Effect of long-term organic farming on weed seed-bank

Weed seed-bank response to organic farming duration under two different crop rotations was investigated. Longterm organic farming increases the diversity of weed species in "good" and "poor" crop rotation by 30 and 57%. The amount of weed seeds in the soil in the ninth year of organic farming in "good" crop rotations decreases by 41.6%, while in "poor" crop rotation it tended to increase compared to the beginning of transition period. Most of seeds found in 0-25 cm soil layer were therophytes, germinate in summer and spread barochory. Both in "good" and "poor" crop rotations and both survey years qualitative (number of species) and quantitative (10(3) seeds per m(-2) of soil layer 0-25 cm.) distribution of weeds into biological groups and types of dispersal was even. Long-term organic farming determined uneven quantitative distribution of weeds into ecophysiological and ecological groups. This farming increases the amount of seeds that belongs to indifferent to soil pH, nitrogen rich and dry-moderately moist soil weed species.