Nitrogen fertilization does not affect non-symbiotic N2 fixation in northern forest soils despite its negative impacts on diazotroph communities

Tree productivity in northern regions is limited by low soil nitrogen (N) availability, and biological N 2 fixation is a crucial N input to these forests. To enhance forest productivity, N fertilization has been proposed as a strategy although it may negatively affect N 2 fixation and the abundance of diazotrophic microorganisms. In contrast to N 2 fixation by the cyanobacteria-moss associations, there is limited understanding of non-symbiotic N 2 fixation in northern forest soils and the free-living diazotrophs involved. To assess the impact of N fertilization on non-symbiotic N 2 fixation and the diazotrophic community in soil, we sampled 15 forest sites along a latitudinal gradient in Sweden that are part of a fertilization experiment. Fertilization started between 41 and 55 years ago, using ammonium nitrate at 100–150 kg N ha −1 every 5th year for the first 25 years and thereafter every 7th year. We measured non-symbiotic N 2 fixation in the soil organic layer in laboratory incubations and analyzed the diazotrophic community. Both the abundance and diversity of diazotrophs decreased in response to N fertilization. However, this decline did not translate into significant changes in non-symbiotic N 2 fixation rates (22.4 ± 4.2 and 22.5 ± 5.7 ng N g −1 dry weight soil h −1 in the control and N treatments, respectively). Yet, N 2 fixation per area increased by 24 % in fertilized plots because of the increase in the organic layer stock caused by higher primary production. Additionally, we observed an influence of fertilization and mean annual temperature on diazotroph community composition across the gradient. Our findings indicate that N fertilization in northern forests strongly affects diazotrophs, the organic layer stock, and N 2 fixation. Although N fertilization positively affected the N 2 fixation rate per area in this experiment, its negative effect on diazotroph diversity might reduce N 2 fixation in the long run. • Nitrogen (N) fertilization increased organic layer stocks in 15 forests in Sweden. • N fertilization reduced diazotroph abundance and diversity in soils. • Diazotroph communities differ in relation to temperature and N fertilization. • Non-symbiotic N 2 fixation per gram of soil remained stable despite N fertilization. • 24 % higher N 2 fixation m −2 with N fertilization due to larger organic layer stock.