Box 2: Rewilding microbes to boost carbon cycling
Soil microbes are key drivers of the global cycling of Soil Organic Carbon (SOC). They both deplete SOC through respiration and accumulate SOC through growth and by stabilizing soil aggregates (Anthony et al. , 2020). Ecto- and endo- mycorrhizal fungi are particularly important for the accumulation of SOC as their hyphae stabilize soil aggregates, making SOC inaccessible for other microbes and limiting respiration (Wei et al. , 2019). Restoration of the natural carbon cycle in revegetated areas is therefore highly dependent on the return of soil microbiota. Although some soil communities in restoration areas track towards remnant sites (Barber et al. , 2017), soil microbes differ in their dispersal capabilities according to the presence or absence of traits needed to survive during airborne dispersal such as spore formation and pigment production (Choudoir et al. , 2018). Consequently, dispersal distances vary greatly between species, with some fungi exhibiting effective dispersal ranges of only ~1 km (Peay, Garbelotto and Bruns, 2010).
Like invertebrates, the return of soil microbes and the functions they provide to revegetated “habitat islands” on degraded farmlands is often assumed to occur passively (Box 1). However, restoration projects may benefit from actively rewilding soil microbes. This could both overcome dispersal constraints and tailor the reconstructed microbe community to a desired trajectory. Local paddock trees are often the last remaining remnant trees on degraded farms. They are potential reservoirs of soil carbon cycling taxa as they contain mycorrhizal fungi and bacterial species adapted to competitive dynamics within local conditions (Wood, Tang and Franks, 2018). This provides a competitive advantage over communities already established in revegetated areas and increases the likelihood of rewilded communities overcoming the biotic barrier.
Rewilding soil microbial communities would entail moving soil from the rhizosphere (area of soil in contact with roots) of a local paddock tree and scattering this around the base of revegetated plants (Figure 4). Local paddock trees may be particularly useful in highly modified systems where remnant patches are non-existent and could contain the last locally adapted source of remnant microbial communities.