Asymbiotic nitrogen fixation on woody roots of Norway spruce and silver birch

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High rates of asymbiotic nitrogen (N) fixation have been measured in woody roots in temperate forests, but this rate has not been quantified in boreal forests. We studied the asymbiotic N2fixation associated with living and decomposing woody roots of Norway spruce (Picea abies (L.) H. Karst.) in three sites in Finland. In addition, tree species effect was studied in one site that included Norway spruce and silver birch (Betula pendula Roth.) monocultures and mixed stands. The rate of N2fixation measured as nitrogenase activity was affected by host tree species; spruce roots were the most active (0.67 C2H4·day–1·(g dry mass)–1in spruce monocultures). The activity was not statistically different in decayed and living root samples, and moisture content did not explain the observed high variability in nitrogenase activity. In a birch–spruce mixed stand, the average N2fixation in woody roots was 0.17 kg N·ha−1·year−1, whereas in Norway spruce dominated sites, the activity ranged from 0.06 to 0.15 kg N·ha−1·year−1. The N2fixation in decaying and living woody roots is an important contributor to the long-term total N balance of the forest. However, the estimated rate of N2fixation is low compared with atmospheric N deposition.