PhD Research Project: CENTA NERC - Understanding Methodological Constraints and Organic Carbon Qual

oil 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.

Research Needs:
a) Methodological challenge:
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 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.

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.

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. This knowledge will create a framework for ecological restoration and management of natural ecosystems for ensuring sustainability.

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.

Funding Notes

In addition to completing an online application form, you will also need to complete and submit the CENTA studentship application form available from www.centa.org.uk.

CENTA studentships are for 3.5 years and are funded by the Natural Environment Research Council (NERC). In addition to the full payment of their tuition fees, successful candidates will receive the following financial support.

Annual stipend, set at £14,296 for 2016/17
Research training support grant (RTSG) of £8,000

CENTA students are required to undertake from 45 days training throughout their PhD including a 10 day placement.

References

1. Sgouridis, F., A. Stott, and S. Ullah. 2016. Application of the 15N-Gas Flux method for measuring in situ N2 and N2O fluxes due to denitrification in natural and semi-natural terrestrial ecosystems and comparison with the acetylene inhibition technique. Biogeosciences 13, 1821–1835
2. Groffman, et al. 2012. Terrestrial denitrification: challenges and opportunities. Ecological Processes 1-11; DOI: 10.1186/2192-1709-1-11
3. Sgouridis, F. and S. Ullah. 2015. Relative magnitude and controls of in situ N2 and N2O fluxes due to denitrification in natural and seminatural terrestrial ecosystems using 15N tracers. Environmental Science and Technology 49, 14110 −14119
4. Cárdenas et al. 2003. Biogenic gas emissions from soils measured using a new automated laboratory incubation system. Soil Biol Biochem. DOI: 10.1016/S0038-0717(03)00092-0
5. *Loick et al. 2016. Denitrification as a source of nitric oxide emissions from incubated soil cores from a UK grassland soil.Soil Biol Biochem. DOI: 10.1016/j.soilbio.2015.12.009
6. Stevens R, Laughlin R (1998) Measurement of nitrous oxide and di-nitrogen emissions from agricultural soils. Nutrient Cycling in Agroecosystems, 52, 131-139
7. Gontharet et al. 2014. Distribution and sources of bulk organic matter (OM) on a tropical intertidal mud bank in French Guiana from elemental and isotopic proxies. Chemical Geology 376: 1-10
8. Huang et al. 2011. Post-harvest residue management effects on recalcitrant carbon pools and plant biomarkers within the soil heavy fraction in Pinus radiata plantations. Soil Biol Biochem. DOI: 10.1016/j.soilbio.2010.11.008