Change of the Weed Seed Bank during the First Complete Six-Course Crop Rotation after Conversion from Conventional to Organic Farming

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Abstract
The modification of the weed seed bank in the top soil (0–20 cm) during a six-year period including a sequence of annual crops and a perennial grass-clover ley in the rotation, were analysed in an organic cropping system after conversion to organic farming. The seed bank of dicotyledonous annual species varied from a minimum of 7200 seeds m−2, the year following a three-year period with perennial ley, to a maximum of 17600 seeds m−2, the year following a three-year period with annual crops. The number of species in the seed bank was essentially constant at 18–21 during the rotation, but the number of emerged species in the field decreased from about 19–20 during the annual crop period to a minimum of eight in third year ley, indicating a reduced input of seeds to the seed bank during the ley period. Dicotyledonous perennial weed seeds in the soil constituted only 2–3% of the seed bank. The correspondence between the soil seed bank and emerged weed plants was not straightforward. For Capsella bursa-pastoris and Chenopodium album, only 41% of the calculated correlations were significantly positive, indicating that an increase in the seed bank only occasionally can predict an increase of the actual weed flora.