Arbuscular mycorrhizal (AM) fungi can improve restoration outcomes by facilitating establishment of native plants. Yet, we know little about the effects of restoration methods on AM fungal communities. We examined how herbicide application and mowing (with thatch removal) of invasive Brassica nigra (black mustard) or adding topsoil from a reference ecosystem (i.e., salvage topsoil) might alter AM abundance and fungal communities in a coastal sage scrubland. Black mustard is a non-mycorrhizal plant that produces chemicals which may inhibit AM fungi. Removing mustard plants and burying mustard seeds via the addition of salvage topsoil, which includes AM propagules, AM host-plant seeds, and organic matter, could potentially improve AM growth and diversity. However, some restoration practices, such as stockpiling salvage topsoil or removing thatch might harm AM fungi. Indeed, removing thatch may expose soil surfaces to increased solar radiation and stockpiling salvage topsoil may hinder hyphal growth, damage AM spores, or alter AM fungal community composition. Therefore, in a restored coastal sage scrubland, we measured the effects of these restoration practices on native and invasive plant cover, AM hyphal abundance, and fungal community composition in plots exposed to either herbicide or mowing versus untreated controls, and compared AM fungal communities at plots with stockpiled or freshly-deployed salvage topsoil.
We found that each restoration practice reduced invasive plant cover, while only freshly-deployed salvage topsoil addition improved native plant richness and cover. All treatments altered fungal community composition. Herbicide increased AM abundance in one field site, potentially because herbicide not only killed aboveground black mustard plants, but also eliminated non-mycorrhizal plant roots via desiccation. Mowing with thatch removal did not improve AM abundance, potentially from either increased solar radiation or remnant black mustard roots inhibiting mycorrhizal growth. Although stockpiling salvage topsoil limited fungal diversity, adding fresh topsoil not only promoted AM diversity and abundance but also increased AM host-plant diversity. Overall, both salvage topsoil and herbicide may deter non-native plant invasion and help re-establish the AM community in restored ecosystems.