Functional implications of the pH-trait distribution of the microbial community in a re-inoculation experiment across a pH gradient

We compared the influence of the microbial community composition and the environmental conditions for the functioning - microbial growth and respiration - and trait distribution - bacterial pH tolerance - of soil microorganisms across a pH gradient. Sterilised soil microcosms, including pH 4.1, 5.2, 6.7 and 8.3, with added plant litter were inoculated with unsterilized soil in a factorial design and monitored during two months. The trait distribution - pH-tolerance - of bacterial communities converged with the pH of the soil environment. Still, the different inoculum communities could result in suboptimal pH-tolerance in all soil pH environments; inoculum communities derived from low pHs had lower than optimal pH-tolerance in high soil pH environments, and vice versa. The functioning of bacterial communities with trait distributions mismatched to the soil pH environment was impaired. The legacy of the initial bacterial trait distribution on bacterial pH tolerance and functioning was detected within one week and remained for two months in all soil pH environments. Fungal inoculum communities derived from low compared to high pHs resulted in higher fungal functioning. Thus, in contrast with bacteria there was no evidence that variation in pH-tolerance influenced fungal performance. Instead the fungal inoculum size appeared to explain these results. Bacteria dominated respiration in high pH while fungi dominated at low pH environments. Consequently, respiration was affected by how well-matched the bacterial trait distribution was to the pH of the soil environment at higher pHs. At low pH, the inoculum size of fungi appeared to determine the respiration.