Nitrogen fixation by Stereocaulon paschale under field conditions

Nitrogen fixation (C 2 H 2 reduction) by the lichen Stereocaulon paschale (L.) Fr. was studied in the field in northern Sweden. Field studies were carried out in a very dry dwarf-shrub-type pine forest with S. paschale as the only nitrogen-fixing lichen species. During May–September 1973 and 1974 nitrogen fixation was measured on 28 occasions at irregular intervals, and a total of about 1200 lichen samples were studied. The mean nitrogen-fixation activity during May–September was 0.85 and 1.04 μg N g −1 dry weight h −1 for 1973 and 1974 respectively. The water content of the thallus was the most important factor influencing the nitrogen-fixation activity. Studies on gain and loss of thallus water served as a basis for calculating the number of hours when nitrogen fixation occurred. In the area investigated, with S. paschale covering 14% of the ground, the species was calculated to contribute 0.1 g nitrogen fixed per square metre during May–September 1974.