Evaluating the Role of Plant Soil Feedbacks on Crop-Weed Competition Between Organic and Conventional Dryland Farms in the Northern Great Plains.

ABSTRACT: Plant soil feedbacks (PSFs) refer to processes by which plants alter belowground ecosystems’ biophysical characteristics, which in turn influence the growth and competitive ability of subsequent plants. Research has shown the importance of PSFs in determining community dynamics in natural systems, but little is known about their magnitude and role in managed agroecosystems. Long-term organic and conventional management in large-scale dryland farms should impact the soil biotic community due to different tillage, fertilization, rotation schemes, and plant diversity, thus modifying PSFs. We conducted a greenhouse trial that tested how PSFs alter plant growth and crop-weed competition using soils from five pairs of organic and conventional farms throughout Montana.  Avena fatua and Amaranthus retroflexus, two common agricultural weed species, were grown in steam pasteurized soil, and inoculated with either biologically active soil or sterilized soil collected from each of the ten farms. This soil-conditioning phase was followed by three response treatments: Triticum aestivum (wheat) grown without competition, or in competition with either A. fatua or A. retroflexus. Germination was strongly affected by the conditioning species, especially for A. retroflexus which had approximately 35% greater germination following A. fatua conditioned soils versus A. retroflexus soils. T. aestivum germination was higher following A. fatua conditioned soils with approximately 7% more seeds germinated. There was supportive evidence that wheat germination was higher for conventional farms.  We will look at plant biomass as an indicator of the strength and direction of the PSFs, and compare this to the above germination rates, to evaluate: [1] if PSFs on crop growth differ between organic and conventional farms, [2] if this crop growth changes depending on species specific soil conditioning, and [3] if crop-weed competitive interactions are differently altered in organic and conventional farms.