Earthworms and weed seed distribution in annual crops

A field study was conducted to determine if earthworm activity would affect the abundance and composition of weed seed banks in annual row-crops. The abundance of weed seeds in surface-deposited earthworm casts was determined in continuous monocultures and rotations that included corn, soybean, and winter wheat, with or without cover crop. Casts were collected weekly over the growing season and the weed seed content determined by direct germination in a heated greenhouse. Weed seed composition in surface casts was compared to that in the seed bank (0-5 cm) and soil surface by the same method. Earthworm cast production varied temporally and among crops and was higher in winter wheat compared to corn or soybean. The effect of crop rotation on cast production was significant only in soybean, with the highest production in monocultures compared to three-crop rotations. Weed seed densities were significantly higher in casts collected from winter wheat than corn or soybean. Comparisons of the composition and relative abundance of seeds in earthworm casts with the surface soil layer and seed bank suggest that earthworms contribute to redistribution of weed seeds over the growing season.