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
3.1. Biomass production and competition for light Total above-ground biomass production was posi-
tively related to nitrogen availability Fig. 1. On the long-term experimental field the positive effect of fer-
tilization was significant in each year. In 1993 and
Fig. 2. Relative transmission photosynthetically active radiation at the ground level on the regeneration field G1, G2 and on the
long-term experimental field N0, N1 at the time of maximum vegetation coverage; with 95 confidence intervals; a: N1 differs
significantly from N0, b: G2 differs significantly from G1, ns: no significant difference; p 0.05.
1994 biomass was significantly greater in G2- than in G1-plots, but in 1992 there was no significant ni-
trogen effect on the regeneration field. Differences between G1 and N1 were small indicating a similar
N-availability in both.
The relationship
between N-availability
and biomass of the weed community was less strong.
In 1992 both fields and 1993 only regeneration field weed biomass was higher in plots with a large
N-supply, but differences were only significant in 1992. In 1994 and 1993 on the long-term experimen-
tal field productivity of the weed community was lower in high N-plots.
High soil nitrogen supply led to a reduced PAR at the soil surface and to an increased competition
for light Fig. 2. Transmission of light was strongly related to total above-ground biomass production.
PAR-values at the time of maximum vegetation cov- erage differed extremely between the years. In the
maize crop 1993 only 1 of the above canopy ra- diation reached ground level in N1, G1 and G2-plots.
Transmission values of winter wheat were about 10 in the same plots.
3.2. Mortality and seed production of weeds Seedling emergence mostly increased with N-
availability Fig. 3. On the regeneration field num- ber of seedlings was higher in G2- than in G1-plots,
but the difference was not significant in 1993. On the long-term experimental field the positive nitrogen
A. Bischoff, E.-G. Mahn Agriculture, Ecosystems and Environment 77 2000 237–246 241
Fig. 3. Dynamics of the weed density plantsm
2
during three vegetation periods and its dependency on N-supply; significant difference of maximum values: p 0.05, p 0.01, p 0.001, ns: not significant.
Table 2 Survival rates and seed production of surviving Chenopodium
album and Lithospermum arvense individuals 1992 Winter
1993 Maize 1994 Spring
wheat barley
Survival Seeds
Survival Seeds
Survival Seeds
Plant Plant
Plant Chenopodium album
N0 0.97
81.7 0.36
3660.7 0.63
3.1 N1
0.97 316.0
a
0.28 966.0
a
0.58 20.0
G1 0.10
3.8 0.40
314.7 0.13
1.2 G2
0.05 21.1
0.05
b
5.0
b
0.13 0.1
Lithospermum arvense N0
1.00 356.1
1.00 278.3
1.00 18.0
N1 0.97
478.1
a
0.92 92.8
a
0.89 35.0
a a
N1 differs significantly from N0.
b
G2 differs significantly from G1, p 0.05.
effect was significant in 1992; in 1994 the high- est weed density was observed in the unfertilized
N0-plots. In N-rich plots a smaller proportion of emerged weeds survived until the end of vegetation
period. Detailed demographic results were obtained for C. album and L. arvense. High N-supply increased
mortality of C. album and L. arvense populations on both fields Table 2, but the majority of differences
was not significant. Despite a lower survival rate, fe- cundity of the surviving plants was sometimes higher
in N1 and G2-plots than in N0 and G1-plots. Seed production of L. arvense was significantly increased
by nitrogen in 1992 and 1994, but reduced in 1993. Nitrogen effect on the seed production of C. album
differed largely between the years. In 1992 the rela- tionship between N-availability and number of seeds
per plant was positive; in 1993 it was negative and in 1994 results of the long-term experimental field and
the regeneration field were contrary.
3.3. Species composition In Table 3 the mean values of three single years
are presented to compare the species composition of above-ground vegetation and soil seed bank. The ef-
fect of N-fertilization N0 versus N1 and of differ- ences in soil N-content G1 versus G2 differed with
species. Nitrophilous weeds with high indicator values for N Ellenberg, 1992 gained from a high N-supply,
particularly Galium aparine, Solanum nigrum, Urtica urens. The majority of species with low indicator val-
ues showed a clearly reduced coverage and soil seed bank, such as Euphorbia exigua, Anagallis arvensis
and Silene noctiflora. Despite low indicator values coverage of Descurainia sophia and Lithospermum
arvense was higher in N-rich plots.
Remarkably, many weeds, frequent in the N1-plots, did not occur in the G1-plots, although soil nitrogen
level was nearly the same. Weeds described as char- acteristic for the Central German Chernozem Region
Hilbig, 1973 such as Consolida regalis, Euphorbia exigua, Lithospermum arvense, Papaver rhoeas,
Silene noctiflora and Veronica polita were absent or
242 A. Bischoff, E.-G. Mahn Agriculture, Ecosystems and Environment 77 2000 237–246
Table 3 Species composition of the regeneration G1, G2 and long-term experimental field N0, N1; coverage and soil seed bank 10 cm deep
1992–1994 average values
a
Indicator Coverage
c
Soil seed bank m
2
value: N
b
N0 N1
G1 G2
N0 N1
G1 G2
Species occurring only or much more frequently on the long-term experimental field Euphorbia exigua
4 0.9
0.2 .
. 52
. .
. Lithospermum arvense
5 1.0
1.6 .
. .
5 .
. Consolida regalis
5 0.2
r .
. .
. .
. .
Anagallis arvensis 6
0.7 .
. .
31 31
. .
Chaenorhinum minus 5
r .
. .
. .
. .
Veronica polita 7
6.7 6.8
r 620
692 .
. Veronica hederifolia
7 7.2
5.0 r
145 114
r Papaver rhoeas
6 5.4
4.1 r
2294 1671
r Sinapis arvensis
6 0.2
0.1 r
. 47
. .
Silene noctiflora 5
5.7 2.8
r r
150 72
. .
Viola arvensis –
10.7 7.3
r r
951 558
. .
Euphorbia helioscopia 7
2.3 2.2
r r
31 21
43 .
Polygonum lapathifolium 8
2.6 3.7
0.1 r
31 72
86 .
Galium aparine 8
0.5 5.5
0.1 0.1
16 26
. 26
Lamium amplexicaule 7
3.2 3.2
0.2 r
258 362
52 26
Cirsium arvense 7
11.3 5.2
0.3 .
r .
. Fallopia convolvulus
6 29.3
27.8 0.4
0.5 98
134 r
Species occurring only or much more frequently on the regeneration field Rumex crispus
6 .
. 0.6
3.6 .
. 26
95 Atriplex patula
7 .
. 0.8
r .
. .
. Malva neglecta
9 .
. 0.3
r .
. .
. Urtica dioica
9 .
. 0.3
. .
. 164
. Ballota nigra
8 .
. 0.2
r .
. .
. Arctium tomentosum
9 r
0.2 0.1
. .
r Artemisia vulgaris
8 r
r 2.8
2.3 .
. 611
327 Urtica urens
8 r
r 0.3
1.2 r
78 629
Thlaspi arvense 6
r r
4.1 6.2
r 284
586 Matricaria maritima
8 0.1
r 1.5
0.4 5
198 155
Sonchus oleraceus 8
0.2 r
1.8 1.1
5 5
95 69
Solanum nigrum 8
0.9 7.2
14.0 28.8
62 698
2093 Species occurring on both fields
Chenopodium album 7
15.3 12.7
4.5 3.1
1116 1183
629 560
Descurainia sophia 6
5.0 8.0
0.8 6.1
171 207
586 1567
Chenopodium ficifolium 7
4.7 6.8
4.4 3.4
868 1721
1679 2110
Amaranthus retroflexus 7
0.5 2.2
1.8 0.5
10 52
379 215
Stellaria media 8
0.1 1.0
0.6 0.6
196 227
52 26
Fumaria officinalis 7
0.7 1.9
0.1 r
. .
17.2 .
Polygonum aviculare 6
0.8 0.7
0.4 0.7
10 36
. .
Lactuca serriola 4
0.9 0.7
0.2 0.1
r .
. Sonchus arvensis
– 1.4
r r
r 10
. .
. Capsella bursa-pastoris
6 0.1
r 0.1
0.1 5
. 26
103 Mercurialis annua
8 0.2
r r
r .
. .
. Chenopodium hybridum
8 r
0.2 r
r .
. .
. Echinochloa crus-galli
8 r
r r
0.2 r
. 26
Taraxacum officinale 8
0.2 r
r .
. .
. .
Hyoscyamus niger 9
0.1 .
r r
r .
.
a
Additional species occurring sporadically without crops: On both fields: Achillea millefolium, Agropyron repens, Arctium minus, Atriplex nitens, Avena fatua, Conyza canadensis, Epilobium adnatum, Galium spurium, Galinsoga ciliata, Galinsoga parviflora, Lamium
purpureum, Poa annua, Poa pratensis, Rumex obtusifolius, Senecio vulgaris, Setaria viridis, Sisymbrium altissimum, Sisymbrium loeselii, Sonchus asper, Trifolium repens, Trifolium pratense, Veronica persica; Only on the long-term experimental field: Anethum graveolens,
Apera spica-venti, Diplotaxis tenuifolia, Euphorbia peplus, Myosotis arvensis, Papaver dubium, Salsola kali, Senecio vernalis, Veronica agrestis; Only on the regeneration field: Anthriscus caucalis, Atriplex tatarica, Brassica napus, Cirsium vulgare, Datura stramonium,
Geranium pusillum, Lepidium ruderale, Medicago sativa, Plantago major, Sambucus nigra, Sisymbrium officinale.
b
See Ellenberg 1992: Scale from 1 to 9 indicator of sites from extremely poor to rich in available nitrogen.
c
calculated from vegetation relev´es, r: coverage 0.1, r not in the experimental plots but sporadically within the fields.
A. Bischoff, E.-G. Mahn Agriculture, Ecosystems and Environment 77 2000 237–246 243
Table 4 Species composition of the different plots compared by qualitative
Sørensen and quantitative Czekanowski similarity indices; N: average value of long-term experimental field, G: average value
of regeneration field
Sørensen-Index Czekanowski-Index
1992 1993
1994 1992
1993 1994
N0–N1 79.5
86.5 73.0
82.4 74.7
49.7 G1–G2
74.6 80.0
70.8 62.5
63.4 76.0
N–G 64.4
66.7 66.7
18.0 27.3
15.9 N1–G1
57.0 71.1
53.3 10.8
39.9 23.7
extremely rare on the regeneration field. Instead, ni- trophilous weeds dominated both G1 and G2 plots.
Similarity indices, especially the quantitative one Czekanowski, showed a low similarity between N1
and G1 plots compared with N0 and N1 or G1 and G2 Table 4. Species composition depended much more
on the experimental field than on nitrogen supply.
3.4. Availability and dispersal of seeds Differences in nitrogen cannot entirely explain dif-
ferences in species composition. Repeated sampling of the seed bank and detailed above-ground vegetation
analysis between 1992 and 1994 indicated that many characteristic weeds of Central German Chernozem
Region had no soil seed bank at all on the regenera- tion field e.g. Consolida regalis, Euphorbia exigua,
Lithospermum arvense, or seed number in the soil was too small to establish stable populations e.g.
Papaver rhoeas, Silene noctiflora, Veronica polita, see Table 3.
Fig. 4 shows the distribution of seed sources of ‘missing’ species in the neighbourhood of the regen-
eration field. Findings were divided into small groups of 10 or less individuals and larger ones of 50 or more
plants. Populations of intermediate size were recorded as several small ones. All species common on the
long-term experimental field but absent from the re- generation field were found in the surroundings. The
populations were often small and confined to the field margins. Single individuals of Silene noctiflora, Con-
solida regalis and Lithospermum arvense were found in a distance of 50–200 m. Papaver rhoeas was quite
common around the regeneration field. The species was not plotted in Fig. 4 because it was recorded at
36 locations. Larger populations of all investigated species were not found within 300 m.
The artificial introduction of Lithospermum arvense seedlings on the regeneration field in 1993 provides
information about seed dispersal Fig. 5. The plants were competitive and produced 490 seeds per indi-
vidual. Altogether 7500 viable seeds reached the soil. Two months after release, diaspores were only found in
the nearest seed traps maximum distance 0.5 m. Har- vest of the crop terminated the dissemination analysis
by seed traps. Until 1994 only one seedling emerged. Another 26 seedlings were recorded in the spring of
1995, all within a maximum distance of 2.5 m from the source. The seedlings did not attain the reproduc-
tive stage because the subsequent tillage for sowing maize destroyed them. No further seedlings appeared
until autumn 1997.
4. Discussion
