Maintaining or increasing the ability of boreal forests to sequester soil organic carbon (SOC) in the long term could play an important role for climate change mitigation. However, there are many uncertainties regarding the dynamics of SOC in managed boreal forest soils, particularly with regard to its stability which is thought to be mediated by the activity of microorganisms. The aim of this project is to measure the impact of logging practices used in Quebec (Canada) on the stocks and stability of SOC in boreal forest. More specifically, it seeks to identify whether there are cohorts of microorganisms that are associated to long-term SOC stabilization in these soils, and to identify if logging practices affect these communities. To do this, we have measured SOC stocks and natural abundance of stable isotopes of carbon (13C) in the soil to a depth of 55 cm along a gradient of logging intensities in boreal balsam fir-white birch stands. Soils were further analyzed for particle-size fractionation as an index of SOC stability and microbial communities were characterized using a metabarcoding approach.
Results/Conclusions
Preliminary results suggest that SOC stocks (forest floor and 0-55 cm mineral) follow this trend 6 to 9 years after logging: clearcut (~209.74 Mg of C ha-1)> control stands (~185.28 Mg of C ha-1)> irregular shelterwood systems (~153.22 Mg of C ha-1). Contrary to what was previously thought, SOC stocks in the 15–35 cm mineral horizon seem to be the most sensitive to logging. For each depth of the mineral soil, enrichments in 13C suggest that SOC in continuous cover irregular shelterwood (~-25.21‰) underwent more cycles of microbial use compared to old-growth forests (~-26.05‰) or a more important relative activity of fungi vs bacteria. These unexpected results still need to be validated with other measures (i.e., microbial mineralization, SOC stability data, soil temperature, fungi: bacteria ratio) as specific microclimate conditions could explain these results (rather than harvesting treatments per se). We discuss the impact of logging practices on SOC stocks and stability in managed boreal forest, and the factors promoting this stability, including the communities of microorganisms.