Change of the weed seed bank during the first complete six-course crop rotation after conversion from conventional to organic farming

The modification of the weed seed bank in the top soil (0-20 cm) during a s ix-year period including a sequence of annual crops and a perennial grass-c lover ley in the rotation, were analysed in an organic cropping system afte r conversion to organic farming. The seed bank of dicotyledonous annual spe cies 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 th e 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. Dicotyledo nous perennial weed seeds in the soil constituted only 2-3% of the seed ban k. The correspondence between the soil seed bank and emerged weed plants wa s not straightforward. For Capsella bursa-pastoris and Chenopodium album, o nly 41 % of the calculated correlations were significantly positive, indica ting that an increase in the seed bank only occasionally can predict an inc rease of the actual weed flora.