Wells NS, Clough TJ, Johnson-Beebout SE, & Buresh RJ. Land management between rice crops affects
soil inorganic nitrogen balance in a tropical rice system. Nutrient Cycling in Agroecosystems
Supplementary materials: Ammonia volatilisation in the fallow paddy environment
Ammonia volatilisation is the product of multiple equilibrium reactions, the net effect of which is that the
proportion of available N lost to the atmosphere depends on the partial pressure of NH3 at the surface of
the soil or floodwater (Sherlock and Goh 1985; Fillery and Vlek 1986; Vlek et al. 1980). Aqueous NH3
(NH3(aq) is in dynamic equilibrium with NH4+, and the balance between the two is determined by pH and
temperature (Eq. 1):
(1)
𝑁𝐻3(𝑎𝑞) = 𝑁𝐻𝑥(𝑎𝑞) /(1 + 10
(0.09018+
2729.92
+𝑝𝐻)
𝑇
)
where NHx(aq) is the sum of NH4+ and NH3 in mol l-1, and temperature (T) is in K (see Sherlock and Goh
(1985) for the complete derivation of this equation). Because extracting soils with KCl effectively
removes both NH4+ and NH3 all values measured during the experiment can be assumed to represent NHx.
An increase in the size of the NHx pool, temperature, or pH will therefore cause an increase in
volatilisation. The actual rate of volatilisation then depends on the equilibrium between NH3(g) and
NH3(aq), as the partial pressure at the soil/water surface must exceed that in the air in order for
volatilisation to occur at a significant rate. This equilibrium is defined by the Henry’s law equilibrium
constant (K):
(2)
𝑁𝐻3(𝑎𝑞) = 𝐾 𝜌𝑁𝐻3(𝑔)
with, 𝑙𝑜𝑔𝐾 = −1.69 + 1477.77/𝑇
where pNH3(g) is the partial pressure of NH3(g) at the soil/water surface.
These two equations were used to estimate the pNH3 in the different fallow treatments over time using the
recorded soil temperature, pH, and NH4+ data. Straw sub-plots were excluded from this analysis because
soil pH was not recorded in them. The concentration of NH4+ in the top 2 cm of the soil was used for
aerobic treatments, and that in the floodwater was used for anaerobic treatments.
As expected, pNH3(g) varied significantly between treatments over time (p<0.001), reflecting the
differences in NH4+ concentrations. However, the mean value for all treatments over time was 4.6x10-6
±1x10-6 mg N m3, comparable to background concentrations of atmospheric NH3 found in agricultural
regions (e.g., Zbieranowski and Aherne 2012). It is therefore concluded that the low NHx concentration
and neutral pH found in all of the treatments will make NH3 volatilisation physically unfavourable,
meaning that it does not need to be considered as a meaningful N removal pathway under these
experimental conditions.
References
Fillery IRP, Vlek PLG (1986) Reappraisal of the significance of ammonia volatilization as an N loss
mechanism in flooded rice fields. Fertil Res 9 (1-2):79-98. doi:10.1007/bf01048696
Sherlock RR, Goh KM (1985) Dynamics of ammonia volatilization from simulated urine patches and
aqueous urea applied to pasture .2. Theoretical derivation of a simplified model. Fertil Res 6
(1):3-22. doi:10.1007/bf01058161
Vlek PLG, Stumpe JM, Byrnes BH (1980) Urease activity and inhibition in flooded soil systems. Fertil
Res 1 (3):191-202. doi:10.1007/BF01053131
Zbieranowski AL, Aherne J (2012) Ambient concentrations of atmospheric ammonia, nitrogen dioxide
and nitric acid across a rural–urban–agricultural transect in southern Ontario, Canada. Atmos
Environ 62 (0):481-491. doi:http://dx.doi.org/10.1016/j.atmosenv.2012.08.040