Soil history as a primary control on plant invasion in abandoned agricultural fields

1. Abandoned agricultural (AA) fields are often invaded by exotic plants. This observation has been difficult to explain because agricultural practices change nearly every aspect of an ecosystem. Restoring native plants to AA fields is likely to require a prioritized understanding of the many mechanisms through which agriculture encourages exotic and discourages native plant growth. 2. Using 660 experimental plots in three sites in Methow Valley, Washington, USA, we determined the relative role of neighbour removal, propagule addition, plant-soil feedback, soil disturbance and fungal restriction to explain why exotics cover 38% of the ground in AA fields and 3% of the ground in non-agricultural (NA) fields. 3. After three growing seasons, neighbour removal improved exotic growth from 3% to 11% cover in NA fields but had no effect in AA fields. Propagule addition did not increase exotic growth above natural recruitment. Differences in soil history, a proxy for plant-soil feedback, explained an increase in exotic growth from 9% in NA fields to 39% in AA fields. Soil disturbance improved exotic growth from 9% to 16% cover in NA fields but had no effect in AA fields. Fungicide reduced exotic growth from 39% to 28% cover in AA soils but had no effect on exotic growth in NA soils. Native plant growth never differed by more than 5% cover among treatments. 4. Soil carbon, nitrogen (organic, inorganic and mineralization), phosphorus concentrations and fungal biomass were better associated with plant type (exotic or native) than agricultural history, suggesting that exotics facilitated their own growth by maintaining small beneficial fungal populations and fast nutrient cycling rates. 5. Synthesis and application. Soil history was more important than neighbour removal in determining exotic and native plant distributions. Where exotics rely on plant-soil feedback or legacies of agricultural disturbance, native plant restoration may require soil-based management. In these cases, changing mycorrhizal fungal abundance, increasing soil pathogen loading and slowing nutrient cycling rates may help restore native plants to invaded fields.