values are partly due to differences in 

 physical properties of the sediments that 

 lead to increased rates of water 

 percolation. In addition, the taller, 

 more vigorous plants are more efficient in 

 oxidizing the sediments than are the short 

 plants. This results in a complex 

 feedback system in which plants, redox 

 level, and nitrogen availability interact 

 to control marsh production (Howes et al. 

 1981, 1986). If productivity of a stand 

 of Spartina is stimulated by nutrient 

 additions, there is increased oxidation of 

 the sediments by the plants. The more 

 biomass of the plants increases, the more 

 oxidation occurs, which should 

 (theoretically) lead to more uptake of 

 nitrogen and increased productivity. A 

 substantial part of the observed increase 

 in oxygenation of sediments is caused by 

 water removal from the sediments by 

 transpiration of Spartina . As the water 

 is removed, the sediment does not decrease 

 in volume but the spaces previously 

 occupied by water become filled with air 

 (Dacey and Howes 1984). The sediment 

 still retains the majority of its pore 

 water much like a sponge which has been 

 allowed to drain. Sediment oxidation may 

 also be aided by transport of gases in gas 

 spaces inside the plant (Teal and 

 Kanwisher 1966), or by release of organic 

 oxidants such as glycolate from the roots 

 (Armstrong 1967), or by both. 



The opposite tendency (i.e., the 

 sediment becomes more reducing) is the 

 result of microbial decomposition. 

 Because of the limited ability of oxygen 

 to move through sediments by diffusion, 

 oxygen is usually absent below the top few 

 millimeters of marsh muds. The 

 decomposers below this depth depend 

 principally on the reduction of sulfate 

 for their energy. They "respire" using 

 sulfate rather than oxygen and produce 

 sulfide as a product. The redox of salt 

 marsh soils is closely correlated with 

 sulfide concentration. The balance 

 between the very high reducing power 

 resulting from microbial activity and the 

 oxidative action of higher plants 

 determines the redox state of the soil, 

 which in turn affects nutrient uptake. 

 Usually the reducing activities of the 

 micro-organisms prevail and, although the 

 plants may make the soils less reduced, an 

 oxidized state is rare and the majority of 



marsh sediments are highly reduced. 

 Spartina roots can respire anoxically, but 

 in conditions of extreme waterlogging and 

 reduction, the plants cannot compensate, 

 so production is severely reduced and 

 dieback may occur (Mendelssohn et al. 

 1981). 



Sulfide, which is responsible for the 

 low redox values of salt marsh soils, is 

 toxic to wetland plants. This has been 

 demonstrated for rice by Joshi et al. 

 (1975) and for Spartina by Mendelssohn 

 et al. (1982). In very reduced marsh 

 sediments, such as those where short 

 plants of Spartina grow, sulfides 

 undoubtedly contribute to the inhibition 

 of further growth by counteracting the 

 oxidizing activities of roots and perhaps 

 by poisoning them. 



To explain the conclusive results of 

 fertilization experiments in Great 

 Sippewissett Salt Marsh, one must 

 understand the relationships between 

 sediment redox and Spartina physiology. 

 The Sippewissett marsh experiments suggest 

 that under reducing conditions, much less 

 nitrogen can be picked up by plants than 

 is possible in oxidized soils. Thus, in 

 reduced sediments, only a greatly 

 increased concentration of dissolved 

 nitrogen (such as is provided by 

 fertilizing) allows uptake to occur at 

 rates similar to those found in oxidized 

 soils. This hypothesis is supported by 

 the findings of Linthurst (1980), who 

 showed in greenhouse experiments that 

 while the addition of nitrogen doubled the 

 biomass of Spartina , nitrogen addition 

 plus aeration of the rooting medium 

 increased biomass by a factor of 4.5. He 

 suggested that Spartina production in the 

 marsh is regulated by a combination of 

 nitrogen, salinity, pH, and aeration. 



5.1.2. Other Autotrophs 



Total production of the salt marsh 

 system is the sum of the production of the 

 higher plants and that of all the other 

 autotrophs, including the algae living on 

 surfaces, phytoplankton in the water, 

 photosynthetic sulfur bacteria, and 

 chemoautotrophic iron (and sulfur) 

 bacteria. The contributions of none of 

 these autotrophs have been accurately 

 measured and are assumed to be small 



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