Site preparation and fertilization of wet forests alter soil bacterial and fungal abundance, community profiles and CO2 fluxes

Nitrogen (N) fertilization and mechanical site preparation are used to increase productivity of forests and prevent paludification in wet climates. However, these practices may also alter soil microbial community structure and rates of decomposition and respiration. The effects of mounding, drainage and fertilization were investigated at a mounded hybrid spruce site and a drained coastal western redcedar - western hemlock site in British Columbia (B.C.), Canada. CO2 efflux was measured at four dates over a one-year period using static closed chambers. Total bacterial and fungal ribosomal marker abundance and structure were assessed using quantitative PCR and terminal-restriction fragment polymorphism (T-RFLP), respectively. Mounding reduced soil water content of planting sites, but also removed much of the forest floor layer from the rooting zone and led to a significant decrease in CO2 efflux rates, as well as bacterial and fungal abundance and diversity. Drainage increased CO2 fluxes and led to significant differences in T-RFLP profiles in drained and undrained samples. Fertilization had variable effects on the microbial community and CO2 fluxes at the two sites. Site preparation and fertilization can improve regeneration of wet forests that would otherwise be subject to paludification, but can lead to shifts in carbon fluxes and microbial community structure. Major disturbances to soil structure and composition (e.g., following mounding) can alter the microbial community structure in a way that suppresses microbial diversity and function for up to one year following treatment.