Controls on N2O production

N2O can be both produced and consumed during denitrification; the magnitude and direction of N2O exchange between the soil and atmosphere therefore reflect the net amount of these two opposing processes. The rate of production depends not only on the amount of nitrogen that is nitrified and/or

Rigid Contact Lens Container
Fig. 5.1. Possible ecological niches for nitrogen transformation pathways in fertilized soils. (Redrawn from Wrage et al., 2001.)

denitrified, but also on the ratio of N2O generated per unit of nitrogen processed. The amount of nitrogen that is nitrified and/or denitrified is related to the size and activity of the nitrifier and denitrifier populations, the amount of nitrogen available and the competitive pressure from other biological activity (e.g. nitrogen uptake by plants). Nitrogen availability is governed by external nitrogen additions (e.g. organic and inorganic fertilizers) and, because microbial decomposition of organic matter provides NH+ for nitrification and nitrification provides NO- for denitrification (Fig. 5.2), by the rate and magnitude of antecedent micro-bial processes.

Conditions that govern the rate of nitrification tend also to influence the ratio of N2O produced per unit of NH+ (N2O/NH+ ratio) consumed, but often in opposing directions. For example, high NH+ and oxygen availability favours nitrification but reduces the

N2O/NH+ ratio. Although the temperature range of nitrification is typically reported as 5-40°C (Bremner, 1997), populations may adapt to prevailing climatic conditions. In the cool subhumid region of Canada, for example, Malhi and McGill (1982) observed measurable nitrification at -4°C and peak rates at 20°C. The N2O/NH+ ratio appears to increase as soil temperature and soil pH increase (Bremner, 1997).

Denitrification is favoured by the absence of oxygen and by adequate levels of soluble carbon and NO-. The optimum pH range for denitrifiers is ~6.0-8.0, although denitri-fier populations appear to adapt to long-term pH conditions (Parkin et al., 1985) and activity can occur at pH as low as 3.5 (Aulakh et al., 1992). Denitrifiers favour temperatures between 5°C and 40°C but, as with nitri-fiers, populations may adapt to prevailing climatic conditions (Malhi et al., 1990). The N2O/NO3- ratio is very dynamic and sensitive

Fig. 5.2. A simplified view of the nitrogen cycle in a cropping system. Adoption of no-till (NT) can influence numerous processes in this cycle: (i) industrial fixation (via amount of nitrogen fertilizer needed); (ii) mineralization (via reduced disturbance); (iii) immobilization (via residue placement); (iv) nitrification (via effects on soil moisture and processes generating NH+); and (v) denitrification (via effects on aeration).

Fig. 5.2. A simplified view of the nitrogen cycle in a cropping system. Adoption of no-till (NT) can influence numerous processes in this cycle: (i) industrial fixation (via amount of nitrogen fertilizer needed); (ii) mineralization (via reduced disturbance); (iii) immobilization (via residue placement); (iv) nitrification (via effects on soil moisture and processes generating NH+); and (v) denitrification (via effects on aeration).

to the factors controlling denitrification rate. The ratio tends to be related positively to the NO-/soluble carbon ratio and oxygen level, and negatively to temperature and pH.

N2O production at the cellular level is determined by the interaction of many regulating factors. Oxygen status, for example, is strongly governed by soil water content; since oxygen travels more slowly through water than through air, oxygen diffusion into the soil decreases as soil water-filled pore space increases. Emissions of N2O from nitrification peak between 50% and 60% water-filled pore space (WFPS), while emissions from denitrification peak between 70% and 90% WFPS (Aulakh et al., 1984; Davidson et al., 1986; Stevens et al., 1997). Emission rates drop rapidly when WFPS exceeds 90% because denitrification goes to completion (N2) in the absence of oxygen, and the transmission of N2O to the soil surface is impeded, resulting in greater opportunity for further reduction.

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