Chemistry 109 



TABLE 4-13 Nitrogen Concentrations and Uptake Rates in Tundra 

 Ponds in 1970 and 1971* 



*Concentrations are expressed as /ig N liter" 



' and uptake rates as jUg N liter- hr- 



the sediments are 50% water and contain 60^8 NH3-N liter ' in the 

 Carex beds and 2,500 ^g NH3-N liter ' in the rest of the pond. This is 18 

 mg N m"Mn the top 20 cm of sediment in the Carex beds and 500 mg 

 outside the beds. Additional quantities of exchangeable inorganic 

 nitrogen, 810 mg N m"^ in Carex beds and 1400 mg N m"^ outside the 

 beds, are available as well. Obviously the sediment algae will have 

 adequate NH3 in the pore water but the Carex roots will not (they can 

 remove all the interstitial NH3 once a day). Unfortunately, our 

 measurements of uptake of ^'^N by isolated roots gave an extremely high 

 value of 7.4 g N m"^ day^^ which is hundreds of times too high to agree 

 with the productivity values. Similar results were obtained by Morris 

 (1978) for Spartina roots. It is likely that lowered oxygen concentrations 

 in situ cause a low rate of N uptake. 



