Nitrous oxide is produced and consumed by bacterial activity associated with the soil nitrogen cycle. Production of N2O can occur in association with both nitrification and denitrification. During nitrification NH| is oxidized to NOg , and in the process NO and N2O can be produced as intermediate by-products. Under anaerobic conditions (or in anaerobic microsites within an otherwise aerobic soil) nitrate is reduced to N2, again with the intermediate by-products NO and N2O. The NO and N2O that are dissolved in the soil water phase will diffuse out of the soil into the atmosphere if they are not chemically transformed (oxidized to form nitrate or reduced to form N2). There are three general factors that affect the rate of N2O production and its emission from soils: (1) availability of substrate (ammonium and nitrate), which affects the total flux rate of material through the nitrogen cycle and therefore the amount of nitrous oxide formed (Keller et al., 1988); (2) the efficiency of N2O production and consumption during nitrogen cycle processes. This is in turn controlled by the relative rates of nitrification and denitrification, which change with environmental conditions; (3) efficiency of N2O diffusion out of the soil, which is controlled by the amount of water in soils (water-filled pore space) and by soil texture (Matson and Vitousek, 1990; Matson etal., 1990; Davidson, 1992; Keller and Reiners, 1994; Davidson and Schimel, 1995). A number of recent studies have examined the importance of these factors, as is briefly reviewed below.
A major reason for variation in N20 and NO emissions between different soil systems may be changes in the relative contributions of nitrification and denitrification. Several parameters that would allow prediction of the relative contribution of nitrification and denitrification to the emitted NO, N20, and N2 gases have been proposed. First, it has been proposed that at an optimum water-filled pore space (WFPS) of 60%, nitrification is the dominant process, and NO is the predominant nitrogen gas emitted. At higher values of WFPS (between 60 and 90%), soil aeration becomes limited, denitrification activity approaches a maximum, and N20 becomes the most abundant gas emitted. However, when WFPS is greater than 90%, N20 produced by denitrification is reduced to N2, which then becomes the most abundant gas emitted (Davidson, 1991, 1993; Keller and Reiners, 1994; Dendooven et al, 1996). Second, a widely accepted assumption is that when the N20/N0 ratio is larger than one, denitrification is the main process producing N20 emissions (Davidson, 1993). Third, some studies found that N20 emissions were higher after NO,/ rather than NI fertilization (Keller et al, 1988; Livingston et al., 1988; Bakwin et al., 1990). This suggested that the greater part of N20 is produced by denitrification in soils.
The generalizations presented above have controversial aspects, however, and recent experimental results have shown that the production of N20 by nitrification or denitrification in a particular soil is not easy to predict. For instance, it has been found that nitrifier bacteria can produce significant amounts of NO and N20 under very anaerobic soil conditions (Bollmann and Conrad, 1998). De Klein and Van Logtestijn (1996) found no correlation between denitrification rates and the rate of NOg addition. The available techniques (non-isotope techniques) for estimating the relative contribution of nitrification and denitrification to the emitted N20 are invasive methods that might not adequately represent the soil under natural field conditions. Since most of the techniques used to partition N20 sources are not done in situ but in laboratory incubations, the possibility of creating experimental artefacts is high.
Measurement of the stable isotope composition of N20 is an important tool that can be used to determine the relative contribution of production, consumption, and diffusion to N20 emissions and test some of the ideas discussed above. The advantage of using stable isotopes is that they have the potential of differentiating processes in situ. The isotope effects for nitrification and denitrification are different and vary in accordance with the same factors that control the rate of production and consumption of nitrous oxide.
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