The role of bacteria in soil carbon dynamics of managed boreal forests

Boreal forests are significant net carbon sinks and play an essential role in the global carbon cycle. However, such forests are often subject to management practices such as clear-cutting. After clear-cutting, most root-associated mycorrhizal fungi die along with their tree hosts, opening a niche for saprotrophic microorganisms, including soil bacteria. With the competition eliminated, soil bacterial activity is expected to increase. Conversely, as forests regrow, mycorrhizal fungi suppress soil saprotrophs, potentially decreasing soil organic matter decomposition.

In this study, we utilized two parallel chronosequences in Sweden, each consisting of 18 stands that varied in time since disturbance, representing forest rotational management and a natural reference, a wildfire chronosequence. In each forest stand, we trenched plots and removed vegetation to exclude mycorrhizal fungi. We then measured bacterial growth in soil samples across the chronosequences. We hypothesized that (1) bacterial growth would increase after clear-cutting and forest fire, then decrease as forests regrow due to suppression by mycorrhizal fungi; and (2) bacterial growth would be higher in trenched plots than in non-trenched plots at each forest stand due to the elimination of competition with mycorrhizal fungi.

Contrary to both hypotheses, bacterial growth was lowest following forest clear-cutting and wildfire. With forest regrowth, bacterial growth increased. Interestingly, following clear-cutting, bacterial growth peaked when forest productivity was highest (40–70 years post-clear-cutting). Trenching also decreased bacterial growth along both the rotational forest management and wildfire chronosequences. These unexpected results suggest that bacterial communities are negatively affected by plant removal, likely due to their strong dependence on readily available carbon from root exudates.