Soil conditioning affects interactions between native and invasive exotic perennials of semi-natural grasslands

1. Semi-natural perennial grasslands are of increasing importance as components of multifunctional agroecosystems, combining biomass production with provision of other ecosystem services. Soil legacies from previous land use or exotic species can hinder their establishment, but might be overcome through a multi-stage successional strategy, whereby certain species are used to facilitate native grassland species establishment. We tested this strategy via a feedback experiment examining soil-conditioning effects on interference interactions between native and exotic species. 2. Soils in a former maize-soybean production field in Minnesota, USA, were conditioned for 3 years with native or invasive exotic perennials or a maize-soybean crop rotation. Nitrogen (N) fertilization was an additional treatment in field plots. In a greenhouse, native and invasive exotic perennial grassland seedlings were grown on these soils, in monoculture and in native-exotic species pairs, with and without N fertilization. The impact of soil conditioning and field and greenhouse N fertilization on interactions between native and exotic seedlings in mixture was determined. 3. Neighbouring plants suppressed biomass production in all native and exotic species. The maize-soybean rotation left a soil legacy that enhanced suppression of native species when grown with exotic species, while exotic species suffered no such disadvantage. 4. The strong and specific disadvantage to native species of maize-soybean soils decreased with greenhouse N fertilization, but remained significant, while field N addition did not alter this effect. 5. Synthesis and applications. We found that the negative soil legacy of the maize-soybean rotation for native plant performance in interaction with exotics was greatly diminished in soils conditioned by native or exotic perennial species, irrespective of nitrogen addition. This highlights the potential value of perennial species in conversion from row-crop agriculture to grasslands, because all perennial species alleviated the enhanced suppression of natives observed on maize-soybean soils. We did not find strong evidence that these perennial species were capable of specifically facilitating native species over exotics, but a broader range of species should be evaluated.