104 R. T. Prentki et al. 



TABLE 4-10 Nitrogen Fixation Rates in the Barrow Tundra Pond 

 Sediments, 1972* 



7/31 8/4 8/14 8/17 8/21 8/24 



7.1 14.3 - 25.4 



0.0 - 0.0 



0.0 - 0.0 



9.2 0.0 - 0.0 



2 1 



*Data are expressed as /ig N m- hr- . 



with ''^N proved that the method was indeed measuring fixation and that 

 the correct conversion factors were being used. 



Of the four ponds sampled for nitrogenase activity (Table 4-10), two 

 of them, C and D, contained sediments that showed no activity at all 

 during July and August, 1972. Pond E sediments had activity (0.99 ^mole 

 ethylene produced m "^ hr~') at one sampling but not at two others. Pond 

 B, however, had activities of 0.76, 1.53, and 2.72 ^moles m^ hr"^ on 4, 

 14, and 21 August, respectively. These values are low compared with 

 terrestrial fixation rates of up to 50, but may add up to respectable 

 amounts of nitrogen over the ice-free season. Thus, if we assume that 3 

 moles of acetylene are reduced for each mole of N2 fixed and that daily 

 rates can be calculated by multiplying the observed hourly rates by 20 

 (based on diurnal studies carried out in the summer of 1972), then the 

 average fixation of 0.3 1 mg N m ^^ day ~ ' is equal to 28 mg N m " ^ yr \ 

 Barsdate and Alexander (1975) calculated an average value for terrestrial 

 tundra at Barrow of 48 mg N m ^^ yr ' so the pond value is reasonable 

 (however, three other ponds showed virtually no fixation at all). However, 

 it appears that this fixation in the pond is low in relation to the amount of 

 nitrogen already present in the sediments. One measurement of the 

 interstitial waters gave 2730 Mg NH3-N liter ^^ and 11 Mg NO3-N in the 

 upper 8 cm of a pond sediment (Table 4-9). This is 1 68 mg N m " ^ for the 

 top 8 cm alone (concentrations were even higher in the 8 to 13 cm layer) so 

 it appears that the inorganic nitrogen is abundant and the fixation 

 relatively unimportant in providing N for algal and plant growth. 



Denitrification 



One possible reason that the sediments contained low amounts of 

 nitrate is that denitrification occurs. This process was measured in Pond J 



