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

This reaction is unlikely to occur in most soils due to the requirement for low pH and the fact
that unbound amino acids in soils are present in trace quantities. In Brazilian pasture soils, it
has been estimated that 50% of the NO production originates from abiotic processes (Trebs,
2001). Venterea & Rolston (2000b) reported NO production in sterile agricultural soils to be
highly correlated with HNO2 concentration. Volatilization of HNO2 from the soil aqueous
solution may also contribute to NOx emissions to the atmosphere.
The most important factors controlling the production of NO via abiotic processes are
reported to be organic matter content, concentration of NO2 - and pH (Firestone & Davidson,
1989).
2.4 Consumption of NO and N2O in soils
The major sink for N2O in soils is the reduction to N2 during denitrification. As noted above
the activity of the N2O reductase enzyme is dependent on chemical and environmental
variables. Generally, conditions interfering with N2O diffusion in the soil (see below) appear
to enhance N2O consumption (Chapius-Lardy et al., 2007; Hénault et al., 2001). However, the
factors regulating N2O consumption are not yet well understood and merit further study
(Chapius-Lardy et al., 2007).
Nitric oxide is highly reactive and may be decomposed by several chemical and biological
reactions in the soil. Both autotrophs and aerobic heterotrophs have been shown to be capable
of oxidising NO to NO2 - and/or NO3 - (Conrad, 1995). Alternatively, as noted above, NO may
be reduced in the denitrification sequence. For many denitrifying species once the NO
reductase enzyme is synthesized, it is relatively insensitive to oxygen concentrations so that
NO consumption by denitrification may take place even in well aerated soil (Remede &
Conrad, 1991).
Theoretically there is also the potential for NO to be abiotically oxidised in the soil
atmosphere via the same reactions that occur once it is released into the lower atmosphere, via
reactions with O3 or O2, but it is generally assumed that only the latter is potentially possible
(Conrad, 1995), since there are few if any data on O3 in the soil profile, but even so the NO
concentration would need to be > than about 10 L L -1 and as such is unlikely. Thus chemical
oxidation reactions in the soil are of marginal significance.
Consumption rates of NO vary according to different soil types and other environmental
factors affecting the consumption processes. For example, nearly 95% of the NO produced in
an organic soil via nitrification was oxidised to NO3 - within the soil rather than emitted to the
atmosphere (Dunfield & Knowles, 1999). However in a mineral gley soil the consumption
rate was only 38% of the gross NO production (Dunfield & Knowles, 1999).
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