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Date of Award


Degree Name

Doctor of Philosophy




An experimental field, under conventional (fall plowing, spring disking) and reduced (no preplanting soil disturbance) tillage for over 12 years, was sampled with the objectives of determining the distribution of weed seeds in the soil profile and their location in relation to soil structural units. Soil samples were taken at the depths of 0-5, 5-10, and 10-20 cm in the spring and fall of 1982, air-dried, and classified into seven aggregate size-classes by dry rotary-sieving. Sub-samples were deflocculated and washed through sieves to retain large (> 0.9 mm) weed seeds;In the spring, 74% of all seeds in the reduced tillage and 15% of those in the conventional tillage soil were in the 0-5 cm depth layer. Conventional tillage distributed weed seeds uniformly among soil aggregate classes, while reduced tillage produced more seeds in the non-aggregated fraction of the soil. In the fall, the distribution of weed seeds in the profile and among soil structural units was very similar for both tillage regimes;Growth chamber and laboratory experiments evaluated the effects of seed-soil microsite characteristics on weed seed germination. Corn (Zea mays L.), soybean (Glycine max (L.) Merr.), velvetleaf (Abutilon theoprasti Medic.), and giant foxtail (Setaria faberi Herrm.) seeded among soil aggregates > 1.25 cm showed decreased seedling emergence with decreased frequency of irrigation. The same species seeded inside artificially made soil aggregates showed increased seedling emergence with decreased frequency of irrigation. Aggregates made with 0.4 g of soil, incubated in high relative humidity environments, significantly decreased the germination of corn and soybean seeds placed in their centers. Germination of velvetleaf and giant foxtail seeds was significantly reduced by 0.1 g soil aggregates. Rice (Oryza sativa L.) had 68% emergence in 1.0 g fully moist soil peds, while larger seeded corn and soybean had only 20% and 10%, respectively. The inhibition of germination of giant foxtail seeds by fully moist soil peds was partially reversed by incubating seed-containing aggregates in oxygen-enriched atmospheres;The location of weed seeds in relation to soil structural units, the seed-soil microsite, may have an ecological role regulating weed seed germination and dormancy under field conditions.



Digital Repository @ Iowa State University,

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Mario R. Pareja



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190 pages