Internal N cycle

There are also a number of enzymes that degrade organic N compounds, inside or outside the cell, thereby making N available for assimilation or uptake, respectively. Examples we will consider include urease, amino acid oxidases, and extracellular peptidases (MulhoUand et al., 2002 MulhoUand and Lee, in revision. Chapter 7 by MulhoUand and Lomas, Fig. 2, this volume Table 32.1). In addition, for heterotro-phic organisms, digestive enzymes, especially proteases, are important for internal N cycling, recouping cellular N, and excretion. 

Elevated atmospheric N deposition can modify ecosystem N cycling by increasing ratios of N inputs to internal N cycling and by changing the amounts and forms of N made available to plants and microbes. In addition to these direct effects, N deposition can alter feedbacks between plants and soils. For example, increases in leaf or fine root N concentration occurring... 

In a sense similar to DON, N trace gases that are produced and lost In the course of nitrification could be regarded as leaks of potentially available N (Firestone and Davidson, 1989), whereas losses via denitrification could be more analogous to leaching of nitrate. The nitrification process is internal to the N cycle of many eco.systems, while denitrification utilizes a pool of N that can accumulate when N is available in excess (Vitousek ct al, 1998). 

Figure 3 Schematic diagram of the processes and pools central to the internal cycling of N in the ocean. The isotope effects shown here are based on laboratory studies. Dashed arrows represent assimilation of dissolved species into particulate matter, and solid arrows represent remineralization. Complete consumption of the ammonium pool by assimilation in the surface ocean or by nitrification in the ocean interior causes the relatively high isotope effects associated with these processes to have little effect on N isotope dynamics. However, in regions where ammonium assimilation and nitrification co-occur, their isotope effects will impact the <5 N of their respective products, PN and nitrate. In nitrification, ammonia (NHs), rather than the protonated form ammonium (NH4 ), is oxidized. However, ammonium is the dominant species in seawater, and there is isotope discrimination in the ammonium-ammonia interconversion. Thus, the isotope effects for ammonia oxidation given here and elsewhere in the text refer specifically to consumption of ammonium. The processes surrounding DON production and utilization are not well understood from an isotopic perspective but are thought to play an important role in N cycling.

Figure 5 The effect of different marine N cycle processes on nitrate <5 N and concentration, assuming an initial nitrate <5 N of 5%o. The trajectories are for reasonable estimates of the isotope effects, and they depend on the initial nitrate <5 N as well as the relative amplitude of the changes in nitrate concentration (30% for each process in this figure). A solid arrow denotes a process that adds or removes fixed N from the ocean, while a dashed line denotes a component of the internal cycling of oceanic fixed N. The effects of these two types of processes can be distinguished in many cases by their effect on the concentration ratio of nitrate to phosphate in seawater. The actual impact of the different processes on the N isotopes varies with environment. For instance, if phytoplankton completely consume the available nitrate in a given environment, the isotope effect of nitrate uptake plays no major role in the <5 N of the various N pools and fluxes the effect of nitrate generation by organic matter degradation and nitrification, not shown here, will depend on this dynamic. Similarly, the lack of a large isotope effect for sedimentary denitrification is due to the fact that nitrate consumption by this process can approach completion within sedimentary pore waters.

N. R. Johanson and J. W. Muehlhauser, "MHD Bottoming Cycle Operations and Test Results at the CFFF," paper presented at Second International Workshop on Fossil Fuel Fired MHD, Bologna, Italy, 1989. 

Table 8.9 State cycle length and the number of state cycles for random boolean nets of size N and connectivity k a = pn — 1/2, where Pk is the mean internal homogeneity of all Boolean functions on K inputs (see text).

Correa, A., Marion, N., Fensterbank, L., Malacria, M., Nolan, S.P. and Cavallo, L. (2008) Golden Carousel in Catalysis The Cationic Gold/Propargylic Ester Cycle. Angewandte Chemie International Edition, 47, 718-721. 

Jungbluth, N., Bauer, C., Dones, R., Frischknecht, R. (2005) Life Cycle Assessment for Emerging Technologies Case Studies for Photovoltaic and Wind Power. International Journal of Life Cycle Assessment, 10(1), 24-34. 

Jungbluth N, Tietje O, Scholz RW. Food Purchases Impacts from the Consumers Point of View Investigated with a Modular LCA. International Journal of Life Cycle Assessment. 2000 5(3) 134-142. DOI 10.1007/BF02994077... 

Cu(0) species. Alternatively, the Cu(n) species may first undergo oxidation by an external oxidant (or internal redox process) to a Cu(m) intermediate, and then undergo reductive elimination to provide the product and a Cu(i) species. Re-oxidation to Cu(n) would then, in theory, complete the catalytic cycle, but in practice, most reactions of this type have been performed with stoichiometric amounts of the copper reagent. 

HUher, W., Murphy, R.J., Dickinson, D.J. and Bell, J.N.B. (1996). Life-cycle assessment of treated wood a view from the road. International Research Group on Wood Preservation, Doc. No. IRG/ WP 96-50078. 

---