Effects of soil depth and aggregate size on weed seed distribution and viability in a silt loam soil

The position of weed seeds within the soil matrix plays an important role in seedling emergence and seed survival. The relationship of weed seeds with soil aggregates and soil depth was evaluated in a Waukegon silt loam soil that had been under a long-term, conventional tillage, annual crop management system. Soil aggregates were separated and classified into eight size classes from ≤5 to >12 mm and weed seeds were extracted from the aggregates. Amaranthus spp., Chenopodium album L. (common lambsquarters), Polygonum pensylvanicum L. (Pennsylvania smartweed), Setaria faberi Herrm. (giant foxtail), and Solanum ptycanthum Dun. (eastern black nightshade) accounted for the majority of seeds recovered. In general, seed viability declined from April to June, but increased in October following seed deposition. Seeds of individual species were most abundant in the aggregate size class most closely matching its seed size. However, seeds were commonly found associated with aggregates larger than 9 mm. Highest seed viability was found in the aggregate fraction closest to the seed size, however, S. faberi viability was also high in the >12 mm aggregate size class. Regardless of aggregate size, seed numbers were generally greatest in the upper 5 cm of soil. The results of this research were species-dependent and variable and demonstrated the complexity of weed seed/soil aggregate associations. However, they did show that seed placement within the soil matrix may play an important role in weed population dynamics.