Factors affecting nitric oxide and nitrous oxide emissions from ...

2.6.3 Soil moisture and aeration
The main factor believed to regulate the release of NO and N2O from soil is the availability of
oxygen which is controlled by the partial pressure in the gas phase and the soil moisture
content (Bollmann & Conrad, 1998). Oxygen is generally considered an inhibitor to
denitrification (Knowles, 1982; Payne, 1982). The effect of water on denitrification occurs
through its control over O2 diffusion, with O2 diffusing 1 x 10 4 times slower in water. Thus
wet soils are more anaerobic with higher rates of denitrification and decreased nitrification.
However, even in relatively aerobic soils anaerobic micro sites may exist and denitrification
has been shown to occur in well aerated soils (Müller et al., 1997; Russow et al., 2009). The
response, in terms of NO and N2O fluxes, with respect to soil moisture and soil aeration is
complex and depends on the duration of soil wetting events, the relative intensity of the
wetting events, and the antecedent dry periods. Previous studies have shown that high NOx
fluxes (8-80 ng N m -2 s -1 ) can occur during the first rain of the season (Cardenas et al., 1993;
Johansson & Sanhueza, 1988; Johansson et al., 1988; Levine et al., 1996). For example,
during the transition from the dry to the wet season strong 'pulsing' effects of the NO flux
were observed and NO emissions increased by a factor of up to 60 within hours. Cardenas et
al. (1993) found maximum NO fluxes occurred when the gravimetric moisture content was
between 10-18% in a Venezuelan savannah. It is generally expected that as the soil moisture
content increases that N2O emissions will increase as a result of denitrification, while as soils
become drier N2O emissions will be dominated by nitrification with enhanced fluxes of NO
(e.g. Hou et al., 2000). Similar results have been found in microbial cultures where the
production of NO per cell was highest by autotrophic nitrifiers yet independent of O2
concentration in the range tested (0.5 to 10%), whereas N2O production was inversely
proportional to O2 concentration (Anderson & Levine, 1986).
Due to its influence on soil moisture the soil texture also affects NO and N2O emissions. In
well aerated coarse-textured soils with < 60% WFPS, nitrification, may be considered the
main process involved in NO production and emission (Bollmann & Conrad, 1998; Bouwman
et al., 2002; Davidson, 1992; Mexiner & Yang, 2004; Skiba et al., 1992). Fine-textured and
poorly aerated soils (> 60% WFPS) provide favourable conditions for the denitrification
process (Groffman & Tiedje, 1991). Thus, soil physical characteristics (i.e. aeration) and
particle size are important factors affecting the upward movement of NO and N2O. The
diffusion and transport of gases increases with decreasing water content and increasing
particle size. There are no studies that have examined the effect of soil moisture on the
emissions of NOx from ruminant urine patches.
12