Tree species-specific wood traits control diazotrophic community composition in deadwood

Deadwood is a nitrogen (N) poor environment and its decomposition rate is limited by low N bioavailability. Diazotrophs reduce such N limitations as they are capable of fixing N2 which employs the functional gene nifH. The present study investigates the diazotrophic community composition in deadwood logs of 13 different tree species in temperate forests. Directly after sampling, deadwood was incubated with water (total fraction) or bromodeoxyuridine (BrdU), and latter labeled the metabolically active fraction. Nucleic acid extracts from both fractions were used to assess the nifH gene-based diversity by amplicon sequencing. The active fraction was a subset of the total fraction, which differed in abundance and with a lower richness. Wood traits such as phosphorous, molybdenum, magnesium, glucose concentration, and C:N ratio were positive factors of richness and community composition of the active fraction. Bradyrhizobium and Methylocapsa dominated the total fraction, while the active fraction was mainly governed by Frankia and Treponema regardless of tree species studied, suggesting that they are well adapted to C sources and nutrient-poor conditions in deadwood. The great diazotrophic diversity (50 genera) in the active fraction indicates an enormous potential for biological N fixation during deadwood decay.