School of GeoSciences

School of GeoSciences

Richard Nair

PhD Topic: Interactions Between Atmospheric Nitrogen Deposition and the Forest Carbon Cycle

Supervisors: Maurizio Mencuccini (University of Edinburgh) Mike Perks (Forest Research NRS), and Peter Levy (CEH Edinburgh)

I am a final year PhD student in the School of Geosciences within the Global Change Research Group in the School of Geosciences. My current research focuses on using the stable isotope of nitrogen, 15N as a tracer in field and nursery mesocosm experiments to quantify how nitrogen allocation within the soil-plant system may differ when it is applied by different methods.

Prior to coming to Edinburgh I studied for a BSc (in Biology) and MRes (in Ecology and Environmental Management) at the University of York. During these degrees I have worked at the Millennium Seed Bank, Kew and at Forest Research Alice Holt. My general research interests are in global change ecology, nutrient cycles and trace gas fluxes. I am particularly interested in the use of stable isotopes to address questions in these fields and in the past have used a mass spectrometer in the field to measure 13CO2 / CO2 fluxes from soil core treatments and custom built stem respiration chambers. Currently I am working primarily with measuring biomass 15N / N content under different nitrogen deposition treatements as part of my PhD project

There is an ongoing debate about the role of nitrogen deposition in explaining a net carbon sink in temperate and northern forests. The global nitrogen (N) cycle is heavily modified by human activity and reactive, plant-available forms of N are emitted to the atmosphere from agricultural, industrial, and transport sources. As this is deposited in ecosystems far from the original source, there is the potential for this extra nitrogen to release constraints on biological activity in N-limited systems, which may promote growth. Forests contain large amounts of carbon (C), due to long-lived, woody biomass, and may have a strong growth response to N deposition.

However, the net C uptake in forests may also be due to a variety of other factors and the strength of a C response depends on how photosynthesis (carbon uptake) and respiration (carbon release) respond to N deposition. For a strong C response N must also cause a strong response in wood production, rather than leaves, twigs, or fine roots. Previous experiments indicate that trees are relatively poor competitors for additional N, and may not use much of this nutrient to produce high C, woody tissues. 

Most stable isotope approaches have applied 15N directly to the forest floor and have not included interactions with the canopy in their method of N application, which has been suggested to promote a higher retention of N by trees than is calculated using throughfall-based N budgets or soil-applied 15N accountancy. Estimates of the importance of canopy uptake vary and are not usually directly compared to soil applications of N. The application of 15N tracers in deposition also mean it is difficult to trace N from internal pools of decomposing litter, where 15N natural abundance tends to be highly conserved, but which represent most of the available N to trees, even under heavy N deposition.

During my project I have attempted to address both of these problems in a series of experiments on Sitka Spruce where the differences in fate of 15N-labelled N amendments to the canopy can be directly compared to the fate of nitrogen The first phase of my PhD concentrated on creating 15N-labelled litter (leaves and twigs) to use in field experiments. I then moved on to two further time-series experiments: a mesocosm experiment on pot-bound saplings where in combination treatment I varied the method of N-application and 15N content of treatments to allow a comparison the fate of deposition and litter N under canopy or soil deposition, and a field experiment where a more realistic system could be studied, but canopy treatments were not possible. I hope to update this page soon with further information about my results and their publication!

If you have any questions about my work, please contact me:

Institute of Atmospheric and Environmental Sciences School of GeoSciences Crew Building, Kings Buildings University of Edinburgh Edinburgh EH9 3JN, UK