**2.1 Eexperimental design**

Memuro continuous cropping experiment (42°53' N, 143°04' E) was conducted from 1980 to 1995. Soybean (*Glycine max*), adzuki bean (*Vigna angralis*), sugar beet (*Beta valgaris*), potato (*Solanum tuberosum*) and spring wheat (*Triticum aestivum*) were cultivated in the experiments. The eleven plots were established in each crop (Fig. 1). One plot was rotation plot, and other plots were continuous cropping plots (Table 1). Soybean was cultivated only chemical fertilizer in rotation plot (R) and the control plot in continuous cropping plots (C0). Wheat straw manure was applied in every year from 1980 at 15, 30, 50 t/ha, respectively (W15, W30, W50). Burk compost was applied in every year from 1981 at 15, 30, 50 t/ha, respectively (B15, B30, B50). D-D (1,3-dichloropropene) was applied from 1990 as a soil fumigation. In the soil fumigation plots, wheat straw manure was applied in every year from 1980 at 0, 15,30t/ha (F0, F15, F30).

(Neve et al., 2004). Soil fumigation sterilizes Fungus (Asano et al., 1983) and nematodes. This experiment is a good example to study the influence of the soil microbe on soybean. About a

C0 W15 W30 W50 B15 B30 B50 F30 F15 F0 R

Sugar beet

W15

B50

W30

F30

W50

F15

B15

F0

F0

B15

F0

W50

F15

F15

W30

F30

F30

W15

B50

B50

C0

B30

R

R

R R

The cropping sequence of rotation plot is sugar beet – Potato - Adzuki bean - Spring wheat - Soybean.

Memuro continuous cropping experiment (42°53' N, 143°04' E) was conducted from 1980 to 1995. Soybean (*Glycine max*), adzuki bean (*Vigna angralis*), sugar beet (*Beta valgaris*), potato (*Solanum tuberosum*) and spring wheat (*Triticum aestivum*) were cultivated in the experiments. The eleven plots were established in each crop (Fig. 1). One plot was rotation plot, and other plots were continuous cropping plots (Table 1). Soybean was cultivated only chemical fertilizer in rotation plot (R) and the control plot in continuous cropping plots (C0). Wheat straw manure was applied in every year from 1980 at 15, 30, 50 t/ha, respectively (W15, W30, W50). Burk compost was applied in every year from 1981 at 15, 30, 50 t/ha, respectively (B15, B30, B50). D-D (1,3-dichloropropene) was applied from 1990 as a soil fumigation. In the soil fumigation plots, wheat straw manure was applied in every year

C0

B30

B30

Rotation Potato

subject picked up in this experiment, the knowledge of past were surveyed.

Continuous cropping ○○ ○ ○ ○ ○ ○ ○ ○○ Rotation ○

Soil fumigation ○○○

Wheat straw manure 15 30 50 30 15

Burk compost 15 30 50

Table 1. Treatments in Memuro continuous cropping experiments.

10m 3m 5m

Spring wheat Spring wheat

B15

F0

B15

B15

F0

Fig. 1. The treatment plots in Memuro continuous cropping experiment.

\* The application rate of organic matter is expressed in t/ha. Soil fumigation (D-D) was applied from 1990 to 1995.

Adzuki bean

W15

B50

W15

W15

B50

Soybean

**2.1 Eexperimental design** 

from 1980 at 0, 15,30t/ha (F0, F15, F30).

C0

B30

C0

2.7m

 4.7m

 5.2m

B30

C0 R

W50

F15

W50

W50

F15

W30

F30

W30

W30

F30

**2. Memuro continuous cropping experiment** 

#### **2.2 The effect of continuous cropping to soybean cyst nematode (SCN)**

The time course changes of egg density of SCN were showed in Fig. 2A. Closed symbols showed the value of rotation plot, and open symbols showed that of continuous cropping plots. The nematode susceptibility cultivar "Kitami-shiro" was used from 1980 to 1991, and nematode-resistant cultivar "Toyo-musume" was used from 1992. D-D was applied from 1990. In the continuous cropping plots, the egg density of SCN tended to decrease from 1985. Thereafter, the egg density in the rotation plot was higher than that in the continuous cropping plots.

○ : C0, M15, M30, M50, □ : B15, B30, B50, △ : F0, F15, F30, ● : R. Graph A: The changes of all plots. Graph B: The changes of plots in which the egg density peaked at 1981.

Graph C: The changes of plots in which the egg density peaked at 1984. The unit of the egg density of SCN is number / g dry soil.

Fig. 2. The time course changes of egg density of SCN.

Farming System and Management 255

Soybean yield was measured from 1980 to 1995. The time course changes of yield of R and C0 plots were showed in Fig. 4A. In Hokkaido, soybean yield decrease by cool summer damage. It was a cool summer in 1983 and 1993, and soybean yield decreased. To examine the effects of continuous cropping and organic matter application, analysis of variance (ANOVA) was conducted for the soybean yield data. First, the yield of continuous cropping plots except soil fumigation plots (C0, W15, W30, W50, B15, B30, B50) were converted into the index by the yield of rotation plot (R). For the index of the continuous cropping + organic matter application plots (W15, W30, W50, B15, B30, B50), ANOVA was conducted as treatment replication (Fig. 4B). The solid line is a value of 1980, and the dotted line is solid line - least significant difference. In the year when index significantly decreased than that of 1980, soybean yield probably decreased by continuous cropping. For reference, the indexes

The mean of indexes of organic matter application plots did not decrease significantly except 1983. However, the indexes of C0 of 1983~1985 were lower than a dotted line. This time was almost coincided with the time when the egg density of SCN increased. The indexes of C0 increased again afterwards. The egg density of SCN decreased, too. Therefore, it is suggested that the yield decrease of continuous cropping was influenced by SCN. The

For the soybean yield data, ANOVA was conducted as year replication. Using a yield of all plots except soil fumigation plots (C0, W15, W30, W50, B15, B30, B50, R) of 1981~1995, the effects of continuous cropping and organic matter application were investigated. Using a yield of all plots in 1990~1991, the effects of continuous cropping, organic matter application and D-D on nematode susceptibility cultivar "Kitami-shiro" were investigated. Using a yield of all plots in 1992~1995, the effects of continuous cropping, organic matter application and

By ANOVA for the data of 1981-1995, yield decreased significantly by continuous cropping, and the decrease was approximately 20% (Fig. 5A). Organic matter application increased yield. By ANOVA for soil fumigation period (Fig. 5B, 5C), yield did not decrease significantly by continuous cropping. However, the trend in Fig. 5A was similar in those figures. Yield decreased by continuous cropping, and increased by organic matter application. By D-D, yield increased at the same level as the rotation plot. D-D might remove the effect of continuous cropping as a nematocide. However, the egg density of SCN declined before D-D application period. The effect of D-D was found to "Toyo-musume" that was nematode resistant variety. Therefore, it was suggested that D-D influenced the factor

SCN forms the cyst containing a large number of eggs (Ichinohe, 1955a). Two or three generations of SCN can grow up in the soybean growing period of Hokkaido (Ichinohe, 1955a). SCN inhibits rhizobial adherence, too (Ichinohe 1955a). The damage of SCN is most remarkable if SCN invaded to soybean at 2-3 weeks after sowing (Okada, 1968). The damage of SCN is reduced by fertilization (Okada, 1966). SCN reduces the growth of soybean, but

**2.3 The effects of treatments to soybean yield** 

organic matter application probably increased soybean yield.

D-D on nematode resistant cultivar "Toyo-musume" were investigated.

**3. Factors to influence continuous cropping soybean** 

**3.1 Soybean cyst nematode (SCN)** 

**2.3.1 Time course change** 

of C0 were shown.

**2.3.2 Treatment effects** 

except SCN.

■:plots in graph B, □:plots in graph C.

Fig. 3. The placement of soybean continuous cropping plots in Graph B and C.

Fig. 2B and 2C show the time course changes of egg density of SCN in the continuous cropping plots. Fig. 2B shows the plots which the egg density of SCN was highest in 1981- 1982. Fig. 2C shows the plots which the egg density of SCN was highest in 1984. The placement of each plots are showed in Fig. 3. The closed squares show the plots in Fig. 2B, and the open squares show the plots in Fig. 2C. The plots which belonged in Fig. 2B were located in the south side of experiment field, and the plots which belonged in Fig. 2C were located in the north side. The difference of time course changes of egg density probably depended on the position of plots. In both plots, the egg density of SCN decreased by 2 - 5 years continuous cropping.

Graph A: The changes in yield of C0 and R. ○ : C0, ● : R.

Graph B: The time course changes in yield index.

□ : C0 / R × 100. ◇ : The means of (W15, W30 W50, B15, B30 or B50) / R × 100,

Solid line: the value of ◇ in 1980.

Dotted line: solid line - least significant difference from Stutentized range. \*\* is significantly in 1%.

Fig. 4. The time course changes of yield and yield index of soybean.

Fig. 2B and 2C show the time course changes of egg density of SCN in the continuous cropping plots. Fig. 2B shows the plots which the egg density of SCN was highest in 1981- 1982. Fig. 2C shows the plots which the egg density of SCN was highest in 1984. The placement of each plots are showed in Fig. 3. The closed squares show the plots in Fig. 2B, and the open squares show the plots in Fig. 2C. The plots which belonged in Fig. 2B were located in the south side of experiment field, and the plots which belonged in Fig. 2C were located in the north side. The difference of time course changes of egg density probably depended on the position of plots. In both plots, the egg density of SCN decreased by 2 - 5

<sup>A</sup> Kitami-shiro

1980 1985 1990 1995

1980 1985 1990 1995

Toyomusume

Year

Year

Fig. 3. The placement of soybean continuous cropping plots in Graph B and C.

0

50

Yield index (%)

Graph A: The changes in yield of C0 and R.

Solid line: the value of ◇ in 1980.

Graph B: The time course changes in yield index.

◇ : The means of (W15, W30 W50, B15, B30 or B50) / R × 100,

Dotted line: solid line - least significant difference from Stutentized range.

Fig. 4. The time course changes of yield and yield index of soybean.

○ : C0, ● : R.

□ : C0 / R × 100.

\*\* is significantly in 1%.

100

150

B\*\*

Yield (t/ha)

■:plots in graph B, □:plots in graph C.

years continuous cropping.

#### **2.3 The effects of treatments to soybean yield 2.3.1 Time course change**

Soybean yield was measured from 1980 to 1995. The time course changes of yield of R and C0 plots were showed in Fig. 4A. In Hokkaido, soybean yield decrease by cool summer damage. It was a cool summer in 1983 and 1993, and soybean yield decreased. To examine the effects of continuous cropping and organic matter application, analysis of variance (ANOVA) was conducted for the soybean yield data. First, the yield of continuous cropping plots except soil fumigation plots (C0, W15, W30, W50, B15, B30, B50) were converted into the index by the yield of rotation plot (R). For the index of the continuous cropping + organic matter application plots (W15, W30, W50, B15, B30, B50), ANOVA was conducted as treatment replication (Fig. 4B). The solid line is a value of 1980, and the dotted line is solid line - least significant difference. In the year when index significantly decreased than that of 1980, soybean yield probably decreased by continuous cropping. For reference, the indexes of C0 were shown.

The mean of indexes of organic matter application plots did not decrease significantly except 1983. However, the indexes of C0 of 1983~1985 were lower than a dotted line. This time was almost coincided with the time when the egg density of SCN increased. The indexes of C0 increased again afterwards. The egg density of SCN decreased, too. Therefore, it is suggested that the yield decrease of continuous cropping was influenced by SCN. The organic matter application probably increased soybean yield.
