PhD Thesis, 2010 - University College Cork

Literature review
Due to the natural complexity of the soil structure, the moisture, oxidation and
aeration conditions differ significantly across even a few millimetres (size of the soil
aggregates). Typically, a surface of the aggregate is better aerated and prone to quickly
changing conditions of water and gas content, while on the other hand, conditions
inside aggregates are tightly controlled by the size of the intra-aggregate pores and as a
result high-moisture reduced environments tend to dominate. This means that all
transformational processes are present in most natural soils at any given time; however,
depending on the macroscopic conditions certain processes will dominate (Bateman
and Baggs 2005). The macroscopic conditions, such as average water content or
temperature of the soil horizon cannot change suddenly and therefore the intensity of
the nitrous oxide flux typically lags behind any triggering events. To the best of our
knowledge there was no information reported on the duration of the lag periods for any
kind of trigger events.
The importance of lag periods is now becoming clear when considering the
importance of the high-intensity fluxes, referred to as “burst”, “peaks” or “events”
(Clayton et al. 1997; Leahy et al. 2004), further clarification for these terms will be
given in the Chapter 4. Such fluxes are the opposite of “background emissions”, which
are characterised by low intensities, long duration and high frequencies of occurrence.
These bursts were reported to comprise up to 85% of the annual flux from the fertilised
grassland (Hsieh et al. 2005) on the basis of difference with the background flux.
While qualitative descriptions of the peaks are not uncommon, it is hard to find any
quantitative definition of the peak events. Further investigation into causes and
phenomenon of the emission peaks is imperative if the understanding of the temporal
patterns of the nitrous oxide to be improved.
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