The effects of nitrogen and diaspore availability on the regeneration of weed communities following extensification

The regeneration of weed communities after cessation of fertilization and herbicide use was investigated on an arable held representative of the Central German Chernozem Region. As a result of high, but spatially heterogeneous input of farmyard manure and slurry until 1983, there were large differences in soil nitrogen supply within this 'regeneration field' during the investigation period (1992-1994). At the same time studies were carried out on a nearby long-term experimental field that reflects the potential species-rich weed community with and without N-fertilization. Total biomass production was greater in plots of high N-supply and light transmission into the canopy was reduced. Biomass of surviving weed plants was positively related to N-availability but mortality increased. Thus, a large N-supply may delay the regeneration of weed communities because it increases the risk of extinction of small initial populations. Despite favourable growing conditions in the low N-plots of the regeneration field many typical weeds were absent. As dispersal of these species was too slow in relation to the distance of the closest populations diaspore input was not sufficient for re-establishment. A field experiment showed that Lithospermum arvense seeds were dispersed within 2.5 m distance over 2 years, whereas the next population was found 300 m away from the regeneration field. In species poor agricultural landscapes, regeneration of weed communities following extensification is often limited by dispersal. (C) 2000 Elsevier Science B.V. All rights reserved.