5.3.2. Phosphorus 



Phosphorus is an essential element 

 for organisms and often limits production 

 on land and in freshwater, though rarely 

 in coastal waters. It enters marshes 

 bound to sediment particles and dissolved 

 in ground and tidal waters. Experimental 

 additions of phosphorus alone had no 

 effect on marsh production, though when 

 added to marsh plots already receiving a 

 high dose of nitrogen, phosphorus did 

 increase plant growth (Teal 1984). Two 

 generalizations can be made about the 

 relationship between phosphorus and salt 

 marshes (see Nixon 1980 for a recent 

 review): (1) marshes seem to act as 

 phosphorus sinks, accumulating phosphorus 

 in their sediments; (2) marsh sediments 

 lose some of their phosphorus both from 

 pumping by Spartina and from diffusion, 

 and may well serve as a source for 

 reactive phosphorus to the surrounding 

 waters (Nixon 1980). Nitrogen is 

 generally the factor thought to limit 

 plant production in coastal oceans areas; 

 however, in situations where phosphorus is 

 limiting to plankton production (such as 

 in an enclosed lagoon where an active iron 

 cycle may remove phosphorus from the water 

 as ferric phosphate), a neighboring marsh 

 could support productivity by supplying 

 phosphorus from marsh sediments. 



5.3.3. Sulfur Cycle 



Because of the abundant supply of 

 sulfur in seawater, sulfur is never 

 limiting to marsh organisms. This 

 abundance is exemplified by one of the 

 characteristic odors of the salt marsh: 

 dimethylsulfide, a reduced sulfur 

 compound. Hydrogen sulfide, which is 

 obvious as the rotten egg smell 

 occasionally apparent (especially on 

 disturbed marshes), also attests to 

 sulfur' s abundance. Hydrogen sulfide is 

 toxic to higher plants, and even those 

 plants that grow in wetlands (e.g. rice, 

 Spartina ) are harmed when their tolerance 

 is exceeded (Joshi et al . 1975). 



Howarth (1980) measured the annual 

 cycle of sulfate reduction in Great 

 Sippewissett Salt Marsh, and found the 

 same sort of seasonal cycle as was evident 

 in other microbial annual cycles except 

 that the maximum sulfate reduction rate 



was displaced towards the fall. There was 

 a time lag between maximum temperature and 

 maximum sulfate reduction activity. The 

 substrate for sulfate reduction is organic 

 matter from decaying or leaking roots and 

 rhizomes. Most of these die in the fall 

 which explains the increased reduction at 

 this time. 



Some of the sulfide produced is 

 fairly rapidly bound up as pyrite, a form 

 in which the sulfur is not toxic to the 

 higher plants (Howarth 1980). But the 

 ability of Spartina roots to oxidize 

 sulfide in sediments is the principal 

 mechanism by which it lives in an 

 environment that would otherwise have 

 toxic levels of hydrogen sulfide. 

 Spartina also has the ability to take up 

 dissolved sulfide and apparently oxidize 

 it enzymatical ly within the roots, another 

 possible mechanism for resisting toxicity 

 (Carlson and Forrest 1982). 



The amount of energy available to 

 organisms through sulfate reduction is 

 very much less than is available through 

 oxidation of the same organic compound 

 with oxygen. For example, the oxidation 

 of glucose in the presence of oxygen 

 provides a little over 39 kilojoules per 

 gram (kJ/g) of glucose carbon; oxidation 

 via the sulfate reduction cycle provides 

 only about 8 kJ/g. The 31 kJ/g difference 

 does not, of course, disappear. Since the 

 organic matter is oxidized all the way to 

 carbon dioxide and water, there is no 

 energy left in organic matter. The 

 "missing" energy, as one would suspect, is 

 locked up in the sulfide. This sulfide 

 diffuses to the oxidizing layers in the 

 marsh sediments where it is then 

 reoxidized (either chemically or by 

 sulf ide-oxidizing organisms) to produce 

 sulfate. The energy produced by this 

 reaction is over 30 kJ/gC, the difference 

 between what was available to the sulfate 

 reducers and what would have been 

 available had the organic matter been 

 oxidized by an aerobic organism. The 

 oxidation of sulfide may be incomplete and 

 produce thiosulfate or other intermediate 

 products. Correspondingly, less energy is 

 yielded at each step in the process, but 

 the sum of energy from all the steps will 

 remain about the same. 



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