of marsh sediment and available to 

 Spartina roots. Another minor component 

 of the total nitrogen budget was the loss 

 of ammonia to the air from the marsh 

 surface. 



Nitrogen is fixed from the atmosphere 

 by nitrogen-fixing bacteria associated 

 with the roots of the grasses and by algae 

 growing on the surface of the marsh. 

 Rates of nitrogen fixation between 

 different parts of the marsh vary as much 

 as between different marshes (Valiela 

 1982). About 10% of the nitrogen input 

 may result from nitrogen fixation 

 primarily by the bacteria associated with 



roots of Spartina (Teal et al. 1979). 

 While this is a relatively small 

 percentage, it is very important to the 

 plant because it occurs just at the site 

 of uptake and, therefore, is most readily 

 available for the plant's use. 

 Denitrification is the microbial process 

 that returns nitrogen to the air as 

 nitrogen gas. There are several smaller 

 biological components of the budget: 

 organic nitrogen is transported out of the 

 marsh by shellfish harvesting by humans or 

 by fish swimming out of the marsh; organic 

 nitrogen is transported into the marsh by 

 feces deposition by birds, such as gulls, 

 that have fed outside the marsh but come 

 there to rest. 





MONTH 



Figure 22. Net exchanges of inorganic 

 nitrogen between Great Sippewissett Salt 

 Marsh and Buzzards Bay (top) and the bay 

 and ground water (bottom). Heavy black 

 bar indicates period of Spartina 

 senescence. The bottom graph compares the 

 input of nitrate from ground water to the 

 summed total exchanges of all forms of 

 inorganic nitrogen by tides (Valiela and 

 Teal 1979). 



Seasonal changes in the nitrogen 

 cycle at Great Sippewissett Salt Marsh 

 gave us insight into the processes which 

 controlled it. At the beginning of the 

 most active growing season for the 

 grasses, there was substantial import of 

 ammonium from the bay to the marsh (Figure 

 22). At that time the Spartina needed 

 nearly 40 kg N/day. The ground water 

 input of inorganic nitrogen amounted to a 

 little less than 10 kg N/day; the tides 

 supplied about 8 kg N/day to the marsh. 

 The resulting deficit of over 20 kg N/day 

 had to be made up by processes within the 

 marsh itself. A little later, in August, 

 when the plants had matured, flowered, and 

 were beginning to become senescent, as 

 much as 12 kg N/day of ammonium were 

 exported from the marsh to the bay via the 

 tides. Not only was most of the ground 

 water nitrogen not being intercepted by 

 the marsh, but the plants were leaching 

 nitrogen into the water. 



Denitrification in a New England 

 marsh, like nitrogen fixation performed by 

 bacteria, varies in response to the marked 

 seasonal temperature changes (Figure 23). 

 Denitrification rates were highest (5 mg 

 N/m 2 /hr) in tidal creek bottoms which 

 carry out most of the total 

 denitrification for the entire marsh. The 

 short Spartina parts of the marsh 

 accounted for most of the remaining 

 denitrification. When denitrification for 

 the marsh as a whole is compared with the 

 input of nitrate from ground water and the 

 export of nitrate to the bay, it is 

 apparent that nitrate is exported only 

 during those seasons when denitrification 



38 



