Nitrogen metabolism of wood decomposing basidiomycetes and their interaction with diazotrophs as revealed by IRMS

Abstract

Isotope ratio mass spectrometry (IRMS) is an advanced method to investigate carbon (C) and nitrogen (N) in organic samples. In particular, the N content, its isotope signature and the C/N ratio reveal important facts of nutrient cycling, niche separation and ecological food webs. In this study, the characteristics of N turnover of wood decomposing microorganisms were investigated. The growth of the white rot causing basidiomycete Trametes versicolor is enhanced after addition of ammonia or urea, whereas the brown rot causing Oligoporus placenta is not accelerated. In addition, an interaction of each fungus with atmospheric N 2 assimilating (diazotrophic) bacteria was investigated. Cultivation experiments with a gas mixture of 15N 2/O 2 and subsequent IRMS analysis of dry biomass of the diazotrophs Azotobacter croococcum, Beijerinckia acida and Novosphingobium nitrogenifigens revealed that they assimilated up to 12% of their N from N 2. The experiments reflected N availability as a prerequisite for efficient growth of decomposing basidiomycetes and diazotrophs. Fungal–bacterial co-cultivation experiments showed that depending on the growth characteristics and bacterial N 2 assimilation activity N is transferred from certain bacteria into fungal biomass. Thus, the experiments gave a first indication of an interaction between wood decomposing basidiomycetes and diazotrophs, which is a novel pathway of fungal N acquisition.