Spartmo altermfloro 



1000 r 



•? 800 



§ • 



* 400 - 



Figure 17. 



al ternif lora 



Peak biomass of Spartina 

 in experimental plots to 

 which nitrogen alone (+N) or nitrogen and 

 phosphorus (+N+P) were added at rates of 

 2.5 g N/m 2 /week and 1.5 g P/m 2 /week. 

 Controls were not fertilized. (Teal and 

 Valiela, unpubl. data, Great Sippewissett 

 Salt Marsh, MA). 



added without nitrogen has no effect. 

 Fertilization increases marsh production 

 as a whole two- to threefold and converts 

 the least productive parts of the marsh 

 almost to creek bank production levels. 

 At that point, further growth of Spartina 

 may be light-limited rather than nutrient- 

 limited. Similar results have been seen 

 in many other salt marshes (Sullivan and 

 Daiber 1974; Broome et al. 1975; Gallagher 

 1975; Chalmers 1979). 



Measurements of nitrogen reductase 

 (an enzyme involved in nitrogen uptake), 

 comparison of nutrient content of Spartina 

 from various parts of marshes from Nova 

 Scotia to Louisiana (Stewart et al. 1973; 

 Stewart and Rhodes 1978; Mann 1978; 

 Mendelssohn 1979), and experimental 

 results from nutrient enrichment studies 

 all lead to the conclusion that salt marsh 

 plants are usually nitrogen-limited in 

 most parts of natural marshes. 



Sediment redox. 



In view of the 



studies suggesting nutrient limitation, it 

 seems paradoxical to find that the amounts 

 of dissolved ammonium and phosphate in 

 interstitial waters of salt marsh 

 sediments are very high. These levels are 

 more than sufficient to provide all that 

 Spartina can take up if the roots are in 

 an oxidized environment (Valiela and Teal 

 1978; Morris 1980). Nitrate nitrogen 

 concentrations range from to 50 ug-at/1 

 and ammonia nitrogen from 10 to 500 



ug-at/1. For phosphate, the range is 5 to 

 20 ug-at/1 (Valiela and Teal 1974). The 

 concentrations of these ions in seawater 

 are usually less than 1 ug-at/1. 



Nitrogen uptake rates by Spartina in 

 an experimental, oxidized medium are 

 faster than uptake rates in the usually 

 reduced sediments in the field (Morris 

 1980). In cultivated rice, which also 

 grows in anoxic soils, nutrient uptake 

 rates depend on the oxygen concentration 

 of the soil (Ponnamperuma 1972). Spartina 

 shows very reduced uptake of dissolved 

 inorganic nitrogen when the oxygen content 

 of the growing medium is low (Morris and 

 Dacey 1984). These observations suggest 

 that redox conditions at the roots are 

 involved in limiting nutrient uptake 

 (Linthurst 1979; Howes et al. 1981). In 

 experiments in Georgia, Wiegert et al. 

 (1983) drained marsh soils with plastic 

 tile lines that carried water from the 

 soils to the creeks; Spartina production 

 was increased presumably because of 

 increased sediment oxidation. 



Stands of taller plants grow in 

 relatively more oxidized sediments while 

 short plants are found in more reduced 

 situations (Figure 18). The higher redox 



-200 -100 

 I 



Eh (mV) 



+200 



_^ 



_1_ 



+300 +400 

 I I 





Figure 18. Redox (Eh) profiles in a) 

 sediments with tall and short Spartina 

 alterniflora , and b) an area of marsh in 

 which grass was smothered and a nearby 

 area in which the grass was becoming 

 reestablished (Howes et al. 1981). 



29 



