Trace gas emissions of nitrous gases
Nitrous gases are formed under anaerobic conditions by the activity of denitrifying microbials (see section Denitrification). These gases diffuse from the anaerobic fractions of the soil to the aerobic parts, travel vertically through the soil profile and leave the soilsurface through the uppermost soil layer to the atmosphere. Nitrous gases are also formed when ammonium is nitrified to nitrate. An overview over the trace gas fluxes and associated nitrogen pools is given in Figure 5.4.
Figure 5.4. Nitrogen trace gas fluxes in the soil profile.
NO and N2O is formed during the nitrification process. These nitrogen fluxes are calculated as:
where gmfracNO is the maximum NO fraction parameter and NNH4→NO3 is the nitrification rate. f(θ), f(T) and f(pH) are the response functions for soil moisture, temperature and soil pH respectively. The formation of N2O, i.e. NNO3→N2O, is calculated with the same equation by exchanging gmfracNO to gmfracN2O and excluding the term f(pH).
The response function for soil moisture is calculated as:
where θ(z) is the soil moisture content, θs(z) is the water content at saturation, and gθsatcritNO and gθsatformNO are parameters. The parameters gθsatcritNO and gθsatformNO are exchanged to gθsatcritN2O and gθsatformN2O to calculate the response function for the formation of N2O. See viewing function Moisture response function nitrification.
The response function for soil temperature is calculated by:
where gTmaxNxO, gToptNxO and gTshapeNxO are parameters and T(z) is the soil temperature. See viewing function Temperature response function nitrification.
The response function for soil pH is calculated as:
where gpHcoef is a pH coefficient and gpH(z) is the soil pH in different soil layers. See viewing function pH response function nitrification.
When the nitrous gases are emitted directly to the atmosphere from the layer in which they were formed, the diffusion of nitrous gases from anaerobic fractions are calculated as:
(6.69)
and analogously for N2O and N2. f(O2) is an oxygen diffusion exchange function.
This equation is slightly modified when vertical trace gas diffusion through the soil is simulated:
(6.70)
and analogously for N2O and N2. The negative sign indicates that the flow is directed from the anaerobic to the aerobic parts of the soil.
The oxygen diffusion exchange function is calculated as:
where fAnvol is the volumetric anaerobic fraction of the soil and odiffred is an oxygen diffusion reduction parameter. See viewing function Oxygen diffusion exchange function.
Odiffrate is the oxygen diffusion rate calculated as:
where dO2 is a tortuosity parameter, oO2diffrate is the oxygen diffusion rate at 20 °C, DO is the diffusion coefficient in free air and fa is the air porosity calculated as:
(6.73)
where θs and θ are the saturated and actual soil moisture content respectively.
The flux of nitrous trace gases to the atmosphere when transport through the soil is not considered, is equal to the sum of the nitrous gases formed by nitrification and denitrification:
(6.74)
and analogously for N2O and N2.
Vertical transport of trace gases through the soil, when simulated explicitly, is a function of the oxygen diffusion rate:
and analogously for N2O and N2. The negative sign indicates that the gas flow is always directed upwards in the soil.
For the uppermost layer eq.(6.75) is modified into:
(6.76)
where Δz in this case symbolises thickness of layer 1.