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
2.1. Study sites Study sites were located in the Central German
Chernozem Region about 15 km southwest of Halle. They were characterized by low rainfall average an-
nual precipitation: 492 mm and typical Chernozem soils.
The crop rotation on both experimental fields was: spring barley 1991 – winter wheat 199192 – maize
1993 – spring barley 1994. 2.1.1. Regeneration field
Between 1962 and 1983, large amounts of farm- yard manure and slurry from a nearby animal produc-
tion unit were deposited on the field. This fertilization ceased in 1984, whereas herbicide application contin-
ued until 1990. Afterwards the field became part of the Bad Lauchstädt experimental station of the UFZ
Centre for Environmental Research Leipzig, Halle and was divided into 105 plots of 25 m
2
5 m × 5 m. From the first soil analysis in 1986 until the final
assessment, large differences in soil nitrogen content have been recorded within the regeneration field
Table 1. Investigations were carried out in the G1- area, consisting of the six plots with the lowest soil
A. Bischoff, E.-G. Mahn Agriculture, Ecosystems and Environment 77 2000 237–246 239
nitrogen content, and in the G2-area which included the six plots of highest nitrogen availability.
The soil N-content declined remarkably since 1986. The seasonal dynamics of inorganic soil N-content
differed from fields regularly receiving mineral nitro- gen fertilizer Table 1. In the course of each veg-
etation period, the amount of inorganic nitrogen in- creased by mineralization from the organic N-pool.
Large amounts of nitrogen were not available until growth of arable plants terminated.
2.1.2. Long-term experimental field The long-term experimental field was part of the
Etzdorf experimental station of the University of Halle. It was used as a study site to investigate herbi-
cide and fertilizer effects on arable weeds since 1976. The present paper deals only with the unsprayed plots
originally serving as controls. One part of the field received 80–120 kg mineral nitrogen fertilizer N1
per hectare and year dependent on the crop species, the other one was not fertilized with nitrogen N0.
Each nitrogen level comprised five plots 10 × 10 m
2
arranged in a block design cf. Bischoff, 1996. Dur- ing the investigation period 1992–1994 the mineral
soil nitrogen content was similar in N1 and G1-plots Table 1.
2.2. Methods To determine above-ground biomass production,
crop and weeds were harvested three times a year from one circle per plot ∅ = 50 cm. The mate-
rial was separated into the individual species and weighed after drying. In two permanent quadrats per
plot each 0.5 m × 0.5 m the number of weeds was assessed five times a year and the percentage of re-
productive plants was estimated. At three locations per plot photosynthetically active radiation PAR
was measured near soil surface using a line quantum sensor LI-COR, LI-191SA, length 1 m. Relative
transmission was calculated from radiation values in and above canopy of the above canopy radiation.
To determine soil seed bank, eight soil samples per plot were taken 10 cm deep with a corer of 3.7 cm
diameter. Four of them were mixed to provide two composite samples per plot. They were placed in
plastic boxes under outdoor conditions and seedling emergence was recorded Kropac, 1966. Vegetation
analysis of the whole plot area was carried out us- ing the Braun–Blanquet-method ‘vegetation relevé’
modified by Barkman et al. 1964. 50–100 individuals of Chenopodium album and
Lithospermum arvense were marked in the different plots for several studies in population biology. C.
album was common in both fields. L. arvense being previously absent on the regeneration field was arti-
ficially introduced in 1993 by planting 45 juvenile individuals at three leave-stage of maize. The plants
were positioned in the western margin of the field G1-area to observe dispersal into the crop stand.
The number of seeds was assessed and viability was analysed by Tetrazolium-Test. Dispersal was studied
by collecting seeds in traps of 10 cm diameter until crop harvest and by recording the following seedling
emergence. The traps were arranged in 0.5 m intervals east of the source plants cf. Fig. 5. The field was
ploughed in November.
The distribution of species absent on the regener- ation field Chaenorhinum minus, Consolida regalis,
Euphorbia exigua, Lithospermum arvense, Papaver rhoeas, Silene noctiflora and Veronica polita was
investigated in a 1 km radius.
To analyse nitrogen content one soil sample depth = 0–20 cm per plot was taken three times a
year. Mineral nitrogen was colorimetrically assessed by flow injection analysis. Detailed methods and
results are published in Körschens and Mahn 1995.
2.3. Analysis To compare species composition of the differ-
ent plots, two indices of similarity were calculated. The Sørensen-Index is qualitative and considers
only the occurrence of species Goodall, 1973. The Czekanowski-Index, also called quantitative
Sørensen-Index, is calculated on the basis of the mean coverage.
Sørensen : 2c
a + b ×
100 where a, b is the number of species only occurring in
relevé a, b and c is the number of species occurring in both relevés.
Czekanowski : 2ma
ma + mb ×
100
240 A. Bischoff, E.-G. Mahn Agriculture, Ecosystems and Environment 77 2000 237–246
Fig. 1. Total biomass and biomass of weeds black on the regener- ation field G1, G2 and on the long-term experimental field N0,
N1 at the time of maximum vegetation coverage; with 95 con- fidence intervals; a: N1 differs significantly from N0, b: G2 differs
significantly from G1, ns: no significant difference; p 0.05.
where ma, mb is the total coverage of all species oc- curring in relevé a, b and mc is the total coverage
of species occurring in both relevés for each species minimum value of relevé a and b.
For calculation of mean coverage values Braun– Blanquet levels were converted into percentage of
coverage. Braun–
r +
1 2a 2b 3
4 5
Blanquet- level
coverage 0.01 0.2 2.5 10 20 37.5 62.5 87.5
Normal distribution and equality of variances were analysed by Kolmogorov–Smirnov- and Levene-test.
The results did not necessitate transformation of the presented data before analysis. Differences between
the plots of the long-term experimental field were eval- uated by ANOVA factors: block and fertilization.
Differences between G1- and G2-plots of the regener- ation field were analysed by a simple t-test. Chi-square
test was performed to compare frequencies.
3. Results
