Nitrates ammonification

Seitz H-J, H Cypionka (1986) Chemolithotrophic growth of Desulfovibrio desulfuricans with hydrogen coupled to ammonification with nitrate or nitrite. Arch Microbiol 146 63-67. 

Burger M, Jackson LE (2003) Microbial immobilization of ammonium and nitrate in relation to ammonification and nitrification rates in organic and conventional cropping systems. Soil Biol Biochem 35 29-36... 

Autotrophic activity. Because of the low C N ratio and its declining value as carbonaceous residues are degraded there is substantial ammonification. With all mean treatment times greater than the doubling time of Nitrobacter sp. nitrification will occur provided that oxygen is not limiting. Smith and Evans (19) found that with DO levels above 15% of saturation, nitrification continued until the culture was limited by a fall in pH level. Up to 40% of the slurry ammonia was oxidised. The autotrophic activity never achieved steady state and cycled between periods of activity when the pH value was above about 5.5 and periods of inactivity when the pH value fell below 5.5. Complete nitrification of all ammonia only occurred if the pH value was controlled at about 7 by the addition of alkali. When the DO level was held within the range of 1 to 15% of saturation a system of simultaneous nitrification and denitrification was established. The reduction of nitrate allowed the pH value to remain above 6 and nitrification to continue. Thus more than 70% of the ammonia was oxidised. If the DO level was held below 0.1% of saturation, nitrification was inhibited (unpublished). 

The involvement of HA during bacterial conversion of nitrate to NH3 (known also as the nitrate ammonification phase of the nitrogen cycle) has been studied at the molecular level as part of an effort to delineate the mechanism of conversion of nitrite to NH3 by a group of multiheme cytochromes of bacterial origin. The overall reduction reaction is depicted in equation 3 for cytochrome c-nitrite reductase " ... 

Nitrate and Ammonium. The transformations of nitrogen species may occur under suitable microbial catalysis (5, 36). Nitrate reduction may result in formation of either elemental nitrogen or ammonium. Mass balances over a whole lake have indicated the importance of the denitrification process for the elimination of nitrogen from lakes (37). The conditions for the dis-similative ammonification of nitrate are poorly known (36). Ammonium is also released by the mineralization of biomass. 

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. 

Jprgensen, K.S. (1989) Annual pattern of denitrification and nitrate ammonification in estuarine sediment. Appl. Environ. Microbiol. 55, 1841-1847. 

Patrick, O., Slawayk, G., Garcia, N., and Bonin, P. (1996) Evidence of denitrification and nitrate ammonification in the river Rhone plume (northwest Mediterranean Sea). Mar. Ecol. Prog. Ser. 141, 275-281. 

Rysgaard, S., RisgaardPetersen, N., and Sloth, N. P. (1996). Nitrification, denitrification, and nitrate ammonification in sediments of two coastal lagoons in Southern France. Hydrobiologia 329, 133-141. 

Figure 21.1 Microbial nitrogen cycling processes in sedimentary environments on a coral reef (A) nitrogen fixation (B) ammonification (C) nitrification (D) dissimilatory nitrate reduction and denitrification (E) assimilatory nitrite/nitrate reduction (F) ammonium immobilization and assimilation. Adapted from D Elia and Wiebe (1990). Anammox (the anaerobic oxidation of NH4" with NO2 yielding N2 ) is not represented, as it has not yet been shown to occur on coral reefs, but may be found to be important in reef sediments.

Rysgaard, S., Risgaard-Petersen, N., and Sloth, N. P. (1996). Nitrification, denitrification, and nitrate ammonification in sediments of two coastal lagoons in Southern France. Hydrobiol. 329, 133—141. Sand-Jensen, K. (1977). Effects of epiphytes on eelgrass photosynthesis. Aquat. Bot. 3, 55—63. Sand-Jensen, K., and Borum, J. (1991). Interactions among phytoplankton, periphyton, and macrophytes in temperate freshwaters and estuaries. Aquat. Bot. 41, 137—175. 

Nitrate reduction studies have focused overwhelmingly on denitrification at the expense of other NO sinks such as dissimulatory NO reduction to NH4 (also known as DNRA or nitrate ammonification). The ecological implications of reducing NOJ to NH4, versus N2 are vastly different because NH4 is more readily retained in the ecosystem, and it is a form that is readily assimilated by biota. Thus, DNRA contributes to eutrophication by reducing the quantity of fixed nitrogen that is returned to the atmosphere as N2. 

Oxidation of ammonia to nitrite, N02, and nitrate, N03, is called nitrification the reverse reaction is ammonification. Reduction from nitrite to nitrogen is called denitrification. All these reactions, and more, occur in enzyme systems, many of which include transition metals. A molybdenum enzyme, nitrate reductase, reduces nitrate to nitrite. Further reduction to ammonia seems to proceed by 2-electron steps, through an uncertain intermediate with a -fl oxidation state (possibly hyponitrite, N202 ) and hydroxylamine ... 

Interestingly, nitrification is a process that actually generates acidity, equivalent to two H+ ions for every ion of NO3 produced by the oxidation of NH4+. However, the ammonification of organic nitrogen to ammonium consumes one H+, as does the uptake of nitrate from soil by plant roots. Therefore, nitrification only acidifies soils if the ammonium substrate is added directly, for example through fertilization or by atmospheric deposition, or if the nitrate is not taken up by plants and leaches from the soil. 

Ammonification is the process by which the organically bound nitrogen of microbial, plant, and animal biomass is recycled after their death. Ammonification is carried out by a diverse array of microorganisms that perform ecological decay services, and its product is ammonia or ammonium ion. Ammonium is a suitable source of nutrition for many species of plants, especially those living in acidic soils. However, most plants cannot utilize ammonium effectively, and they require nitrate as their essential source of nitrogen nutrition. 

The reverse of nitrification, nitrate reduction (ammonification of NO ), can also occur. At pe < 6 (Figure 8.4) organic material can reduce NOf and N02 by bacterial mediation to NH. This is in competition with denitrification. 

Two types of dissimilatory nitrate reductases, localized in either the cytoplasmic membrane or the periplasm, are widespread among bacteriaT" The cytoplasmic membrane-bound nitrate reductase (Nar) catalyzes the first steps of denitrification (NOs N02 NO N2O N2) and nitrate ammonification (NOs"N02 NH3). These reactions are the terminal electron-transfer reactions during anaerobic respiration, the enzymes being part of a redox loop generating a proton-motive force capable of driving ATP synthesis.Periplasmic nitrate reductase (Nap) participates in cellular redox processes, aerobic denitrification, and nitrate scavenging. ... 

FIGURE 8.5 Oxidation and reduction reactions of nitrogen in wetlands. Numbers 1-7 refer to pathways of nitrogen reactions. 1 = ammonification 2 = immobilization 3 = nitrification 4 = denitrification 5 = dissimilatory nitrate reduction to ammonia 6 = dinitrogen fixation and 7 = ammonia volatilization. 

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