Soil biota legacies of alfalfa production vary with field conditions and among varieties and species

Plants can have strong effects on soil biota and thus subsequent plant growth in those soils. In agroecosystems, these microbe-mediated plant-soil feedbacks (PSFs) can have strong positive or negative effects on subsequent crop growth, but how multiple factors including the identity of the species involved, crop diversity and soil characteristics influence PSF is poorly understood. To address this knowledge gap, we focus on alfalfa (Medicago sativa), the globally most common forage legume. We sampled plants and soils from 24 alfalfa stands evenly split between alfalfa monocultures and grass-alfalfa mixtures of varying ages. Using the soils from these sites, we conducted an experiment to quantify how the conditioning plant community and soil characteristics altered PSF effects on four alfalfa varieties and five additional native and non-native grass and legume forage species. Alfalfa monocultures generated more negative PSF than mixtures overall, indicating dilution of antagonistic soil biota in more diverse stands. Differences among varieties and species were idiosyncratic: one alfalfa variety had positive PSF in mixture soils, whereas one legume species (Trifolium pratense) had positive PSF in monoculture and another (Onobrychis viciifolia) had positive PSF in mixture. Among varieties, PSFs were mediated by soil texture, soil phosphorus, and alfalfa fiber content. Similarly, PSFs among species were mediated by alfalfa fiber and nitrogen content, stand age, plant species richness and weed abundance. These relationships, however, were highly dependent on the variety or species selected and whether the field was seeded to monoculture or mixture. Interestingly, PSFs of native species were negatively impacted by weed abundance, suggesting that weeds may have deleterious effects on native plants via soil biota legacies, thus limiting the utility of native species when reseeding pastures to restore productivity (i.e., pasture rejuvenation). Combined, these results show that the impacts of soil biota on crop productivity depend on previous cropping practices and the variety or species of subsequent crops. As plant-soil feedback is context dependent, variety and species selection combined with diversity-related management practices are critical for successful pasture rejuvenation. Such an approach may sustainably reduce the impact of negative soil biota in agroecosystems.