Non-symbiotic nitrogen fixation in forest ecosystems

Since the first attempts to increase forest growth using nitrogen (N) fixation, biological N fixation has been intensively studied. Numerous studies that focused on symbiotic N fixation and rates were well quantified. However, the rates and importance of non-symbiotic N fixation in forest ecosystems remains incomplete. In this review, reports of non-symbiotic N fixation from various studies throughout the world were examined and the ecological significance of non-symbiotic N fixation in temperate forest ecosystems was investigated. The reported rates of non-symbiotic N fixation varied greatly, and ranged from < 0.01-5 kg N ha^-1 year^-1. However, an average input of 2-3 kg N ha^-1 year^-1 could be expected when all ecosystem components were included. In several studies, non-symbiotic N fixation seemed to decrease with stand age. Some methodological considerations such as incubation time and conversion factor in the acetylene reduction method were presented. Also, it should be noted that more detailed studies on non-symbiotic N fixation in boreal and tropical forests are needed to understand the importance of N fixation in these forest ecosystems.