Case study: grasslands – issues and solutions, research networks.

How we measure N₂O using chamber techniques

L. M. Cardenas, D.R . Chadwick, T. H. Misselbrook, N. Donovan, K. A. Smith, R. M. Rees , M. Bell,

C . J . Watson , K. L. McGeough, G. McNeill, R. Williams, J. M. Cloy , R. E. Thorman,  M. S. Dhanoa

Nitrous oxide plays a dual role in the atmosphere as a greenhouse gas and via its influence on stratospheric ozone chemistry. The main source of N₂O is agricultural soil, with an estimated 96 kt emitted from this source in the UK in 2012 (ca. 83% of the total UK N₂O emissions). Microbial transformations such as nitrification, denitrification and chemodenitrification are responsible for these emissions.  Soil texture and structure and land management practices (including presence of livestock) create spatial variations in the main drivers of these transformations -- soil wetness, aeration, temperature and mineral N content -- and consequently in the size of the emissions.  Daily and seasonal variation in soil conditions (chemistry, physics and biology) and thus in emissions also occurs. This variability makes it a difficult challenge to quantify emissions and currently makes the soil source the largest contributor to the overall uncertainty of the UK greenhouse gas inventory. A UK government-funded project aims to improve the quantification of this source by measuring emissions from nine sites with contrasting soil, climate and land management combinations. Gas sampling is based on the static chamber technique, with a limited number of autochambers for comparison. A statistical approach applied to earlier measurements was used to validate the technique used (Chadwick et al., 2014). Here, we describe the methodology used, detailing the procedure for sample collection and analysis. We also present the results of the statistical analysis showing that our assumptions for chamber deployment and headspace sampling are justified, as the uncertainty in fluxes introduced is far smaller than that resulting from non-representative spatial sampling. We also include a comparison of the daily mean estimated by the static chambers and the results from the autochambers.

Chadwick et al (2014). EJSS 65, 295-307.