Nitrogen fixation - Arctic tundra 



Vera Alexander* University of Alaska 



Don ScheU University of Alaska 



Maggie Billington University of Alaska 



Linda Peyton University of Alaska 



G.B. Tlirelkeld University of Alaska 



The purpose of this study was to determine the significance of nitrc^en fixation as a nitrogen 

 input into the tundra ecosystem. Since the rate of decomposition of organic material is believed 

 slow, nitrc^en limitation can reasonably be expected. If this is the case, a large proportion of nit- 

 rogen fixers should occur. Many tundra plants belong to groups known to fix nitrogen, including 

 those lichens having blue-green algae as the algal component, the blue-green a^ae that occur in 

 moist areas on the tundra surface, the legumes, and nonlegume nodulated plants such as Dryas. 

 The actual contribution of these to the tundra nitrogen budget is not known. 



This study was divided into two main phases. First, a survey of plants in the arctic tundra 

 IBP site area and in an alpine tundra site was made, testing as many species as possible for 

 nitrogen fixation. Second, nitrogen input through fixation was estimated on a surface area basis for 

 some of the plots at the Barrow site. 



Nitrogen fixation measurements were carried out using the acetylene-reduction method, with 

 confirmation of positive results using '^N. Details of the methodologies will be presented in sub- 

 sequent reports. 



Two distinct types of experiments were carried out in Barrow. Selected plants were tested for 

 nitrogen fixation and segments of soil with associated vegetation from the plots used fa the other 

 terrestrial work in this area were measured. In addition, numerous nitrogen fixation determinations 

 were made on the ponds used for the aquatic Barrow work, including water samples, concentrated 

 organisms from the water, and pond sediments. 



Cores, 0.01 m^ were removed from a small number of control and manipulated plots to test for 

 nitrc^en fixation. From each cae, squares were cut with 3-cm-long sides; these were subjected 

 to testing using acetylene-reduction and nitrogen-15, with duplicate acetylene reduction experiments 

 for some cores. Blanks were also run; in these no acetylene was added, so that any ethylene pro- 

 duction by the tundra vegetation or soils could be detected and subtracted from the nitrogen fixa- 

 tion values. 



For the site 2 plots, all the control plots showed nitrogen fixing activity, with rates of about 

 0.2 X 10-' (ig-at/cm^ hr. This is equivalent to 2/xg-at/m^ hr, which would account for a major nitro- 

 gen input into this system. The conversion of ethylene production rates to equivalent nitrc^en 

 fixation was done according to established procedures, but the value obtained should be considered 

 an approximate value only. No ethylene production in the absence of acetylene was detected at 

 any time. The manipulated plots showed a reduction of nitrc^en fixation rate in the case of the 

 clip and clear plot, and a complete absence of fixation in the two fertilized plots tested (Table 

 XXIV). 



Plants present at site 2 and vicinity were collected and subjected to acetylene reduction, and 

 in some cases nitr(^en-15, tests fa nitrc^en fixing ;ictivity. The plants and lichens which yielded 

 neg;Uive results are listed below: 



Salix pulchra Astralagus alpinus L. 

 Petasites (rigidus Dryas integrilolia 

 Saxifraga flagellaris Alectoria bicolor (Eliih.) Nyl. 

 Saxi^raga cernua Dactylina arctica (Hook.) Nyl. 



Principal author 



56 



