Chemistry 103 



TABLE 4-9 Concentration of Nitrogen Compounds in the Sediment of 

 Pond J, 1973, for a Transect from the Edge to the Center 

 of the Pond 



followed a similar pattern except that the amounts were much lower (1 to 6 

 Mg N in the plant beds and 4 to 1 1 ^g N in the pond center). From this 

 evidence, it appears that sediment nitrogen is being cycled by macrophyte 

 uptake. 



The exchangeable nitrogen is another potential source of nitrogen in 

 the sediment. This pool is measured by extraction with 0.5 N HCl for 1 hr 

 at 25°C and includes the relatively small amount of nitrogen in the pore 

 water. In Pond C, the exchangeable nitrogen in the top 16 cm was more 

 than 99% ammonia and measured 26 Mg N (g dry wt)~' in the Carex bed 

 and 45 Mg N (g dry wt) " ^ in the center of the pond. 



Nitrogen Fixation 



One process by which nitrogen could be increased in the ponds is 

 nitrogen fixation. This process was studied with the acetylene reduction 

 method, an indirect technique which yields results closely correlated with 

 nitrogen fixation (Stewart et al. 1967). The details of the method used here 

 are reported in Schell and Alexander (1970); briefly, samples were taken 

 and placed in a small tube, acetylene was added and the samples incubated 

 in the light for 6 to 24 hours. After this, gas samples were taken and the 

 amount of ethylene formed was determined in a gas chromatograph 

 equipped with a flame-ionization detector. Some experiments carried out 



