130 L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142
fall often inadequate but also over 60 of annual pre- cipitation occurs in the 3 months between July and
September, often in the form of intense thunderstorms which cause tremendous amounts of erosion. Hence
the problem for agricultural production is not so much the absolute scarcity of rainfall but rather its uneven
seasonal distribution Li, 1998.
In the prevailing rainfed farming system, winter wheat Triticum aestivum L. monoculture is a com-
mon practice. This practice is characterized by a 2–3 month summer fallow from the wheat harvest in
end of June or early July through to sowing in late September and is generally considered to increase
soil water storage that is available for use by the subsequent crops. However, it often lowers the over-
all precipitation-use efficiency because of the large amount of water evaporated from the bare soil during
the fallow period Gao and Zhang, 1992; Zhu et al., 1994. The reason is that the fallow period closely
parallels the rainy season and lies in the height of summer-time in the region, thereby having a high
evaporative demand under high rainfall and temper- ature. Moreover, the soil lies bare during the fallow
period and will be at risk to the erosive action of heavy rainstorms. So severe soil erosion during this
period is also a great threat to sustainable production. Another negative effect of wheat monoculture on crop
production stems from pest, weed and disease infes- tation Hanley and Ridgman, 1978; Al-Fakhry, 1990;
Cook, 1990; Gao and Zhang, 1992; Zhang, 1992. It is widely recognized that the practices of crop rotation
are among most effective methods for alleviating ero- sion, improving the water availability and maintaining
high yields Follett and Stewart, 1985; Williams and Renard, 1985; Wang, 1989; Francis and Clegg, 1990;
Amir et al., 1991; Caporali and Onnis, 1992; Liu and Liu, 1992; Shan, 1993. However, the successful im-
plementation of crop rotation practices hinges upon certain environmental e.g. geographic, climatic and
edaphic, technological and socio-economic factors. Particularly, environmental factors almost completely
dictate the crop and its management selection. In semi-arid climates, choosing proper crop types and
cultivars is of great importance to the development of effective cropping systems and management prac-
tices. This choice often entails taking into account not only crop productivity and water-use efficiency but
also the time of sowing, duration of the growth cycle, growth property of seedlings and soil conservation
requirements. On the other hand, a successful crop rotation system is not only a profitable agricultural
system but also a beneficial soil conservation tool.
As part of a long-term research effort aimed at establishing a sustainable rainfed farming system in
the semi-arid and sub-humid regions of northwest China, this paper presents a detailed study on the
water use patterns and agronomic performance for some cropping systems with and without fallow crops
in a semi-arid environment. The objectives of this study were to: 1 determine the grain and above-
ground biomass production and water-use efficiency of individual crops grown in the rotation; 2 analyze
the seasonal and inter-annual patterns of soil water storage and utilization as well as water stress for
the four major rotation crops such as winter wheat, corn, potato and millet; 3 determine the grain and
aboveground biomass production and water-use effi- ciency for different rotation systems and evaluate the
capacities of the rotation systems with and without fallow crops to utilize soil water storage in conjunc-
tion with seasonal precipitation; 4 establish whether the introduction of fallow crops into the wheat mono-
culture significantly influences the quantity of water stored in the soil that will be used by the subsequent
wheat crop; and 5 discuss the characteristics of soil conservation for different rotation systems.
2. Materials and methods
2.1. Study site description The study was conducted on rainfed land in the
semi-arid region of Xifeng 35
◦
40
′
N, 107
◦
51
′
E, elev. 1298 m a.s.l., Gansu province, northwest China. The
soil of the study area is a sandy loam with an av- erage field water-holding capacity of 0.223 kg kg
− 1
and wilting point of 0.07 kg kg
− 1
. Soil bulk density in the 0 to 2 m depth ranges from 1.1 to 1.4 Mg m
− 3
, with a weighted average of 1.3 Mg m
− 3
. In this region, the long-term 1948−1988 average annual precipita-
tion is 561 mm, over 60 occurring during the period July−September. Average annual pan evaporation is
1504 mm, about three times higher than precipitation. Average annual mean temperature is 8.3
◦
C, and mean temperatures in the hottest July and coldest January
L. Fengrui et al. Agriculture, Ecosystems and Environment 79 2000 129–142 131
Table 1 Weather conditions for the three major years of experiment in comparison with the average values for the 40 years, 1948−1988
Weather variable 1989
1990 1991
Average Annual rainfall mm
483 759
443 562
Annual mean temperature
◦
C 8.2
8.9 9.1
8.3 Annual mean soil surface temperature
◦
C 10.1
10.8 11.1
9.8 Annual accumulated temperature ≥0
◦
C 3379
3610 3567
3446 Annual solar radiation MJ m
− 2
5040 5280
6120 5489
Annual pan evaporation mm 1184
1217 1401
1503 Annual growing season days
256 266
252 255
months are 21.3
◦
C and −5.3
◦
C, respectively. Aver- age annual solar radiation is 5489 MJ m
− 2
. The mean length of the annual growing season is 255 days.
The field work took place during the years 1988−1991. Table 1 summarizes weather conditions
for the 3 years 1989, 1990 and 1991. Compared with the average values for the 40-year period 1948–1988,
there was low rainfall 483 mm and low solar radia- tion 5040 MJ m
− 2
in 1989, high rainfall 759 mm and high mean temperature 8.9
◦
C in 1990 and low rainfall 443 mm and high mean temperature
9.1
◦
C in 1991. Averaged over the 3 years, annual precipitation, mean temperature, solar radiation and
length of the growing season were 562 mm, 8.8
◦
C, 5480 MJ m
− 2
and 255 days, respectively, being close to the average values for the 40 years.
2.2. Experimental design and treatments Eight principal crops of the region were chosen as
rotation crops, including winter wheat Triticum aes- tivm L., cv. ‘Xifeng No. 16’, corn Zea mays L., cv.
‘Zhongdan No. 2’, potato Solanum tuberosum L., cv. ‘Tainshu No. 1’, millet Penisetum glaucum L. R.
Brown, cv. ‘Changnon No. 1’, sorghum Sorghum bi- color L. Moench, cv. ‘Jinzhong No. 405’, soybean
Glycine max L. Merr., cv. ‘Jindou No. 2’, winter rapeseed Brassica napus L., a local cultivar and flax
Linum usitatissimum L., cv. ‘Tianya No. 1’. Three early-maturing crop species such as broomcorn mil-
let Panicum miliaceum, a local cultivar, soybean cv. ‘Bayueza’ and annual common vetch Vicia sativa L.
were used as the summer fallow crops.
According to 3-year crop rotation schemes, all crops were grown in sequence at a fixed seeding rate
slightly higher than that employed by local farm- ers. Wheat was sown annually in late September,
with 180 kg ha
− 1
and harvested in the end of June or early July. Corn was sown annually in late April,
with 30 kg ha
− 1
and harvested between 18 and 25 September. Potato was sown annually in late April,
with 600 kg ha
− 1
and harvested between 15 and 25 September. Millet was sown annually in early May,
with 22.5 kg ha
− 1
and harvested in late September. Rapeseed was sown in late September in 1989, with
30 kg ha
− 1
and harvested in early July. Sorghum was sown in late April in 1990 and 1991, with 37.5 kg ha
− 1
and harvested in late September. Soybean was sown in late April in 1990 and 1991, with 60 kg ha
− 1
and har- vested in late September. Flax was sown in late April in
1990 and 1991, with 60 kg ha
− 1
and harvested in mid September. The fallow crops were sown in the stand-
ing wheat and rapeseed residue immediately after the wheat and rapeseed harvest, and harvested between 15
and 25 September, with a seeding rate of 75 kg ha
− 1
in soybean, 37.5 kg ha
− 1
in broomcorn millet and 90 kg ha
− 1
in common vetch cutting for forage. Thirty-two 3-year crop rotation systems were estab-
lished on sixty-four 3.5 m × 10 m plots with a 1.2 m space between plots. Plots were arranged as a random-
ized complete block with two replicates. The 32 rota- tion systems were divided into two groups: one with
fallow crops e.g. W+CV−Sg−C wheat+common vetch−sorghum−corn and the other without fallow
crops e.g. C−P−W corn−potato−wheat. Wheat monoculture served as the experiment control. Wheat
had been grown in the study plots before the experi- ment.
Because the soil of this area is rich in K and deficient in N and P, farmyard manure was exten-
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
