132 L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142
sively applied to the experiment plots once a year before planting. Apart from this, a moderate amount
of chemical fertilizer was applied according to soil test recommendations: fertilizer N urea at a rate
of 86 kg ha
− 1
and fertilizer P superphosphate at a rate of 54 kg ha
− 1
. No potassium was applied. The fallow crops were fertilized with N urea at a rate
of 69 kg ha
− 1
in broomcorn millet and P superphos- phate at a rate of 33 kg ha
− 1
in both soybean and common vetch. Other field management practices
were identical to those employed by local farmers.
2.3. Measurements Throughout the experiment, all crops were har-
vested manually to determine grain yield and above- ground biomass production grain yield plus crop
residue. For each rotation system, grain and biomass yields were determined for each crop. Soil water con-
tent was monitored in the following depths: 0−0.1, 0.1−0.2, 0.2−0.3, 0.3−0.5, 0.5−0.75, 0.75−1.0,
1.0−1.5 and 1.5−2.0 m for each plot at 10-day in- tervals with a neutron moisture meter throughout
the experiment period. Soil bulk densities in these layers were measured twice: one at the beginning
and another at the end of the experiment using sam- ples obtained with an auger. Precipitation data were
recorded at the meteorological station in the study site. A more detailed description of the experimental
design and treatments is given by Li and Gao 1994.
2.4. Data analysis Actual evapotranspiration ET for each crop, de-
fined as the amount of precipitation for the period be- tween sowing and harvesting the particular crop plus
or minus the change in soil water storage in the 2 m soil profile, was computed by the soil water balance equa-
tion Xin, 1986; Zhu and Niu, 1987. Runoff was es- timated using a simple model proposed by Forest see
Lhomme, 1991. According to one study in the same region Xin and Zhao, 1992, the maximum penetra-
tion of rainfall in a non-cropped field in a wet year is about 2.5 m but less than 2 m in a cropped field. Hence,
percolation loss below 2 m can be assumed negligi- ble. Studies have indicated that the amount of water
obtained by crops from the movement of groundwater upwards approaches zero when the level of ground-
water is below 4 m Zhang, 1980; Yuan, 1984. There- fore, this variable was also assumed negligible because
the groundwater level in the study area is below 10 m. Water-use efficiency WUE, expressed as grain yield
or biomass production per unit ET, was first calcu- lated for each crop, then the value for each rotation
system was determined based on data of individual crops. To analyze quantitatively the seasonal variation
in soil water storage and utilization, the relative de- gree of utilization of soil water storage for crops was
determined using the following formula:
RDU = SWS
s
− SWS
h
SWS
s
× 100
where RDU is the relative degree of utilization of soil water storage, SWS
s
is the amount of soil water stor- age at sowing and SWS
h
is the amount of soil water storage at harvest. The numerator i.e. SWS
s
− SWS
h
of the equation would allow the estimation of the net soil water storage consumed by crops within a partic-
ular time interval. In addition, the Dunnett-type test of Levy 1975
was employed to determine the differences between the experimental control and other rotation systems.
Simple linear regression analysis was used to deter- mine the relationships between evapotranspiration and
growing-season precipitation.
3. Results and discussion
3.1. Yield, ET and WUE of individual crops in the rotation
There were considerable variations in yield, ET and WUE among years Table 2 . For example, the
ET value of wheat was 390, 445 and 471 mm for the three cropped seasons, respectively, with an aver-
age of 435 mm. In one study at the same region, Pu 1992 reported mean ET value of 446 mm in wheat,
being close to the present study. The ET value of corn was 403, 475 and 365 mm for the 3 years, with
an average of 414 mm. Rhoades and Bennett 1990 reviewed experiments from different climatic regions
and reported ET values of corn ranging from 375 to 964 mm. The present data were within the range of
these values. Average WUE values over the 3 years
L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142 133
Table 2 Yield, evapotranspiration ET and water-use efficiency WUE for crops grown in the rotation systems the value for each crop is an
average over two replicates Crop
SWS at sowing SWS at harvest
ET Growing-season
Yield WUE
mm, 2 m mm, 2 m
mm rainfall mm
Mg ha
− 1
kg m
− 3
Grain Biomass
Grain Biomass
Eight rotation crops Winter wheat
1988–1989 464
365 390
291 3.27
9.06 0.84
2.32 1989–1990
446 397
445 396
5.39 14.89
1.21 3.76
1990–1991 478
408 471
401 3.04
8.35 0.65
1.77 Mean
463 390
435 363
3.90 10.77
0.90 2.62
Winter rapeseed 1988–1989
460 354
470 364
1.45 3.28
0.31 0.70
Corn 1989
388 346
403 361
9.29 18.83
2.31 4.67
1990 397
446 475
524 5.99
12.43 1.26
2.62 1991
379 342
365 328
7.30 14.13
2.00 3.87
Mean 388
378 414
404 7.53
15.13 1.86
3.72 Potato
a
1989 402
421 310
329 7.48
9.93 2.41
3.20 1990
425 441
508 524
8.42 10.04
1.66 1.98
1991 379
388 320
329 4.85
6.52 1.52
2.04 Mean
402 417
379 394
6.91 8.83
1.86 2.41
Millet 1989
372 357
319 304
6.67 13.33
2.09 4.18
1990 380
433 488
541 3.66
12.11 0.75
2.48 1991
363 340
325 302
4.73 12.49
1.45 3.84
Mean 372
377 377
382 5.02
12.64 1.43
3.50 Sorghum
1990 372
444 551
623 8.47
17.79 1.54
3.23 1991
425 411
366 352
5.83 15.91
1.59 4.35
Mean 399
428 459
488 7.15
16.85 1.57
3.79 Soybean
1990 342
432 434
524 2.46
5.68 0.57
1.31 1991
360 363
316 319
1.31 3.62
0.41 1.15
Mean 351
398 375
422 1.88
4.65 0.49
1.23 Flax
1990 402
434 313
345 1.35
6.60 0.43
2.11 1991
413 423
281 291
1.42 4.20
0.51 1.49
Mean 408
429 297
318 1.39
5.40 0.47
1.80 Three fallow crops
Common vetch 1989
425 452
186 213
2.76 1.48
1990 395
445 222
272 4.41
1.99 1991
409 421
121 133
3.36 2.78
Mean 410
439 176
206 3.51
2.08 Broomcorn millet
1989 383
425 150
192 1.49
5.90 0.99
3.93 1990
402 446
211 255
1.83 6.22
0.87 2.95
Mean 393
436 181
224 1.66
6.06 0.93
3.44 Soybean
1990 395
445 214
264 2.38
6.33 1.11
2.96
a
5 kg freshpotato seeds are equal to 1 kg grain yield; SWS, soil water storage.
134 L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142
were 0.90 kg m
− 3
for wheat and 1.86 kg m
− 3
for corn. By comparison, the wheat value was lower than the
value 1.48 kg m
− 3
reported by Zhu et al. 1994 in a sub-humid region of north China but the corn value
was very close to the value 1.94 kg m
− 3
reported by Zhu et al. 1994. These data compare favourably
with values reported by other researchers. Musick and Porter 1990 found WUE values for autumn-planted
irrigated wheat ranging from 1.0 to 1.2 kg m
− 3
, but they also cited a number of studies reporting values
from 1.4 to 1.6 kg m
− 3
. Rhoades and Bennett 1990 reviewed studies on corn and reported values of
1.2 kg m
− 3
for Bushland, TX, 1.7 kg m
− 3
for Davis, CA and 1.9 kg m
− 3
for the southern Negev region of Israel. The WUE values for wheat from the present
study are in the low range of the values reported in the literature, but the WUE values for corn are in the
upper range of reported values.
In a rainfed cultivation, the water for evapotranspi- ration is furnished partly by growing-season precipi-
tation and partly from soil water stored before sowing. However, the relative contribution of soil water stor-
age to evapotranspiration defined as the percentage of soil water consumed by crops relative to evapo-
transpiration varied significantly among years. For wheat, the relative contribution was 25, 11 and 15
for the three cropped seasons, respectively, with an average of 17. The relative contribution also varied
appreciably among crops, particularly between win- ter and summer crops. Soil water storage decreased
during the wheat growing season, although soil water content remained relatively stable during the winter
months mid-November to the end of March because of temperatures 0
◦
C, low soil evaporation and the wheat plant being dormant during the period. On av-
erage, about 83 of the water consumed by wheat was derived from growing-season precipitation and
the remainder from pre-sowing soil water storage. In contrast, soil water storage tended to increase during
the summer crop growing seasons. For corn, the av- erage growing-season precipitation over the 3 years
was 404 mm and actual ET was 414 mm, with only 3 of the water consumed by corn being derived
from pre-sowing soil water storage. For potato and millet, the average growing-season precipitation over
the 3 years was 394 and 382 mm and actual ET was only 379 and 377 mm. Hence, the water consumed
by these two crops was furnished completely by growing-season precipitation. This was also the case
for the fallow crops. Because the growing periods of the fallow crops closely paralleled the rainy sea-
son July through September, their growth depends only on growing-period precipitation. In the case of
broomcorn millet, growing-period precipitation was 192 and 255 mm in 1989 and 1990 and actual ET
was 150 and 211 mm in the respective years. Con- sequently, soil water storage in both years increased
by 42 and 44 mm which was available for use by the subsequent crops.
The above result was supported by the outcome of regression analysis on the relationship between ET
and growing-season precipitation P. A non-linear re- lationship existed between ET and P for wheat ET =
241.64 × 1.0016P, R
2
= 0.96, suggesting that vari-
ation in ET was not only associated with growing- season precipitation but also related to pre-sowing
soil water storage. A close positive linear rela- tionship was found between ET and P for corn,
potato and millet ET = 118.72 + 0.6784P, R
2
= 0.87;
ET = −33.22 + 1.0315P, R
2
= 0.99; ET = 90.18 +
0.7350P, R
2
= 0.98, respectively, indicating that these
crops depend mainly on growing-season precipitation. 3.2. Seasonal and inter-annual patterns of soil water
storage and utilization The relative degree of utilization of soil water stor-
age RDU varied significantly in the dry and wet years Table 3. For wheat, the RDU values in the 0−0.5,
0.5−1.0 and 1.0−2.0 m layers were 81, 44 and 26 in the dry year and 54, 39 and 17 in the wet year at
the minimum soil water content. The dry year RDU increased by 50 0−0.5 m, 14 0.5−1.0 m and
51 1.0−2.0 m compared with the wet year.
Similar pattern can be found when comparison was made at harvest time. RDU also significantly varied
among crops. RDU was greatest in wheat, followed in descending order by corn, potato and millet in both
dry and wet years. In 1989, a dry year, wheat not only depleted 81 and 44 of soil water stored in the
0−0.5 and 0.5−1.0 m layers but also depleted 26 of soil water stored in the 1.0−2.0 m layer at the mini-
mum soil water content. At this time, however, corn depleted 61, 31 and 7 of soil water stored in the
0−0.5, 0.5−1.0 and 1.0−2.0 m layers, potato depleted 62 and 12 of soil water stored in the 0−0.5 and
L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142 135
Table 3 Seasonal patterns of soil water storage and utilization in different soil layers m for the four rotation crops in a representative dry 1989
and wet 1990 year
a
Wheat Corn
Potato Millet
0−0.5 0.5−1
1−2.0 0−0.5
0.5−1 1−2.0
0−0.5 0.5−1
1−2.0 0−0.5
0.5−1 1−2.0
SWS at sowing Dry year
140 116
214 135
127 215
145 135
246 144
134 236
Wet year 142
130 220
150 144
261 146
139 235
143 139
246 At minimum soil water content
Dry year SWS mm
26 65
158 53
88 201
55 119
267 90
137 257
RDU 81.4
44.0 26.2
60.7 30.7
6.5 62.1
11.9 –
37.5 –
– Wet year
SWS mm 65
80 182
129 131
254 116
137 267
114 145
266 RDU
54.2 38.5
17.3 14.0
9.0 2.7
20.5 1.4
– 20.3
– –
At harvest Dry year
SWSmm 29
66 162
88 104
197 116
137 265
128 146
266 RDU
79.3 43.1
24.3 34.8
18.1 8.4
20.0 –
– 11.1
– –
Wet year SWS mm
95 113
194 139
135 252
141 138
265 145
142 261
RDU 33.1
13.1 11.8
7.3 6.3
3.4 3.4
0.7 –
– –
–
a
SWS, soil water storage; RDU, relative degree of utilization of soil water storage.
0.5−1.0 m layers and millet depleted only 38 of soil water stored in the 0−0.5 m layer. These data suggest
that the water-consuming depth for wheat is deeper than 1 m, whereas the main water-consuming depth is
in the 1 m zone for corn and potato and in the 0.5 m soil profile for millet. Studies have shown that winter
wheat roots reach to 0.36 m in December, 0.83 m in January, 1.6 m in March and 1.9 m in April Zhu, Niu
and Zhao, unpublished data, as was also found in the present study.
The calculated RDU values at harvest time were significantly lower than at the minimum soil water
content in both dry and wet years. The difference was most pronounced in the three summer crops. This is
because the summer crops were grown during the pe- riod of high precipitation which occurred in the late
part of the growing season, thus resulting in increased soil water storage at harvest time. For this reason, it is
not possible to characterize the real status of soil water utilization for these crops simply by using the RDU
values calculated at harvest time as an index. A feasi- ble approach for assessing the capacity of the summer
crops to utilize soil water storage is to use a comple- mentary set of indices, i.e. not only considering the
RDU values calculated at harvest time but also con- sidering the RDU values calculated at the minimum
soil water content.
3.3. Spatial and temporal variation in water stress It is commonly accepted that the upper limit of the
optimum soil water content for plant growth is the field water-holding capacity, with the lower limit about
70 of the field water-holding capacity Pu, 1992; Yu, 1992. When soil water content is below 60
of the field water-holding capacity, it may hinder the growth and development of crops. According to the
field water-holding capacity of 0.223 kg kg
− 1
in the experiment field, a soil water content of 0.134 kg kg
− 1
i.e. approximately 60 of the field water-holding ca- pacity was set as the critical moisture content. On the
basis of this critical value, the onset, duration and end of the water stress period in different soil layers for the
above four crops were determined in both dry and wet years Table 4. To characterize the severity of water
stress, the extent of water deficit was divided into four levels: slight deficit with soil water content ranging
from 50 to 59 of the field water-holding capacity,
136 L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142
Table 4 Onset, duration and end of the water stress period in different soil layers for the four rotation crops in a representative dry 1989 and wet
year 1990 Layer m
Dry year Wet year
Duration days Emergence of minimum soil water content
Onset End
Onset End
Dry year Wet year
Dry year Wet year
Wheat 0.0−0.5
17 May 13 July
8 May 26 June
58 50
Mid-June Late-May
0.5−1.0 24 May
16 July 24 May
6 June 54
14 Late-June
Mid-May 1.0−2.0
8 June 14 July
None None
37 None
Early-July Early-June
Corn 0.0−0.5
10 June 14 August
None None
66 None
Early-August Early-July
0.5−1.0 5 August
22 August None
None 18
None Mid-August
Mid-August 1.0−2.0
None None
None None
None None
Late-August Late-August
Potato 0.0−0.5
15 June 9 August
None None
56 None
Early-July Early-July
0.5−1.0 None
None None
None None
None Early-August
Mid-July 1.0−2.0
None None
None None
None None
Late-August Late-May
Millet 0.0−0.5
7 June 2 August
None None
57 None
Mid-July Early-July
0.5−1.0 None
None None
None None
Mid-August Early-June
1.0−2.0 None
None None
None None
None Late-August
Late-May
moderate deficit with soil water content ranging from 40 and 49 of the field water-holding capacity, se-
vere deficit with soil water content ranging from 30 to 39 of the field water-holding capacity and ex-
treme deficit with soil water content below 30 of the field water-holding capacity. For wheat, water stress
was observed not only in the dry year but also in the wet year. In the dry year, water stress in the 0−0.5,
0.5−1.0 and 1.0−2.0 m layers emerged respectively on 17, 24 May and 8 June and ended on 13, 16 and 18
July, lasting for about 58, 54 and 37 days. In the wet year, water stress occurred from 8 May to 26 June in
the 0−0.5 m layer and from 24 May to 6 June in the 0.5−1.0 m layer, lasting for about 50 and 14 days. For
the three summer crops, water stress occurred only in the dry year. The periods of water stress took place
from 10 June to 14 August in the 0−0.5 m layer and from 5 to 22 August in the 0.5−1.0 m layer for corn
and from early June to early August in the 0−0.5 m layer for potato and millet.
Winter wheat experienced the most severe water stress among four crops. At sowing, soil water content
in all three layers remained above 60 of the field water-holding capacity for any crop in either the dry or
the wet year. At the minimum soil water content, soil water content in the 0−0.5, 0.5−1.0 and 1.0−2.0 m
layers for wheat was at extreme, moderate and slight deficit, respectively, in the dry year and at moderate
and slight deficit in the 0−0.5 and 0.5−1.0 m lay- ers in the wet year. At this time, however, only in
the dry year was soil water content in the 0−0.5 and 0.5−1.0 m layers at severe and slight deficit for corn
and soil water content in the 0−0.5 m layer at severe deficit for potato and millet. At harvest, only in the
dry year was soil water content in the 0−0.5, 0.5−1.0 and 1.0−2.0 m layers at extreme, moderate and slight
deficit for wheat, but not for the other crops.
3.4. Agronomic performance and water use patterns for different rotation systems
The 16 rotations without fallow crops were signif- icantly p 0.05 greater in annual grain yield and
WUE for grain production than wheat monoculture. Among the 16 rotations with fallow crops, seven pat-
terns produced more p 0.05 grain yield and five patterns had greater p 0.05 WUE for grain pro-
duction than wheat monoculture Table 5. Cropping systems showed a marked increase in evapotranspira-
L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142 137
Table 5 Average annual grain and biomass yields and water-use efficiency WUE for different rotation systems in comparison with wheat
monoculture W−W−W
a
Patterns Grain yield Mg ha
− 1
WUE kg m
− 3
Biomass yield Mg ha
− 1
WUE kg m
− 3
Amount increase Amount increase
Amount increase Amount increase
Winter monoculture W–W–W
3.85 0.94
10.43 2.53
Rotations without fallow crops C−C−W
5.70 48
∗
1.34 43
12.60 21
2.97 17
C−P−W 6.70
74 1.56
66 12.08
16 2.82
11 C−Sy−W
4.44 15
1.10 17
10.23 –
2.54 0.4
C−M−C 6.19
61 1.57
67 14.00
34 3.55
40 C−C−M
6.24 62
1.67 78
14.03 35
3.76 49
C−P−Sy 5.89
53 1.59
69 9.77
– 2.63
4 C−Sy−C
5.77 50
1.54 64
11.71 12
3.12 23
C−M−P 5.64
47 1.50
60 11.57
11 3.07
21 P−P−Sy
5.53 44
1.57 67
7.67 –
2.18 –
P−C−M 6.11
59 1.72
83 11.83
13 3.33
32 P−Sy−C
5.46 42
1.53 63
9.44 –
2.65 5
P−M−P 5.56
44 1.55
65 9.90
– 2.76
9 M−M−C
5.52 43
1.56 66
12.80 23
3.62 43
M−C−P 5.53
44 1.57
67 11.03
6 3.12
23 M−P−Sy
5.79 50
1.66 77
9.10 –
2.61 3
M−Sy−M 4.82
25 1.44
53 11.34
9 3.39
34 Mean
5.68 48
1.53 63
11.19 7
3.01 19
Rotations with fallow crops W+CV−W+CV−W+CV
3.74 –
0.64 –
13.03 25
2.23 –
W+CV−Sg−C 6.59
71 1.48
57 14.35
38 3.21
27 W+CV−C−Sg
4.38 14
0.98 4
13.70 31
3.07 21
W+CV−M−Sy 2.88
– 0.67
– 9.08
– 3.19
26 W+CV−F−W
3.00 –
0.66 –
9.57 –
2.11 –
W+CV−C−Sy 3.51
– 0.82
– 8.23
– 1.92
– W+CV−M−Sg
4.25 10
0.89 –
12.36 19
2.59 2
W+CV−C−W 4.19
9 0.86
– 10.04
– 2.07
– W+BM−Sy−C
4.80 25
1.13 20
11.01 6
2.59 2
W+BM−Sg−F 4.98
29 1.09
16 12.37
19 2.70
7 W+BM−C−F
3.73 –
0.92 –
9.25 –
2.28 –
W+BM−P−Sy 5.10
33 1.21
29 9.33
– 2.22
– W−R+CV−P
3.23 –
0.72 –
7.71 –
1.72 –
W−R+CV−M 2.62
– 0.59
– 8.72
– 1.86
– W−R+SY−W
3.52 –
0.68 –
9.16 –
1.78 –
W−R+BM−Sg 4.91
28 1.07
14 12.99
25 2.83
12 Mean
4.09 6
0.90 –
10.65 2
2.40 –
∗
Significance of differences compared with wheat monoculture at p 0.05.
a
W− wheat; C− corn; P− potato; M− millet; Sg− sorghum; Sy− soybean; F− flax; R− rapeseed; CV− common vetch; BM− broomcorn millet; SY− soybean.
tion when the fallow crops were added to the rotation largely because of better utilization of seasonal precip-
itation. On average, the 16 rotations with fallow crops utilized 17 and 27 more precipitation than the 16
rotations without fallow crops and wheat monoculture Table 6.
The present study indicated that the cultivation of fallow crops after the wheat is harvested did not greatly
influence the quantity of water stored in the soil for use by the subsequent wheat crop. In 1990, a wet year, no
significant difference in soil water content at 1 m depth existed between the bare soil no fallow crop and the
138 L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142
Table 6 Water use characteristics of different rotation systems in comparison with wheat monoculture
a
Evapotranspiration Growing-season
Soil water Precipitation-use
mm rainfall mm
supply mm, 2 m efficiency
Wheat monoculture W−W−W
1235 1018
217 60.4
Rotations without fallow crops C−C−W
1272 1209
63 71.8
C−P−W 1284
1026 258
60.9 C−Sy−W
1220 1209
11 71.8
C−M−C 1181
1123 58
66.6 C−C−M
1119 1102
17 65.4
C−P−Sy 1112
1107 5
65.7 C−Sy−C
1124 1122
2 66.6
C−M−P 1129
1118 11
66.4 P−P−Sy
1057 1077
− 20
63.9 P−C−M
1065 1072
− 7
63.6 P−Sy−C
1070 1092
− 22
64.8 P−M−P
1075 1088
− 13
64.6 M−M−C
1061 1074
− 13
63.7 M−C−P
1060 1068
− 8
63.4 M−P−Sy
1048 1058
− 10
62.8 M−Sy−M
1004 1053
− 49
62.5 Mean
1118 1100
65.3 Rotations with fallow crops
W+CV−W+CV−W+CV 1754
1627 127
96.6 W+CV−Sg−C
1339 1268
69 75.3
W+CV−C−Sg 1338
1290 48
76.6 W+CV−M−Sy
1289 1253
36 74.4
W+CV−F−W 1360
1296 136
76.9 W+CV−C−Sy
1284 1253
31 74.4
W+CV−M−Sg 1431
1397 34
82.9 W+CV−C−W
1457 1354
103 80.4
W+BM−Sy−C 1274
1246 27
73.9 W+BM−Sg−F
1372 1397
− 25
82.9 W+BM−C−F
1219 1208
11 71.7
W+BM−P−Sy 1261
1231 30
73.1 W−R+CV−P
1341 1173
168 69.6
W−R+CV−M 1336
1158 178
68.7 W−R+SY−W
1545 1320
225 78.3
W−R+BM−Sg 1376
1179 197
70.0 Mean
1374 1291
76.6
a
, as the percentage of growing-season precipitation as annual precipitation.
fallow crop fields during the period July to September Fig. 1. In 1989, a dry year, no significant difference
in soil water content at 2 m depth was found between the bare soil and the fallow crop fields at 5 weeks
after the cultivation of fallow crops. Even soil water content at 1.2 m depth was slightly higher in the com-
mon vetch field than the bare soil at this time Fig. 2. At the harvest of fallow crops, soil water content at
0.5 m depth was higher in the bare soil than the fallow crop fields, but no great difference was observed in
other soil layers from 0.5 to 2 m between the bare soil and the fallow crop fields Fig. 3. The reason for this
is the effect of plant coverage on significantly reduc- ing soil evaporation and increasing productive tran-
spiration. Because the fallow period coincided with the rainy season and summer-time in the study site,
L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142 139
Fig. 1. Comparison of variation in soil water content at 1 m depth between the bare soil no fallow crop and the fallow crop fields during the period July through September in 1990, a wet year.
the total water consumption by the bare soil was very close to the fallow crop fields as a result of very high
soil evaporation. Also, the cultivation of fallow crops did not significantly influence the grain production of
the subsequent crop. Direct experiment evidence in support of the above finding was that no significant
difference in annual grain yield existed between the wheat monoculture W–W–W and the wheat plus fal-
low crop pattern W+CV–W+CV–W+CV. The an- nual grain yield of the former was 3.85 Mg ha
− 1
, only slightly greater than that 3.75 Mg ha
− 1
of the lat- ter, whereas the annual biomass production of the for-
mer was 10.43 Mg ha
− 1
, being 25 lower than that 13.03 Mg ha
− 1
of the latter. Al-Fakhry 1990 re- viewed studies and reported that it is feasible to culti-
vate forage, oil crops and pulses as break crops in ce- real monoculture in areas with annual rainfall of over
500 mm, in line with the present study. 3.5. Soil conservation characteristics of different
rotation systems Rotation systems also showed the superiority in
fulfilling soil conservation requirements and had the potential for improved agricultural sustainability. As
mentioned above, the major drawback of wheat mono- culture is the danger of severe soil erosion during the
fallow period that coincides with the rainy season. Studies have indicated that over 70 of the soil ero-
sion that occurs in this region is caused by a few in- tense rainstorms events during the rainy season Gao
and Zhang, 1992; Li, 1992. Hence the key to the alle- viation and prevention of soil erosion lies in whether
the soil is covered by plants during this period. Clearly, it is a good way from a pure conservation perspective
to cultivate winter wheat followed by a 2–3 month fal- low crop which covers the ground during the erosion
140 L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142
Fig. 2. Comparison of water content in different soil layers between the bare soil no fallow crop and the fallow crop fields at five weeks after the planting of fallow crops mid-August in 1989, a dry year.
period, thus achieving an year-round plant coverage of the ground. The problem, however, is that produc-
tivity of this cropping system often fluctuates up and down because of a poor match between water supply
and wheat demand. It appears highly feasible in terms of soil conservation and productivity improvement to
cultivate winter wheat followed by a 2–3 month fal- low crop in 1 year and a summer crop cultivation in
the next. In this system, the soil is covered during both easily-eroded rainy periods but lies bare about 6
months every 2 years. As most of this 6-month period is winter with low precipitation snow and tempera-
ture 0
◦
C, not only is soil evaporation very low but also the risk of erosion is much less.
4. Summary
