N release

Potentially mineralizable C and N About 1-5% of organic C and total N Quantities of organic C and N released by indigenous soil microflora during a laboratory incubation. Values are the result of an integration of physical, chemical and microbiological properties of the soil. Indicator of the N fertility of soils and their ability to supply N to crops. 

Calculation of N released from exposure to PGDN at the 8-h AEGL concentrations ... 

Nitrification. Nitrification, the oxidation of NH4+ to N03 , is mediated by bacteria and fungi in both the terrestrial and aquatic portions of watersheds. It is an important process in controlling the form of N released to surface waters by watersheds, as well as in controlling the acid-base status of surface waters (Figure 1). Nitrification is a strongly acidifying process, producing 2 moles of H for each mole of N (NH4+) nitrified. 

About 16% of the total N released by CHC13 after twenty-four hours and extracted by K2S04 is in either ammonium-N or a-amino N. These forms react with ninhydrin, giving a blue-purple color, and measurement of ninhy-drin-N can be used to estimate the amount of biomass C (Amato and Ladd 1988 Joergensen and Brookes 1990). 

Diamonds In bulk collections of large numbers of diamonds, 14N/15N near 400 is typical, confirming their presolar nature by the difference from the terrestrial ratio 272 (but see comments re. Jovian value, above and below). This isotopic ratio (as well as that of C) changes during stepwise combustion of the diamond collection, being much nearer to terrestrial 14N/15N = 272 in the earliest N released. The explanation is not known. 

Mass Fraction of N Released from Acridine Char as a... 

Ammonification is another process that can result in N release. The simplest definition of the process is the release ofNH4 from organic matter (e.g., Herbert, 1999). It can occur by a number of different processes including remineralization by bacteria in the water column and sediments. Photochemical ammonification occurs abiotically when NH4+ is released from organic matter as a result of exposure to UV radiation (reviewed in Bronk, 2002 and Chapter 10 by Gryz-bowski and Tranvik, this volume). Ammonium efflux from cells has also been observed following urea uptake in a number of culture experiments (e.g., Price and Harrison, 1988 Rees and Bekheet, 1982 Uchida, 1976). The release may be due to passive diffusion through the cell membrane and is likely unavoidable because NH3 is lipid soluble. 

Estimates of N2 fixation rates in the global ocean continue to rise as results emerge from studies with the main N2 fixer in the ocean Trichodesmium, the heterocystous endosymbiont Richelia, as well as more recently discovered N2 fixers including unicellular diazotrophic cyanobacteria and bacterioplankton (Capone et al, 1997 HanseU and Feely 2000 Karl et al, 1997 Lipschultz and Owens, 1996 Montoya et al., 2004 Zehr et al, 1998 and 2001). Trichodesmium is involved in N release directly, through release of amino acids, DON, and NH4 (reviewed in Table 8.2). Trichodesmium is also a source of NH4+ and DON as a result of remineralization by associated bacteria, sloppy feeding and excretion by grazers (SeUner, 1992 Sheridan et al, 2002). 

Two other N2 fixers that have been studied are Aphanizomenon and Nodularia. In a study in the Baltic, 7.7 2.1% and 6.7 2.1% of the N2 gas recently fixed by Aphanizomenon and Nodularia respectively appeared in the picoplankton fraction, which indicates N2 fixation and subsequent N release and reincorporation. 

Organism Location Date/ Culture Rate Measured N release rate pmol N colony h N release N2 fixation (%) Reference... 

Trophic cascade effects (top-down control of food webs by predators leading to alternating high/low abundance of each subsequent lower trophic level) can alter the amount and type of N released and regenerated. Copepods can control the... 

A survey of rates of N release from sediments in ocean, coastal and estuarine environments is presented in Table 8.5 we note that this is not an exhaustive... 

Table 8.5 Rates of nitrogen (N) release from sediment. Rates were quantified by measuring concentration changes (CC) in either sediment cores (SC) or benthic chambers (BC)... 

Location Date N release rate mmol N day Chemical species Method Reference ... 

Whether the sediment is oxic or anoxic is also an important determinant of sedimentary N fluxes as well as the forms of N released. In sediments with significant organic matter deposition, oxygen does not generally penetrate far into the sediments such that reduced compounds like NH4 or N2 tend to dominate remineralization (e.g., Pedersen et ah, 1999). Similarly, efflux of NOs is also heavily dependent on oxygen penetration. 

In addition to the microbial community present in aU sediments other organisms can substantially alter the nature of N release from sediment, particularly macroalgae, microphytobenthos, and benthic fauna. 

Different species of macroalgae would be expected to have different N release rates. For example, some macroalgae are known to have large intracellular inorganic N storage pools (e.g., McGlathery et ah, 1996). Release of NH4", as measured with isotope dilution, was observed in Ulva fenestrata (0.08-11.27 pmol N g h ) and Gracilaria pacifica (0.12-0.77 pmol N g h ) when NH4+ was added to the medium. However, no net NH4+ release, over uptake, was observed. Small pulses of free amino acids were released and then rapidly reincorporated (Naldi and Wheeler, 2002), but no protein release was detectable. 

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