Phosphorus has no volatile forms, so 

 sources and losses must occur through 

 water flow across the marsh. Studies in 

 Georgia salt marshes have shown that P 

 accuTiulates in estuarine sediments, 

 fonning an enormous reservoir of many 

 years supply (Pomeroy and Wiegert 1981). 



In aerobic soils P rapidly becomes 

 unavailable because it is tied up with 

 Fe, Ca and aluminum (Al). But under 

 anoxic conditions the ferric phosphates 

 are reduced to the more soluble ferrous 

 form, phosphate anions can exchange 

 between clay and organic anions, sulfides 

 can replace phosphate in ferric 

 phosphates, and hydrolysis of phosphate 

 compounds can occur. 



The P budget for a delta salt marsh 

 is presented in Figure 64. Extractable 

 (and presumably available) P averages 

 between 4 and 3 g/m^ in the sediment over 

 the year (Brannon 1973). Since the annual 

 demand for P by the emergent plants is 

 only about 2.6 g/m^ there does not seem to 

 be any lack of P for plant growth. About 

 2.3 g/m^ is brought in with sediments, and 

 1.7 g/m^ is lost to deep sediments. This 

 leaves a balance of 0.6 g P/m^ exported, 

 again probably as organic P. 



Sul fur 



The S cycle is interesting not 

 because S has been reported to limit 

 plant growth in marshes, but because of 

 its important role in energy transfer. 

 This is a new and still not fully 

 understood role. When oxygen and nitrate 

 are depleted in flooded soil s, sul fate can 

 act as a terminal electron acceptor and is 

 reduced to sulfide in the process. (This 

 gives the marsh its characteristic rotten 

 egg odor). 



In anoxic salt marsties sulfate is a 

 major electron acceptor. In fresh marshes 

 where the supply of sulfate is limited, 

 C is reduced to methane instead. The 

 sulfide radical is a form of stored energy 

 that can be tapped by S bacteria in the 

 presence of oxygen or other oxidants 

 (Howarth et al. 1983). 



In a northeast Atlantic coast marsh 

 the energy flow through reduced inorganic 

 S compounds was equivalent to 70 percent 



of the net belowground primary productiv- 

 ity of the dominant grasses. Apparently 

 most of the stored sulfides are reoxidized 

 annually, by oxygen diffusing into the 

 substrate from the marsh grass roots 

 (Howarth and Teal 1979), but there is a 

 possibility of soluble sulfides being 

 flushed from the marsh to become a source 

 of biological energy elsewhere. In the 

 marsh cited above, Howarth et al . (1983) 

 estimated that 2.5 to 5.3 moles of reduced 

 S/m^/yr are exported by pore water 

 exchange with adjacent creeks. This 

 amounts to about 3-7 percent of the 

 S reduced in the sediment, and as much as 

 20 - 40 percent of net aboveground pro- 

 duction. 



No one has investigated whether the 

 export of reduced S compounds is signifi- 

 cant in Mississippi delta marshes. 

 Brannon (1973) measured the total S 

 content of salt marsh sediments (Figure 

 49) and found the same kind of seasonal 

 variation reported by Howarth et al . 

 (1983). A crude estimate of the amount of 

 reduced S lost to deep sediments by marsh 

 subsidence shows it to be in the neighbor- 

 hood of 1 g (0.3 mol)/m^/yr. This is 

 about the same amount of S deposited by 

 precipitation in southeastern forests 

 (Swank et al . 1984). We have no idea of 

 the reduced S flux from the marsh. 



STORIES 



The role of severe storms on marshes 

 has received little attention, mostly 

 because their occurrence is unpredictable 

 and their immediate effects difficult to 

 document. Storms occur with remarkable 

 frequency on the delta plain. A 1.5-m 

 wind tide occurs about every 8 years. 

 (Figure 12), and smaller storms are annual 

 events. Most of the sediment is deposited 

 in the coastal marshes during these high 

 water periods or during winter storms 

 (Figure 32). 



Day et al . (1977) reported that 

 Hurricane Carmen in 1974 defoliated swamp 

 forests in its path two months earlier 

 than nornial leaf fall. A large amount of 

 organic C, N, and P was flushed from the 

 swamp to the fresh, brackish, and salt 

 marshes of the lower estuary by the 

 accompanying torrential rains. Part of 

 this material undoubtedly resulted from 



77 



