N

FIGURE 13.7 The organization of denitrification enzymes in the cell membrane for gramnegative bacteria (adapted from Ye et al., 1994).

Hours

FIGURE 13.8 The sequence of products formed during denitrification (adapted from Cooper and Smith, 1963).

Each step is enacted by individual enzymes: nitrate reductase (nar), nitrite reductase (nir), nitric oxide reductase (nor), and nitrous oxide reductase (nos). Each is inhibited by O2, and the organization of these enzymes in the cell membrane for gram-negative bacteria is described in Fig. 13.7. At any step in this process intermediate products can be exchanged with the soil environment, making denitrifiers a significant source of NO, in soil solution and important sources of the atmospheric gases NO and N2O.

Each denitrification enzyme is inducible, primarily in response to the partial pressure of O2 and substrate (C) availability. Because enzyme induction is sequential and substrate dependent, there is usually a lag between the production of an intermediate substrate and its consumption by the next enzyme. In pure culture, these lags can be on the order of hours (Fig. 13.8); in the field lags can be substantially longer, and differences in lags among different microbial taxa may significantly affect the contribution of denitrifiers to fluxes of NO and N2O to the atmosphere. That induced enzymes degrade at different rates, and more slowly than they are induced, also leads to a complex response to the environmental conditions that induce denitrification; whether a soil has denitrified recently (whether denitrifying enzymes are present) may largely determine its response to newly favorable conditions for denitrification. Rainfall onto soil that is moist, for example, will likely lead to a faster and perhaps stronger denitrification response than will rainfall onto the same soil when it is dry (Groffman and Tiedje, 1988; Bergsma et al. 2002).

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