River nitrification

Nitrification as a deoxygenation process is normally significant only in river systems receiving pollutional inputs of ammonia. The kinetics of nitrification are less clear than those for carbonaceous deoxygenation (33). 

Oxygen is consumed by micro-organisms as they utilise the substrate and also as a result of nitrification, but oxygen is gained by the river by processes of surface aeration, so that... 

The nitrogen supplies on land consist of the assimilable nitrogen in the soil VS2 0.19-104tkm-2, in plants (12 1091), and living organisms (0.2 1091). A diversity of nitrogen fluxes is formed here of the processes of nitrification, denitrification, ammonification, fixation, and river run-off. The intensities of these fluxes depend on climatic conditions, temperature regime, moisture, as well as the chemical and physical properties of soil. Many qualitative and quantitative characteristics of these dependences have been described in the literature (Hellebrandt et al., 2003). Let us consider some of them. 

In some cases, the effects of complex environmental mixtures could be accounted for in terms of concentration-additive effects of a few chemicals. In sediments of the German river Spittelwasser, which were contaminated by chemical industries in its vicinity, around 10 chemicals of a cocktail of several hundred compounds were found to explain the toxicity of the complex mixture to different aquatic organisms (Brack et al. 1999). The complex mixture of chemicals contained in motorway runoff proved toxic to a crustacean species (Gammarus pulex). Boxall and Maltby (1997) identified 3 polycyclic aromatic hydrocarbons (PAHs) as the cause of this toxicity. Subsequent laboratory experiments with reconstituted mixtures revealed that the toxicity of motorway runoff could indeed be traced to the combined concentration-additive effects of the 3 PAHs. Svenson et al. (2000) identified 4 fatty acids and 2 monoterpenes to be responsible for the inhibitory effects on the nitrification activity of the bacteria Nitrobacter in wastewater from a plant for drying wood-derived fuel. The toxicity of the synthetic mixture composed of 6 dominant toxicants agreed well with the toxicity of the original sample. 

Land use changes, such as conversion of tropical forests to cattle pastures, affect biological N fixation, mineralization, nitrification, and denitrification (Davidson et al. 1993, Keller and Reiners 1994, Matson et al. 1987, Montagnini and Buschbacher 1989, Neill et al. 1995). Matson et al. (1987) estimated that N mobilized annually from deforestation was equivalent to more than half of the industrial N fixed globally and greater than the total amount of N delivered to oceans by rivers. 

Lipschultz et al. (1985) documented the light inhibition of NH3 oxidation in the Delaware River and concluded that this effect influenced the spatial distribution of nitrification in the estuary. Depending on their depth, light is not usually a problem for nitrification in sediments. In shallow sediments, light may have an indirect positive effect on nitrification rates by increasing photosynthesis, and thus increasing oxygen supply to the sediments (Lorenzen et al., 1998). 

Bianchi, M., Bonn, P., and Fehatra (1994). Bacterial nitrification and denitrification rates in the Rhone River plume (northwestern Mediterranean Sea). Marine Ecology-Progress Series 103, 197-202. 

Bianchi, M., Fehatra, and Dominique, L. (1999). Regulation of nitrification in the land-ocean contact area of the Rhone River plume (NW Mediterranean). Aquatic Microbial Ecology 18, 301—312. 

Magalhaes, C. M., Joye, S. B., Moreira, R. M., Weibe, W. J., and Bordalo, A. A. (2005). Effect of salinity and inorganic nitrogen concentrations on nitrification and denitrification rates in intertidal sediments and rocky biofifins of the Douro River estuary, Portugal. Water Research 9, 1783—1794. 

Pakulski, J. D., Benner, R., Amon, R., Eadie, B., and Whidedge, T. (1995). Community metabolism and nutrient cychng in the Mississippi river plume — Evidence for intense nitrification at intermediate sahnities. Marine Ecology-Progress Series 117, 207—218. 

Because nitrification occurs only under aerobic (or microaerobic) conditions and denitrification under anaerobic conditions, the two processes are spatially separated. However, if the sites where these processes occur are sufficiently close together, NO transport and consumption are very rapid and the overall process is considered to be coupled nitrification-denitrification. On the basis of a literature review, Seitzinger (1988) concluded that nitrification is generally the major source of NOj" for denitrification in river, lake, and coastal sediments. The same is likely to be true of non agricultural soils that are largely dependent on mineralization and atmospheric deposition for fixed nitrogen. 

Authors calculate that a total of 143 x lO molN/yr. is removed via coupled nitrification/denitrification on the North Atlantic continental shelf. This estimate is expected to underestimate total sediment denitrification because it does not include direct denitrification of nitrate from the overlaying water. The rate of coupled nitrification/denitrification calculated is greater than the N inputs from atmospheric deposition and river discharge combined (Figure 22). 

There are strong nitrification and denitrification in Zhujiang River Estuary sediments and the average nitrification, denitrification, and nitrate reduction rates ranged from 0.32 to 2.43 imnol/(m h), 0.03 to 0.84 mmol/(m h), and 4.17 to 13.06 mmol/(m -h), respectively. The vertical profiles of the sediments showed that the nitrification and denitrification processes mainly took place in the depth from 0 to 4 cm and there were differences at different sampling sites. The rates of nitrification, denitrification, and nitrate reduction were dominated by Eh, nitrate, and ammonium concentrations in sediment and DO in overlying water (Xu et ah, 2005). 

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