proximity of aerobic and anaerobic layers 

 in marsh sediments (Figure 53). In the 

 aerobic layer, oxidation of ammonium to 

 nitrate occurs. This is an extremely thin 

 layer in most delta marshes because the 

 rate of diffusion of oxygen into the 

 flooded soil is not fast enough to supply 

 the demand by the large microbial 

 population. The nitrate can diffuse down 

 into the anaerobic zone where it is 

 reduced to nitrous oxide and nitrogen gas 

 and lost from the marsh ecosystem. 



Nitrate can also be reduced all the 

 way to ammonium, and perhaps as much as 50 

 percent of it is reduced to this form 

 under the environmental conditions of a 

 delta salt marsh (Smith et al . 1932a). 

 Either the oxidized nitrate or the reduced 

 ammonium can be taken up by the emergent 

 grasses, but free nitrate is present in 

 only the thin aerobic layer. Undoubtedly, 

 nearly all the N absorbed by the marsh 

 plants is ammonium. The nitrification of 

 ammonium and its subsequent denitrifica- 

 tion to N2 is facilitated by the vertical 

 movement of the aerobic-anaerobic inter- 

 face as the tide rises and falls. The 

 ions do not even have to diffuse from one 



NH, 



ji. * ^s oa"^  NH:-N+N0-2-N+ NOi-Nj 



t NITRIFICATION ^^'Ji 



LAYER 



NAEROBIC ..„,.,.„„ 

 SOIL UPWARD 



LAYER DIFFUSION 



X DIFFUSION |^|_|t 



DOWNWARD 

 DIFFUSION 



Nj.N^O ■* 



O"*^*""- " MINERALIZATION 



DENITRIFICATION 

 LEACHING ..^ 



-NOa'-N 



Figure 63. A schematic outline of the 

 redox zones in a submerged soil showing 

 some of the N transformations (Copyright. 

 Reprinted from "Nitrogen in Agricultural 

 Soils," 1982, with permission of the 

 American Society of Agronomy). The 

 aerobids layer has been drawn thick for 

 clarity. In reality, it is seldom over 1-2 

 mm in flooded marshes. 



zone to another - the zones migrate to the 

 ions. 



Most of the N in the substrate is 

 organic; mineralization (the decomposition 

 of organic material and release of in- 

 organic nutrients) of this material 

 yields nearly all of the ammonium 

 available for absorption and for 

 nitrification (Patrick 1982). As much as 

 3.8 yg N/ml soil/week (inland) to 11.1 

 ug/ml/week (streamside) is mineralized 

 under optimum conditions (Brannon 1973). 

 This compares to a peak demand by S_. 

 al terni flora of about 2.1 ug/ml/week based 

 on the Tiaximimum growth rates detennined 

 by Ki rby (1971). Kirby's estimate does 

 not include root production so it is an 

 underestimate, but the indication is that 

 mineralization can provide nearly all the 

 inorganic N that the plant takes up. 

 Delaune and Patrick (1979) came to the 

 same conclusion based on average annual 

 rates. 



It is likely, for two reasons, that 

 plant uptake tracks mineralization closely 

 during the active part of the growing 

 season: (1) Nitrogen is limiting plant 

 growth so the plants would be expected to 

 take it up as it became available. (2) 

 During the active growing season, sediment 

 ammo ni urn- N remains at a very low 

 concentration of less than 1 ug/ml , 

 increasing to higher levels of 6 - 7 ug/ml 

 during October and November when the plant 

 growth demand is much reduced (Brannon 

 1973). 



Ammonium not taken up by plants is 

 likely to be lost through deni trif ication. 

 Vegetated marsh plots retained 93-94 

 percent of added labelled ammonium-N in 

 the plant and soil, whereas in soil cores 

 without plants only 56 percent of the 

 labelled N was recovered (Table 30). 

 However, denitrif ication and other gaseous 

 losses of N are reported to be low in 

 delta salt marshes, probably because 

 plants absorb ammonium before it can be 

 denitrified. Smith et al. (1982a) 

 reported that only about 50 mg N/m^/yr are 

 released as N2O, and estimated that about 

 5 g N/m^/yr is released as Nj through 

 deni trif ication. Nitrogen fixation is 

 also relatively minor. Casselman et al . 

 (1981) measured fixation rates of 15 and 

 4.5 g N/m^/yr in a streamside and an 

 inland marsh, respectively. 



75 



