Nonsymbiotic nitrogen fixation in the topsoil of some forest stands in central Sweden

Abstract

Studies on nonsymbiotic dinitrogen fixation (C₂H₂) in soil were conducted in 20 different forest stands in central Sweden. The stands were between 40 and 120 years old and included both coniferous and deciduous types. The relationships between nitrogenase activity and site productivity, stand composition, and soil properties were examined. Based on three samplings during the growing season, a considerable difference in activity was found to occur between the stands. The highest yearly mean activity for a stand was 200 times the lowest value. The activity was positively and significantly correlated to site productivity. The effect of stand composition on the activity in the forest floor was studied on five sites. The activity was similar under spruce (Piceaabies (L.) Karst.) and pine (Pinussylvestris L.), higher under spruce than beech (Fagussylvatica L.), and much higher under birch (Betulapendula Roth) than spruce when compared two by two on identical parent material. The activity under birch was mainly concentrated in the birch leaf litter. Evaluating the influence of soil properties by regression analysis revealed a strong positive correlation between activity and pH. The soil acidity alone explained 85% of the variation in yearly mean activity among the 15 coniferous stands. The conversion factor between C₂H₂ reduction and ¹⁵N₂ fixation was determined for five soils and ranged from 1.6 to 5.6. The N₂ fixation down to a depth of 6 cm during the growing season was estimated to 0.4–1.4 kg ha⁻¹ in these soils.