Agriculture, Ecosystems and Environment 77 2000 237–246
The effects of nitrogen and diaspore availability on the regeneration of weed communities following extensification
A. Bischoff
a,∗
, E.-G. Mahn
b
a
Department of Community Ecology, Centre for Environmental Research Leipzig-Halle, Theodor-Lieser-Str. 4, D-06120 Halle, Germany
b
Department of Geobotany and Botanical Garden, Martin-Luther-University Halle-Wittenberg, Neuwerk 21, D-06108 Halle Saale, Germany
Received 7 April 1998; received in revised form 15 June 1999; accepted 23 July 1999
Abstract
The regeneration of weed communities after cessation of fertilization and herbicide use was investigated on an arable field 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. ©2000 Elsevier Science B.V. All
rights reserved.
Keywords: Arable fields; N-fertilization; Plant re-establishment; Population dynamics; Seed dispersal; Germany
1. Introduction
Intensification of crop production has led to se- vere changes in weed communities during the last
five decades. Various studies in different European re- gions have shown a rapid decline of most species e.g.
∗
Corresponding author. Tel.: +49-345-5585316; fax: +49-345- 5585329.
E-mail address: bischoesa.ufz.de A. Bischoff.
Erviö and Salonen 1987 for Finland, Albrecht 1995 for Germany, Wilson and Aebischer 1995 for the
UK and Andreason et al. 1996 for Denmark. Many weeds have become very rare or totally extinct. Not
only did species composition change, but the structure of whole weed communities has undergone alterations
Mahn, 1984a.
Recently, several proposals were made to reduce the intensity of cultivation either for environmental
benefits or to reduce overproduction. Since 1992 the
0167-880900 – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 8 0 9 9 9 0 0 1 0 4 - 8
238 A. Bischoff, E.-G. Mahn Agriculture, Ecosystems and Environment 77 2000 237–246
European Union has supported a change to environ- mentally more appropriate cropping systems through
financial compensation EU-Directive No. 207892. With the exception of field margin programs, which
were introduced in Germany during the early eight- ies Schumacher, 1980; Otte et al., 1988, little is
known about the regeneration of weed communities following extensification and the factors influenc-
ing this process. The programs were successful at low-yielding sites but often failed in regions with pro-
ductive soils, that had been cultivated intensively for a long time Ritschel-Kandel, 1988; Oesau, 1991. One
reason for the failure in re-establishing species-rich weed communities could be a remaining soil nitrogen
supply caused by former fertilization. Nitrogen avail- ability distinctly affects weed populations Haas and
Streibig, 1982; Mahn, 1988; Hilbig and Bachthaler, 1992. Another problem is the local extinction of sev-
eral weeds even in the soil seed bank Albrecht, 1989; Oesau, 1991. Diaspore sources are often far away
and re-establishment requires long distance dispersal.
In the course of an integrated research project STRAS; see Körschens and Mahn, 1995 investiga-
tions were carried out on representative arable fields of the Central German Chernozem Region character-
ized by fertile soils and high intensity of agriculture. They focus on two questions to analyse the regenera-
tion of weed communities following extensification:
1. What is the effect of nitrogen? 2. To what extent can dispersal limit the re-
establishment? The effect of nitrogen was investigated on a field
regeneration field that had been used to deposit large quantities of farmyard manure and slurry from
a nearby large-scale livestock farm until 1983. As deposition had not been homogenous there were
large spatial differences in N-supply at the start of investigations in 1991. Parts of the field were still
heavily loaded with nitrogen, in others soil nitro- gen content had decreased to a level allowing for
characteristic arable weeds to regrow. Before 1990, herbicides regularly applied for weed control pre-
vented re-establishment. Investigations on a nearby long-term experimental field provided information
about the potential weed community that could be expected under extensive cropping. In this field no
weed control was carried out within the last 20 years.
Table 1 Mineral soil nitrogen content kgha, 0–20 cm; from Körschens
and Mahn 1995 and Pfefferkorn
a
Date of Regeneration
Long-term experimental sampling
field field
G1 G2
N0 N1
051986 58.5
112.3 051992
15.1 18.2
16.9 57.6
071992 30.1
51.2 14.7
20.8
a
With Dr. A. Pfefferkorn’s permission to use his unpublished data.
To compare growing conditions, biomass produc- tion, light transmission and mortality of weeds were
analysed. Diaspore availability was estimated by soil seed bank analysis and by recording diaspore sources
in the surroundings. Lithospermum arvense, a com- mon species of the long-term experimental field, was
artificially introduced on the regeneration field to obtain information about dispersal and establishment.
2. Materials and methods
