areas with high salinity and short Spartina can be found. However, we 

 have found short Spartina growing where the salinity was found to be only 

 about 10 parts per thousand at several different times during a growing 

 season. If the stunted form is produced by environmental factors, then 

 the factor or interaction of factors may vary from one location to 

 another. That is -- at a particular location, high salinity may limit 

 growth, while at another an unfavorable water regime or a shortage of 

 nitrogen or phosphorus or both might be limiting growth. 



An explanation for nitrogen deficiency in the short height zone may be 

 in the development of a thick mat of roots which creates a sod-bound 

 condition. When sediments are deposited and later colonized by Spartina, 

 most substrates contain adequate nitrogen for plant growth except where 

 it is mostly sand, such as the Drum Inlet site. As the fibrous mat of 

 roots develops over the years, all the available nitrogen is absorbed and 

 either exported in the shoot growth, carried over in living root tissue 

 or bound up in dead root tissue. The dead root material is decomposed 

 and mineralized slowly (evidenced by accumulation of organic matter) due 

 to the anaerobic condition of the marsh sediments. Nutrients added from 

 natural sources apparently are not adequate for maximum plant growth. 

 The addition of nitrogen to the marsh probably includes small amounts 

 from rainfall, asymbiotic nitrogen fixation, directly from flooding tidal 

 waters, deposition of feces from filter feeders in the marsh and deposi- 

 tion of inorganic and organic sediments. 



The amount of sediments deposited is probably the chief difference 

 between the nutrients available to the tall and short forms of Spartina. 

 Sediments are deposited regularly along creek banks providing a fresh 

 medium for plant roots to exploit. Unexploited sediments are also 

 exposed by meandering of creeks. The amount of nitrogen supplied would 

 depend on the nature of the sediments. 



It is more certain that the sediment is the dominant factor in the 

 supply of phosphorus to S. alterniflora (Pomeroy, et al . , 1969). This 

 is borne out in the results of our fertilizer experiments which showed a 

 response to phosphorus on sandy substrate but not on finer -textured 

 material. The texture of the sediments is quite important in the phos- 

 phorus-supplying capacity. In eroded soils of humid climates, phosphates 

 are associated with hydrated oxides of iron and aluminum which occur as 

 films on clay particles. When sediments are deposited in a marsh, the 

 reducing conditions cause the solubility of iron and aluminum phosphate 

 to increase. At the high pH (hydrogen-ion concentration) of marsh soils 

 (7.0 to 8.0) calcium phosphates probably become an important form of 

 phosphorus. The amount of phosphorus available to plants in a salt marsh 

 is related to the amount of clay in the substrate. Pomeroy, et al . (1969) 

 concluded that the subsurface-reduced sediments are the source of phos- 

 phorus for Spartina; however, the response to surface-applied fertilizer- 

 phosphorus in this experiment seems to contradict this. Uptake of 

 fertilizer-phosphorus apparently occurred at or very near the sediment 

 surface, indicating that nutrients in freshly deposited sediment would 

 be readily used. 



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