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Dry agriculture 



Moist 



agriculture 



Estuar i ne 



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Coastal 



Open ocean 



Figure 4-7. A compaiison of primaiy productivity for different kinds of ecosystems (adapted from Teal and Teal 

 1969). 



often obtained, however, since critical materials limit 

 the maximum growth rate of the plants. The most 

 common limiting materials are carbon dioxide, which 

 enters the leaves directly from the air, and inorganic 

 nitrogen and phosphorus which are taken up through 

 the roots. Considerable research (VaUela and Tea! 

 1972, Broome et al. 1975) shows that in the salt 

 marsh habitat nitrogen is most often limiting (table 

 4.3). However, in freshwater lakes and streams, 

 phosphorus is usually the limiting nutrient and this 

 may also be true in the fresh and intermediate marsh 

 habitats. 



Recent evidence indicates that inorganic sedi- 

 ments (primarily clays and fme silts) are the major 

 "new" source of nutrients in Louisiana marshes. 

 DeLaune et al. (1977), for example, showed a strong 

 linear relationship between soil density, which is 

 proportional to the inorganic sediment content in the 

 soil, and biomass of smooth cordgrass (fig. 4-8). 



Since nitrogen is most often the limiting nutrient 

 in coastal marshes, it is important to understand the 

 normal sources and losses of this element. The dia- 

 gram in figure 4-9 illustrates the marsh nitrogen cycle. 

 The major source of nitrogen to plants is made avail- 

 able from stored nitrogen in the soil. It comes from 

 inorganic sediments which are carried into wetlands 

 by flooding waters, and from nitrogen dissolved in 

 the water column. Most of this nitrogen is incor- 

 porated as an insoluble organic form in the sediments 

 and becomes available to plants only after it is mi- 

 crobially transformed into ammonia [minerahzation 

 (3) on fig. 4-9] . Although the vegetation (4) absorbs 

 significant amounts (5) for growth, the largest users 

 of ammonia seem to be microscopic organisms (7), 

 which transform plant detritus into a high protein 

 bacterial biomass. The vast pool of atmospheric nitro- 

 gen gas (8) is not available to marsh grasses; however, 

 certain microscopic organisms change it into a usable 

 organic form [nitrogen fixation (9)] . This is a source 

 of "new" nitrogen for the marsh system, but it is 

 normally small in relation to available sediment ni- 

 trogen. In addition, other microorganisms in the 



£ 1,5Q0 



y. -3686+ 7163x 

 r =0 736" 



Soil density (g/cm^) 



Figure 4-8. Relationship between soil density and 

 growth of smooth cordgrass. 



Table 4.3 Above ground yield of smooth cordgrass 

 in September 1973 in a streamside loca- 

 tion in Barataria Bay, Louisiana as 

 affected by applications of nitrogen and 

 phosphorus (Patrick and DeLaune 1976). 



Treatment 



Mean dry-weight 

 yield (kg/ha) 



Nitrogen (200 kg/ha) 

 Phosphorus (200 kg/ha) 

 Control (no N or P) 



19,160 

 16,560 

 16,660 



alternately oxidized-reduced environment of the sedi- 

 ment surface change ammonia to atmospheric nitro- 

 gen [denitrification (10)], where it is lost from the 

 marsh system. Normally, this process also produces 

 small amounts of nitrogen in comparison to the total 

 nitrogen cycled. However, when the marsh is enriched 

 with culturally derived nitrogenous wastes, denitri- 

 fication becomes an important mechanism to reduce 



161 



