Biological nitrogen fixation and nifH gene abundance in deadwood of 13 different tree species

Deadwood is an extremely nitrogen (N) poor plant litter whose decomposition may rely on external N sources. Biological N₂ fixation (BNF) by free-living diazotrophs, encoded with the nifH gene, is a potential pathway of N acquisition in deadwood. Still, the control of this process by tree species specific traits is hardly known. Here, we examined (1) BNF rates and nifH gene abundances in deadwood of 13 tree species after 12 years of decomposition and (2) how BNF was related to nutrient concentrations and non-structural carbohydrates (NSC). Comparing our BNF rates with the literature revealed no difference for angiosperms but gymnosperms. Large variability prevented any significant difference in BNF and nifH gene abundance between tree species identities of deadwood. Still, higher BNF and nifH gene abundances occurred in the group of diffuse-porous angiosperms compared to ring-porous angiosperms and gymnosperms. Positive relationships between BNF rates and molybdenum, phosphor, sulfur, and N suggested co-limitation of BNF by several nutrients in deadwood. Relatively high NSC concentrations indicated good carbon availability for diazotrophs. Furthermore, structural equation modeling highlighted the importance of water content for BNF and nifH gene abundance, although the overall explanatory power was low. In conclusion, BNF is a ubiquitous microbial process in deadwood of native European tree species and the comparison with other studies suggests no down-regulation of BNF by high N deposition in Central Europe.