25SO4~ -

f 24CH2O + 50H+ [12]

formation of NO2 (Table 9.8). The O2 is the terminal electron acceptor and reacts with electrons and H+ to form water [13]. Transport of the electrons and H+ down the respiratory chain for final reaction with O2 to form water generates the proton motive force used to synthesize ATP. The H+ in excess of that used in reacting with O2 and electrons to form water is released to the environment, causing acidification [14]. Reverse transport is used to transfer e from NH + to NADH as in S0 oxidation. Large amounts of NH+ must be oxidized, and large amounts of H+ are released into the environment during N oxidation from NH+ to NO3 (nitrification) to reduce CO2 for CH2O production [15]. In contrast, H+ is not released during oxidation of NO2 to NO3 (Table 9.9, Eq. [16]). Consequently one should not conclude that all chemolithotrophic oxidation reactions release H+ into the soil environment.

As with S oxidation, it is instructive to estimate how much N must be oxidized to provide energy to synthesize microbial biomass of nitrifiers. To this end the quantities of N that would be oxidized for combined energy and C reduction by Nitrosomonas spp. are estimated in Table 9.10 and for Nitrobacter spp. in Table 9.11. From Eq. [14] there could be 270.7/75 = 3.6 mol ATP moE1 of NH+ oxidized (ATP/2e~ = 1.2). The net estimate is that 12 mol of H+ are released, and 4 mol of C are reduced for every 6 mol of NH+ oxidized to NO2 (Eq. [18]). This NO2 rarely accumulates in soil; it is further oxidized to NO3 (Eq. [19]). Less energy is obtained by oxidizing a mole of NO2 (Eq. [16]) than is obtained by oxidizing a mole of NH+ (Eq. [14]). This partly reflects the smaller number of

TABLE 9.8 NH4 Oxidation both for Energy and for Reduction of CO2in Autotrophic Chemolithotrophs such as Nitrosomonas spp.


NH+ -

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