Estimates of nitrous oxide (N<sub>2</sub>O) emissions from grassland at field and national scale are presented. Concentrations of atmospheric N<sub>2</sub>O are current rising due to increasing anthropogenic emissions. This is of concern as N<sub>2</sub>O participates in reactions leading to the destruction of stratospheric ozone and adding to radiative forcing. Quantifying these emissions accurately is a challenge complicated by environmentally driven spatial and temporal variability of its main source, microbial production in soils. In the UK grasslands are the largest source of N<sub>2</sub>O. In this project for the first time N<sub>2</sub>O was measured from typically intensively managed grassland at Easter Bush, S. Scotland from June 2002 to June 2003 almost continuously. Measurements were made using the micrometeorological eddy covariance technique which couples an ultrasonic anemometer with a Tunable Diode Laser fast response gas. Analysis System (TDLAS) to measure the N<sub>2</sub>O concentration (precision ±0.9%, frequency 10 Hz). The spatial coverage attainable using long-term eddy covariance N<sub>2</sub>O flux measurements (10<sup>3</sup>-10<sup>4</sup> m<sup>2</sup>) represents a valuable alternative to the commonly used static chamber method (< 1 m<sup>2</sup>), overcoming the problem of spatial and temporal variability of N<sub>2</sub>O emissions associated with the latter technique. Four fertiliser applications made over this period were investigated in detail together with environmental conditions. Background activity was characterised by small fluxes (±50 ng N<sub>2</sub>O-N m<sup>-2 </sup>s<sup>-1</sup>). However, N<sub>2</sub>O flux varied widely, with a series of daily emission peaks (≤ 3,795 ng N<sub>2</sub>O-N m<sup>-2</sup> s<sup>-1</sup>) following the fertilisation in June 2002 and less pronounced emissions after the other fertilizations in August 2002, March and June 2003. A multi-linear regression relating combinations of soil water content, soil temperature and the soil available nitrogen with N<sub>2</sub>O fluxes was used to study the seasonal variation of the emissions. Good results (R<sup>2</sup> = 0.83-0.9) were obtained with all the parameters but the best predictors resulted soil water content and soil temperature at 7 cm depth. The extraordinarily large pulse observed on 8 June 2002 immediately after the fertilizer application (120 kg N ha<sup>-1</sup>) represents 1.6% of the N applied. After this event clear diurnal emission cycles, peaking at noon, were recorded for about 10 days, allowing an analysis of the environmental factors controlling the loss. In this case turbulent parameters such as sensible heat flux and friction velocity were closely related to the N<sub>2</sub>O flux suggesting that they could be fundamental drivers in the emissions. The estimate of the total cumulative flux was 5.3 kg N ha<sup>-1</sup> y<sup>-1</sup>, representing an emission factor of 1.68%. The data also reveal a small, but significant, uptake during the winter months, highlighting the existence of N<sub>2</sub>O sink activity on grassland.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:649511 |
| Date | January 2005 |
| Creators | Di Marco, Chiara |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13642 |
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