Understanding how weed communities respond to agricultural management is critical for developing effective ecologically-based weed management strategies. We used a community assembly approach to investigate the response of soil weed seed banks to 25 years of management-related filtering in three different row-crop management systems in SE Pennsylvania: two organic systems (one manure-based, the other legume-based) and one conventional system. Weed seed banks were sampled in April of 2005 and 2006 and quantified by direct germination in a greenhouse. We also assessed the filtering effects of weed management practices conducted in each system by allowing or excluding weed management and then measuring the expression of the seed bank in the emergent weed community in August of each year.

Results/Conclusions

Germinable weed seed bank densities and species richness in the final year of the study were over 40% and 15% higher, respectively, in the organic systems relative to the conventional system. Weed seed bank composition and structure differed between the conventional and organic systems, and the relationships between the seed bank and the emergent weed community varied depending on management system and weed control treatment. Primary tillage, weed control, timing of planting, and fertility management were likely the main filters affecting weed community assembly in the three systems. Weed life history, emergence periodicity, seed size, responsiveness to soil nitrogen, and tolerance to soil moisture conditions appeared to be the most important functional traits determining how weed species responded to management-related filters. Our results suggest that management systems can exert strong filtering effects on the assembly of weed communities, and that these effects can persist over relatively long (> one growing season) time scales. Legacy effects from community-level filtering may be more important than previously assumed, and should be incorporated into predictive models of weed community assembly.