Soil legacies differentially influence native grass establishment in an urban prairie restoration

Background/Question/Methods  Understanding priority effects is an essential consideration in restoring native plant communities. Priority effects, i.e., when the growth of an earlier plant species affects the establishment, growth, or reproduction of later plants, can lead to lasting differences in plant species dominance and community composition. Such priority effects can be the outcome of asymmetric competition or plant-induced changes (‘legacies') in soil properties including alterations in nutrient availability and cycling, and microbial communities, including pathogens and mycorrhizal fungi. In this study, we examined how soil legacy effects on arbuscular mycorrhizal fungal (AMF) communities affect the restoration success of an AMF-dependent native grass, Andropogon gerardii. We evaluated soil N and P resources in urban restoration sites conditioned by five or 15 years of growth by either native grasses or weedy grasses and forbs. Next, we conducted a greenhouse experiment in which seedlings of Andropogon were planted into soils from each of the four restoration sites, and measured plant growth and nutrient content, and AMF status. We used these data to test the hypothesis that soil legacies act as persistent agents of selection to differentially influence soil resource availability, plant growth, and AMF abundance.

Results/Conclusions  We found evidence for soil legacies, or priority effects, acting through both short (5 yrs) and long-term (15 yrs) changes in soil nutrients, plant growth, and AMF abundance. In soils conditioned by 5 yrs of weedy grasses, Andropogon plants showed a reduction in root biomass and AMF root colonization in comparison to plants propagated in soils conditioned by native grasses. After 15 yrs of conditioning by weedy grasses, soil N and P fertility was elevated, and Andropogon plants demonstrated a significant increase in plant biomass, nutrient content, and root colonization by AMF. However, these plant –AMF relationships are unlikely to be mutualistic owing to the reduced plant demand on AMF for nutrient acquisition. In contrast, the growth of Andropogon in soils conditioned by native grasses confirmed the presence of locally adapted AMF. These results imply that weedy species priority effects can alter grassland restorations through temporal changes in nutrient availability that feedback to alter plant growth and the symbiotic functioning in AMF. Because native grasslands are historically nutrient-limited and Andropogon ecotypes adapt to local soil and AMF communities, strategies may need to be implemented to reduce soil N and P fertility.