 10 to 12: Finalization of key research questions with experimental design for year 2 and 3.
Months 1 to 06: Incubation of soil cores collected from restore and mature forest sites using 15NGas flux method at Birmingham and He/O2 gas flow soil 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 methods performance and identification of actions for field validation.
- Field validation of 15N flux method, followed by comparative N2 and N2O losses from restored and mature forest soils under changing climate factors.
- Field measurement of gross N mineralization and N2O flux source portioning using 15N tracers. Year 4:
- Months 0 to 6: Data synthesis and thesis write up, defence and publications.
- Month 10 to 12: Data analysis and contingency time in case of sample processing backlogs.
Partners and collaboration (including CASE)
Dr Laura Cardenas at Rothamsted Research, UK agreed to share the sole core-based N2 flux facility for methodological innovations in the UK. 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 that are in demand within the soil science discipline globally.
For further information about the project or the CENTA PhD programme, please do not hesitate to contact the supervisory team by email under the addresses given above.