The boreal forest rhizosphere
The Boreal Plains ecozone covers the largest area in Alberta (Price et al., 2013) and has the highest carbon stocks of all boreal ecozones in Canada (Figure 2). Forest floor soils host an abundance of microorganisms, which are the regulators of carbon processes in soil. The rhizosphere differs from bulk soil because carbon is readily available for microorganisms due to rhizodeposits (Figure 3; Table 1) from plant roots. However, nutrients—namely nitrogen—are limiting.
Decomposition of soil organic matter by microorganisms due to the availability of labile carbon in rhizodeposits, a process called priming, allows microbes to access nitrogen in organic matter (Chen et al., 2014). Priming has been observed in boreal soils (Karhu et al., 2016; Linden et al., 2014; Fan et al., 2013). The priming effect could reduce the carbon sequestration potential of soils (Hungate et al., 2003); altering microbial communities could convert soils from carbon sinks to sources (Carney et al., 2007).
Climate change and the rhizosphere
Research questions
1. Is microbial community composition, determined by phospholipid fatty acid (PLFA) analysis, different in rhizosphere versus bulk soil? I expect that rhizosphere microbial communities will differ from bulk soil due to the association with roots. Rhizodeposits from plant roots may provide a different environment than the bulk soil, resulting in different communities. However, it is possible that root density is so high in forest floor samples that both sample types receive rhizodeposits, and therefore no difference is observed.
2. Does canopy type (aspen versus spruce) affect rhizosphere microbial community composition? I expect rhizosphere communities under aspen canopies will differ from those under spruce canopies because of the potential for tree species to secrete different rhizodeposits and differences in soil moisture, pH, and understory vegetation.
3. What effects will clear-cutting have on total PLFA and microbial community composition? Clear-cut stands will likely have lower total PLFA and different community composition compared to natural stands due to differences in rhizodeposition, tree age, and canopy cover; however enough time may have passed to negate effects of clear-cutting. Comparing clear-cut and natural stands will help inform future harvesting practices to ensure protection of soil carbon.
4. Could observed changes in total PLFA and rhizosphere community composition with vegetation shifts alter soil respiration from priming? To assess the potential for occurrence of the rhizosphere priming effect, it will be necessary to identify microbial groups, in particular fungi and gram negative bacteria, which are likely responsible for priming (Nottingham et al., 2009). This research will help determine whether elevated atmospheric CO2 and vegetation shifts in the boreal forest will alter soil microbial communities. If differences are observed, this may indicate the potential for carbon losses from these systems. If there are no differences, perhaps concerns over carbon losses from boreal soils are exaggerated in terms of soil microbial communities.
2. Does canopy type (aspen versus spruce) affect rhizosphere microbial community composition? I expect rhizosphere communities under aspen canopies will differ from those under spruce canopies because of the potential for tree species to secrete different rhizodeposits and differences in soil moisture, pH, and understory vegetation.
3. What effects will clear-cutting have on total PLFA and microbial community composition? Clear-cut stands will likely have lower total PLFA and different community composition compared to natural stands due to differences in rhizodeposition, tree age, and canopy cover; however enough time may have passed to negate effects of clear-cutting. Comparing clear-cut and natural stands will help inform future harvesting practices to ensure protection of soil carbon.
4. Could observed changes in total PLFA and rhizosphere community composition with vegetation shifts alter soil respiration from priming? To assess the potential for occurrence of the rhizosphere priming effect, it will be necessary to identify microbial groups, in particular fungi and gram negative bacteria, which are likely responsible for priming (Nottingham et al., 2009). This research will help determine whether elevated atmospheric CO2 and vegetation shifts in the boreal forest will alter soil microbial communities. If differences are observed, this may indicate the potential for carbon losses from these systems. If there are no differences, perhaps concerns over carbon losses from boreal soils are exaggerated in terms of soil microbial communities.