Weed seed decay in conventional and diversified cropping systems RG Ó MEZ*, M LIEBMAN † & G MUNKVOLD‡ *Escuela de Agronom ıa, Universidad de Costa Rica, San Jos e, Costa Rica, †Department of Agronomy, Iowa State University, Ames, IA, USA, and ‡Department of Plant Pathology and Microbiology, Iowa State University, Ames, IA, USA Received 21 January 2013 Revised version accepted 16 July 2013 Subject Editor: Jonathan Storkey, Rothamsted Research, UK Summary Diversified cropping systems can have high soil micro- bial biomass and thus strong potential to reduce the weed seedbank through seed decay. This study, con- ducted in Iowa, USA, evaluated the hypothesis that weed seed decay is higher in a diversified 4-year maize–soyabean–oat/lucerne–lucerne cropping system than in a conventional 2-year maize–soyabean rota- tion. Mesh bags filled with either Setaria faberi or Abutilon theophrasti seeds and soil were buried at two depths in the maize phase of the two cropping systems and sampled over a 3-year period. Setaria faberi seed decay was consistently greater at 2 cm than at 20 cm burial depth and was higher in the more diverse rota- tion than in the conventional rotation in 1 year. Abuti- lon theophrasti seeds decayed very little in comparison with seeds of S. faberi. Separate laboratory and field experiments confirmed differences in germination and seed decay among the seed lots evaluated each year. Fusarium, Pythium, Alternaria, Cladosporium and Trichoderma were the most abundant genera colonising seeds of both species. A glasshouse experiment deter- mined a relationship between Pythium ultimum and S. faberi seed decay. Possible differences in seed susceptibility to decay indicate the need to evaluate weed seedbank dynamics in different cropping systems when evaluating overall population dynamics and for- mulating weed management strategies. Keywords: weed management, seedbank, Setaria faberi, giant foxtail, Abutilon theophrasti, diversified crop rotations, soil microorganisms, seed pathogens, organic matter. GÓMEZ R, LIEBMAN M&MUNKVOLD G (2014). Weed seed decay in conventional and diversified cropping systems. Weed Research 54, 13–25. Introduction Crop rotations comprised of crops with different life cycles can have detrimental effects on weed population growth (Liebman & Gallandt, 1997), with different planting and harvest dates among crops preventing or reducing either plant establishment or seed production by weeds (Bastiaans et al., 2008). In diversified crop- ping systems, the use of forage legumes as green manure and livestock manure to provide organic sources of nutrients and organic matter can reduce weed emergence, by affecting small seeded weeds through the release of allelochemicals or by providing substrates for other organisms that inhibit seedling growth (Liebman & Davis, 2000; Mohler et al., 2012). Previous work by Liebman et al. (2008) at a long- term field experiment established in Iowa, USA, assessed the effect of simple and more diverse cropping systems on weed seed densities in soil. Over a 4-year period, decline of an experimentally supplemented seedbank of Setaria faberi Herrm. (giant foxtail) was greater in a 2-year maize (Zea mays L.) – soyabean Correspondence:RG omez, Escuela de Agronom  ıa, Universidad de Costa Rica, San Jos e, Costa Rica. Tel: (+506) 2511 7782; Fax: (+506) 2433 9086; E-mail: gomez.robin@gmail.com © 2013 European Weed Research Society 54, 13–25 DOI: 10.1111/wre.12052