Denitrification occurrence

There has been a long controversy in the literature concerning the lowest O2 concentrations where denitrification can occur (Robertson and Kuenen, 1984). Some believe it can occur at aerobic-anaerobic interfaces (Christensen and Tiedje, 1988 Bonin et al., 1989), while others believe that low O2 conditions inhibit synthesis of the essential enzymes for denitrification (Payne, 1976 Kapralek et al., 1982). It has also been speculated for some time that anoxic microsites may allow for the occurrence of denitrification in aerobic environments (Jannasch, 1960). Laboratory studies indicate that there are differential effects of O2 on the different steps of denitrification, whereby the NO3- reduction step is less sensitive than N( >7 or N2O reduction (Bonin and Raymond, 1990). 

Bonin, P., Omnes, P., and Chalamet, A. (1998) Simultaneous occurrence of denitrification and nitrate ammonification in sediments of the French Mediterranean coast. Hydrobiologia 389, 169-182. 

Nearly all heterotrophic denitrifiers are facultative anaerobes (Tiedje, 1988). Although various workers defined suboxia differently, the upper limit of oxygen concentration for the occurrence of denitrification is probably 5 pM, with quantitatively most denitrification in the marine environment taking place below about 2 pM (Cline and Richards, 1972 Codispoti et al., 2005 Devol, 1978 Murray et al., 1995). The ability to denitrify is not limited to the bacteria, archea and even some fungi are capable of denitrification (Knowles, 1996 Zumft and Korner, 1997). 

The authors show that the observed excess of molecular nitrogen consists of at least two components similar in magnitude (1) AN2 due to excess air formation, and (2) excess N2 as the product of nitrate reduction. The amount of N2 attributed to denitrification, i.e., excess N2 after the correction for atmospheric N2 (equilibrium plus excess air), depends on the model of excess air formation. The occurrence of excess N2 in anoxic meteoric waters does not necessarily indicate denitrification and may be related to excess air. AN2 is only an indicator for denitrification, if excess air and possible fractionation processes are accounted for. 

First evidences indicating toluene consumption by denitrification and its conversion to CO2 are those obtained by Zeyer et al. (1986). Similar results have been reported by several authors (Alvarez Vogel, 1995 Chakraborty et aL, 2005 Elmen et al., 1997 Kobayashi et al., 2000). However, in these works nitrate consumption and N2 production have not been mentioned, thus, the occurrence of denitrification process has not been verified. Evans et al. (1991, 1992) have rep>orted toluene consumption and production of CO2, biomass and intermediaries as benzilsuccinate with a concomitant accumulation of nitrite. Toluene... 

A major control for the occurrence of either reaction is the oxygen availability or redox condition in soils. Nitrogen reduction increases with increasing oxygen limitation. Thus, with decreasing aeration, denitrification shifts more and more towards N2 as the final product. Globally, denitrification returns 54-115 Tg N to the atmosphere as NON2O-I-N2. 

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