Nitrate Flux

Henriksen, K., Hansen, J.I., and Blackburn, T.H. (1981) Rates of nitrification, distribution of nitrifying bacteria, and nitrate fluxes in different types of sediment from Danish waters. Mar. Biol. 61, 299-304. 

Johnstone, J. (1908) Nitrate flux into the euphoric zone near Bermuda. Nature 331, 521-523. 

Martin, A. P., and Pondhaven, P. (2003). On estimates for the vertical nitrate flux due to eddy pumping. Journal of Geophysical Research, Oceans 108, doi 10.1029/2003JC001841. 

On balance, the shelves are not a net source of N to the open ocean. Instead, the North Atlantic has major exchanges with the Arctic Ocean and with the South Atlantic. Ganachaud and Wunsch (2002) estimate southerly nitrate fluxes of 2200 (+/- 3800) and 6600 (+/- 4700) x 10 mol N year- at 7.5°N and 4.5°S, respectively. We take their mid point value of 4400 (+/— 4000) X 10 mol N year as the net transport of nitrate from the North Atlantic to the South Atlantic. A significant uncertainty lies in the net meridional transport of DON in the basinwide N transport budgets in the Atlantic. Rintoul and Wunsch (1991) speculated that the imbalance they quantified in the poleward nitrate flux across subtropical sections may be compensated by unobserved fluxes of organic nitrogen. DON measurements are stiU too sparse and too imprecise to test this hypothesis. 

The nitrate flux from the Arctic to the Atlantic was estimated by Galloway et al. (1996) as 1500 x 10 mol year, with most of the nitrate due to Pacific water... 

Tusseau-Vufllemin, M. H., Mortier, L., and Herbaut, C. (1998). Modeling nitrate fluxes in an open coastal environment (Gulf of Lions) Transport versus biogeochemical processes. J. Geophys. Res. 103, 7693-7708. 

Nozaki Y. and Yamamoto Y. (2001) Radium 228 based nitrate fluxes in the eastern Indian Ocean and the South China Sea and a silicon-induced alkalinity pump hypothesis. Global Biogeochem. Cycles 15, 555-567. 

Since all these gas emissions are byproducts of organic matter decomposition they are natural sources, but the magnitude of these fluxes may be altered by human intervention both directly, via destruction of intertidal sediments, and also indirectly, via increases in atmospheric CO2 levels (Dacey, Drake Klug, 1994) or changes in nitrate fluxes (Malcolm, this volume). Thus while intertidal sediments are very sensitive to climate change, they are not wholly passive since they can themselves influence... 

Nitrate fluxes in the major rivers of the World are correlated with human population density (Figure 4). 

Figure 4. Relationship between log of population density and log of total nitrogen export from regions of the North Atlantic basin (top, a) and of nitrate in the major world rivers (bottom, b). Both relationships are significant, but the relationship for nitrate fluxes in the World s rivers (bottom) is more significant, has lesser scatter and has a steeper slope. For TN fluxes in the North Atlantic basin (top), logTN = 2.2+0.35 og(population density) r = 0.45 p = 0.01. For nitrate fluxes in the World s rivers (bottom), logN03 = 1.15 + 0.621og (population density) r - 0.53 p — 0.00001 (Howarth, 1996).

The nitrate flux in a downward direction is calculated accordingly ... 

Under normal deep-sea oxygen eonditions, the nitrate flux is generally direeted out of the sediment due to nitrifieation and lower denitrifieation as shown by results reported by Hammond et al. 

Fig. 6.25 Plot of diffusive benthic nitrate fluxes against (a) water depth and (b) organic carbon content in surface sediments off the Rio de la Plata mouth (Argentine Basin).

SWI due to the low nitrate flux into the sediment and the high degradation rate of OM in freshwater sediments. 

Applying the flux equation presented above, a nitrate flux of 0.56 0.16 mol a is computed. Using the average biological C N ratio of 6.6, this leads to a carbon fixation rate of 3.7 + 1.0molm a . The estimate thus obtained is a local, annual-scale measure of new production. 

Nitrate flux from aerobic zones to anaerobic sites For nitrate reduction to occur in wetlands, nitrate must be present in anaerobic zones. Thus, nitrate reduction rates are regulated by transport of nitrate either by diffusion or by mass flow from aerobic zones to anaerobic portions of the soil. Similarly, the rate of nitrification and the oxygen availability in the soil regulate nitrate concentrations in aerobic zones of the soil. In wetlands with limited inputs of nitrate from external sources, nitrification and atmospheric deposition are the primary sources of nitrate. In these systems, denitrification rates are tightly coupled to nitrification rates. 

Exchange of dissolved nitrogen species between the soil and water column support several nitrogen reactions. For example, nitrification in aerobic soil layer is supported by ammonium flux from the anaerobic soil layer. Similarly, denitrification in anaerobic soil layer is supported by nitrate flux from the aerobic soil layer and water column (see Chapter 14 for discussion on transport processes). 

Nitrate flux from the aerobic portion of the soil is controlled by (1) labile organic carbon supply in anaerobic portion of the soil, (2) thickness of aerobic soil layer, (3) water column depth, (4) mixing and aeration in the water column, (5) nitrate concentration, and (6) temperature. The flux of nitrate from the floodwater to underlying soil increases with an increase in temperature (Figure 8.59). At low temperatures, nitrate can diffuse to deeper layers into anaerobic zones. Under these conditions, it is likely that nitrate may play significant role in ANAMOX reactions as temperature optima for this reaction is between 10 and 15°C, as compared to denitrification that has temperature optima around 30°C (see Figures 8.39 and 8.47). 

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