Soil conservation effectiveness and energy efficiency of alternative rotations and continuous wheat cropping in the Loess Plateau of northwest China

Agriculture, Ecosystems and Environment 91 (2002) 101–111

Soil conservation effectiveness and energy efficiency of alternative rotations and continuous wheat cropping in the Loess Plateau of northwest China Feng-Rui Li a,∗ , Chong-Yue Gao b , Ha-Lin Zhao a , Xiao-Yan Li a a

Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, 260 Donggang West Road, Lanzhou 730000, PR China b Gansu Grassland Ecological Research Institute, Lanzhou 730010, PR China Received 15 August 2000; received in revised form 15 May 2001; accepted 13 June 2001

Abstract Winter wheat (Triticum aestivum L.) monoculture is common in wheat-growing areas of the Loess Plateau of northwest China. This system is characterized by nearly 3-month summer fallow from wheat harvest at the end of June or early July to sowing in late September. It not only lowers the overall precipitation-use efficiency because of the large amount of evaporation from the bare soil surface during the fallow period but also entails high risk of erosion by summer rainstorms. There is a need to develop more effective cropping systems to replace the current production system. Seven alternative rotations, mainly using wheat, rapeseed, corn, potato, pearl millet, linseed, alfalfa and sweetclover, were established and their use of environmental resources, production performance, energy efficiency, soil fertility sustainability, and soil conservation effectiveness were compared with continuous wheat cropping. The rotations had greater potential use of environmental resources. Despite showing no clear advantage in grain yields, all rotations were significantly higher in total above-ground biomass production and more efficient in energy transformation compared with continuous wheat cropping. After a 3-year cycle, the rotations did not adversely affect soil bulk density but some rotations significantly increased soil water-stable aggregates compared with the initial measurement. For the rotations based on the inclusion of legumes, the availability of N was apparently improved but the total P was substantially reduced compared with the initial measurement and continuous wheat cropping. An assessment of soil conservation effectiveness with a weighted soil conservation effectiveness index (WSCEI) indicated that the rotations performed much better than continuous wheat cropping in conserving soil and water resources. This study also strongly recommend that it is feasible to cultivate winter wheat followed by a 3-month legume fallow crop in year 1 and then a summer crop cultivation in the next. This system provides a soil cover during both erosion-prone rainy periods while leaving the soil bare for about 7 months (October–April) every 2 years. Another alternative is to cultivate winter wheat followed by a 15-month legume crop cultivation in years 1 and 2 and then a summer crop in year 3. This system allows the soil to be covered during three rainy periods while leaving the soil bare for about 7 months every 3 years. As most of this 7-month period is winter with low rainfall (snow) and temperatures below 0 ◦ C, not only is soil evaporation very low but the risk of erosion is also low. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Alternative rotations; Continuous wheat cropping; Energy efficiency; Soil conservation effectiveness; Northwest China

∗ Corresponding author. Tel.: +86-931-7671112; fax: +86-931-8273894. E-mail address: [email protected] (F.-R. Li).

0167-8809/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 8 0 9 ( 0 1 ) 0 0 2 6 5 - 1

102

F.-R. Li et al. / Agriculture, Ecosystems and Environment 91 (2002) 101–111

1. Introduction The Loess Plateau of northwest China is situated between 34 and 40◦ N, and 102 and 112◦ E at an altitude ranging from 700 to 2200 m. It comprises parts of Gansu, Qinghai, Ningxia, Shanxi, Shaanxi and inner Mongolia provinces with a total area of 620 000 km2 , approximately 6% of the national territory (Liu, 1999). The region is governed by a temperate continental monsoon climate, which is characterized by cold winters, windy and dry springs, and a warm and comparatively rain-rich summer followed by a short, cool autumn. Annual rainfall ranges from 200 to 750 mm, with most rainfall falling between June and September. Annual accumulated temperature above 0 ◦ C ranges from 2500 to 4500 ◦ C. The annual frost-free period varies between 120 and 180 days. The major soil type in the cultivated land area is sandy loam of Loess origin, which is loosely structured and highly susceptible to wind and water erosion (Chen et al., 1996). In a large area of the region where no water resources are available for agricultural irrigation, the most widespread land-use system is rainfed farming. Rainfed cropland occupies about 80% of the total cultivated land (Shan, 1993). Since the beginning of the century, particularly over the last several decades, the rapidly growing human population has placed heavy pressure on productive soil resources, forcing farmers to convert more and more forest land and grassland into cropland and at the same time to increase cultivation of steep erodible slopes. Consequently, this has led to an increase in the scale and severity of soil erosion and a reduction in soil fertility, which are major threats to the sustainability of agroecosystems in the region (Li et al., 2000a). In a recent survey, about 280 000 km2 , or 45% of the Loess Plateau was found to be severely eroded (Tang, 1992). Each year, on average, nearly 1600 million Mg of topsoil are lost from the region through run-off and wind erosion, with associated losses of about 38 million Mg of nitrogen, phosphorus and potassium nutrients (Liu, 1999). Although the problem of soil erosion and land degradation has a biophysical root cause related to climatic, edaphic, topographic and geological features of the region, it is strongly tied to poor land-use management (Ren, 1992). Over the last decades, under the promptings of short-term economic benefits, some effective traditional soil-conserving practices

including crop rotation, ley farming, inter-cropping, multiple cropping, stubble mulch management, and applications of farmyard manure have been gradually abandoned in agricultural production (Zhang et al., 1989; Ren, 1992). In many winter wheat-growing areas of the Loess Plateau, continuous winter wheat (Triticum aestivum L.) cropping dominates the rainfed cultivated land (Zhang, 1992). This system is characterized by nearly 3-month summer fallow (from wheat harvest at the end of June or early July to sowing in late September) and is traditionally thought to increase soil water storage that is used by subsequent crops. However, recent studies have indicated that this system not only lowers the overall precipitation-use efficiency because of the large amount of evaporation from the bare soil surface during the fallow period (Zhu et al., 1994; Li et al., 2000b), but also entails a high risk of severe soil erosion by summer rainstorms (Li, 1992; Liu, 1999). Liu (1999) has reported that over 70% of soil erosion that occurs in the Loess Plateau is caused by a few intense rainstorms events. Therefore, developing and adopting more effective cropping systems is the key to bring soil erosion under control and to achieve sustainable crop production. In an earlier paper, the authors reported the results of water use patterns and agronomic performance for some rotations with and without fallow crops at individual crops and cropping systems (Li et al., 2000b). The objective of this study was to compare several wheat-based alternative rotations with continuous wheat cropping by considering their use of environmental resources (rainfall, temperature and solar radiation), production performance, energy efficiency, soil fertility sustainability, and soil conservation effectiveness, in an attempt to select more effective rotation systems as alternatives for farmers in this region.

2. Materials and methods 2.1. Study site The experiment was conducted at the Qingyang Loess Plateau Agricultural Experiment Station from 1988 through 1991. The station is located about 10 km east of Xifeng county (latitude 35◦ 40 N, longitude 107◦ 51 E, elevation 1298 m a.s.l.), Gansu province, northwest China. The climate is typical of a temperate

F.-R. Li et al. / Agriculture, Ecosystems and Environment 91 (2002) 101–111

103

semi-arid monsoon environment. Based on data from 1948 through 1988, the annual mean solar radiation is 5489 MJ m−2 . The annual mean air temperature is 8.3 ◦ C, and the coldest and warmest monthly mean temperatures are −5.0 ◦ C in January and 21.3 ◦ C in July. The annual mean accumulated temperature above 5 ◦ C is 3446 ◦ C. The mean frost-free period is 161 days, and the growing season is 255 days. The annual mean rainfall is 562 mm, over 70% of which falls between June and September. The annual mean pan evaporation is 1504 mm. The soil is a sandy loam of Loess deposits, which belongs to the Los-Orthic Entisols according to the FAO soil classification (Chinese Soil Taxonomy Cooperative Research Group, 1995), basically similar to the American system of soil classification. The properties are: pH (H2 O), 8.1; water-holding capacity, 0.223 kg kg−1 ; permanent wilting point, 0.07 kg kg−1 . Soil bulk densities in the 0–0.1, 0.1–0.2 and 0.2–0.3 m layers were 1.15, 1.25 and 1.30 Mg m−3 , respectively. 2.2. Experimental design In autumn 1988, eight major crop species of the region, including winter wheat (cv. ‘Xifeng No. 16’), winter rapeseed (Brassica napus, a local cultivar), corn (Zea mays L., cv. ‘Zhongdan No. 2’), potato (Solanum tuberosum, cv. ‘Tainshu No. 1’), pearl millet (Pennisetum glaucum (L.) R.Br., cv. ‘Changnon No. 1’), linseed (Linum usitatissinum L., cv. ‘Tianya No. 1’) and two forage legumes alfalfa (Medicago sativa L.) and sweetclover (Melilotus suaveolens lecleb), were used as rotation crops. Three short season summer crops including vetch (Vicia sativa L.), soybean (Glycine max (L.) Merr., cv. ‘Bayueza’) and broomcorn millet (Panicum miliaceum, a local cultivar) were used as fallow crops, which were planted during the summer fallow period from wheat harvest at the end of June or early July to sowing in late September. According to agroecological management principles (Wen and Pimentel, 1992), seven wheat-based alternative rotations were established with continuous wheat cropping as a control. The field layout of these rotation systems is given in Fig. 1. The experimental plot was a rainfed production field with a history of annually cropped winter wheat for several years prior to the study. The eight rotation systems were arranged in a randomized block design

Fig. 1. A schematic diagram showing the field layout of the eight rotation systems. W: winter wheat; R: winter rapeseed; C: corn; M: pearl millet; L: linseed; A: alfalfa S: sweetclover; Sy: soybean; B: broomcorn millet; V: vetch; /: followed by summer fallow crops.

with two replications due to cost limitations. Plot size was 4 m × 10 m with a 1.2 m space between plots. A detailed description of the field cultivation for all crops in the rotations is given in Table 1. Because the soil of this area is naturally rich in K but deficient in N and P, farmyard compost (a mixture of soil and human and animal feces) was extensively applied to all plots once a year before planting, at a rate of about 15 000 kg ha−1 . Aside from this, a moderate amount of chemical fertilizer was applied in terms of soil test recommendations. For rotation crops, fertilizer N (urea) was used at a rate of 86 kg ha−1 and fertilizer P (superphosphate) was used at a rate of 24 kg ha−1 (only P was applied for alfalfa and sweetclover), but no fertilizer K was applied to any crop. The fallow crops were fertilized with N at a rate of 69 kg ha−1 for broomcorn millet, and P at a rate of 14 kg ha−1 for soybean and vetch. Other field management practices were identical to those

104

F.-R. Li et al. / Agriculture, Ecosystems and Environment 91 (2002) 101–111

Table 1 Planting date, seeding rate, row spacing and harvesting date for all crops grown in the rotation Crop

Planting date

Seeding rate (kg ha−1 )

Row spacing (cm)

Harvesting date

Rotation crops Winter wheat

Late September

180

20

Mid August Late April Late April Early May Late April Early July Early July

30 30 600 (fresh) 22.5 60 11.3 11.3

20 45 35 30 30 Broadcast Broadcast

End of June or early July of the next year Early June of the next year Late September Late September Late September Mid September Late September of the next year Late September of the next year

30 30 Broadcast

Late September Late September Late September

Winter rapeseed Corn Potato Pearl millet Linseed Alfalfaa Sweetclovera Summer fallow crops Broomcorn millet Soybean Vetcha a

Early July Early July Early July

37.5 75 90

Harvesting for forage.

used by local farmers. Weeds were controlled by hand weeding. 2.3. Soil and crop measurements To assess the effects of different alternative rotations and continuous wheat cropping on soil fertility, three samples of soil in a 30 cm depth with an increment of 10 cm were taken using a soil auger (diameter 8 cm and height 10 cm) from each plot for measuring soil physical and chemical properties before the onset and the end of the experiment. Soil water-stable aggregates were measured using a wet-sieving method (Alegre and Cassel, 1996); soil organic matter, total N, total P, available N and available P were analyzed using the methods described by Olsen et al. (1954), and Hamazaki and Paningbatan (1988). Soil bulk density was determined using three samples obtained with soil ring kits from each plot. Throughout the experiment, crops were hand-harvested annually from each plot by taking a harvest sample area (2 m × 8 m) of each plot, or accounting for about 40% of the plot size to determine grain yield and yield of crop residue. Root biomass was measured annually for each crop (including the fallow crops) at its final harvest by sampling three alternate 0.25 m2 quadrats to a 30 cm depth. In addition, fresh above-ground biomass present during the July–September period per plot was measured annually on three occasions of mid July, mid August and

mid September by sampling three alternate 0.25 m2 quadrats of each plot. Then a mean of these three occasions was calculated for each plot. At harvest, each crop was sampled to measure separately energy content of its seed and stover using an oxygen-bomb calorimeter (Li, 1998). For each rotation, its total grain yield, total crop residue and total root mass were computed in terms of all crops grown in the 3-year rotation. Climatic data were recorded at the experiment station. A more detailed description of the experimental measurements is given by Li (1998). 2.4. Data analysis 2.4.1. Energy budget To study energy budgets of different alternative rotations and continuous wheat cropping, a complete inventory of the following was prepared: (1) labor input for humans and animals during the crop rotation production involved mainly in planting, fertilizing, weeding and harvesting; (2) farmyard compost and chemical fertilizer inputs; (3) seed input; and (4) grain yield and yield of crop by-products. The energy budget was first calculated for each crop and then the value of each rotation was determined based on individual crops. The values were computed in terms of labor (human and animal power) as work hours, and quantities of seed, farmyard compost, chemical fertilizer, grain yield and crop by-products.

F.-R. Li et al. / Agriculture, Ecosystems and Environment 91 (2002) 101–111

The values of inputs were converted into energy using their energy equivalents recommended by the various researchers (Pimentel et al., 1973; Mitchell, 1979; Freedman, 1982; Hurst and Rogers, 1983). The values of outputs were directly converted into energy using the measured data (Li, 1998). The energy efficiency of each rotation was expressed as an output/input ratio. 2.4.2. Soil conservation effectiveness The basis of soil conservation effectiveness assessment was to interpret soil conservation requirements for cropping systems (Gao et al., 1994). Soil conservation requirements for a cropping system at the study area should include the length of the cropping period in a rotation cycle, i.e. the length of time for the soil to be covered by crops; the amount of root biomass left to the ploughed layer; above-ground biomass production (grain yield plus crop residue); and crop coverage during the erosion-prone rainy period from July through September. On the basis of these assumptions, a weighted soil conservation effectiveness index (WSCEI) was proposed using a set of comprehensive indicators associated with soil conservation requirements for cropping systems. These indicators included: (1) the length of the cropping period in a 3-year rotation cycle, namely the length of time for the soil to be covered by crops (X1 ); (2) the relative rainfall use, defined as a percentage of growing season rainfall relative to annual rainfall (X2 ); (3) the above-ground biomass production (grain yield plus crop residue) (X3 ); (4) the amount of root biomass left to the ploughed layer (X4 ); and (5) crop coverage during the erosion-prone rainy period, as indicated by the mean fresh above-ground biomass present per square meter during this period (X5 ). Before calculation of WSCEI, the observed data for each of the five indicators were standardized using the following formula: Bi =

Xi Ximax

(1)

where Bi is the standardized values of indicators i (i = 1, 2, . . . , 5), Xi the observed values of indicators i and Xi max is the maximum values of indicators i across the eight rotations. Using standardized values, WSCEI was given by: WSCEI =

5
i=1

Wi B i

(2)

105

where Wi is the weight assigned to each of the five indicators in terms of their relative importance to soil conservation requirements (Li and Gao, 1994). 2.4.3. Statistical analyses for soil and crops In this study, the least significant difference (LSD) test was performed to determine the effects of different alternative rotations and continuous wheat cropping on production performance and soil physical and chemical properties.

3. Results and discussion 3.1. Rainfall characteristics The inter-annual and within-season rainfall during the study period is presented in Fig. 2. The first year (1989) was relatively dry; annual rainfall was 483 mm, which was 14% lower than the long-term annual average for the site. The second year (1990) was wet; annual rainfall was 759 mm, which was 26% higher than the long-term annual average. The third year (1991) was quite dry; annual rainfall was 443 mm, which amounted to only 79% of the long-term annual average. Crop growth and biomass production in 1989 and 1991 were presumably limited to some extent by low rainfall availability. 3.2. The use of environmental resources Potential use of environmental resources (rainfall, temperature and solar radiation) for different alternative rotations and continuous wheat cropping was evaluated by comparing their relative resource use, which is expressed as a percentage of growing season rainfall, accumulated temperature above 5 ◦ C and solar radiation relative to annual values. For rainfall, the relative resource use of the seven rotations ranged from 79 to 90%, with an average of 84%, being 30–39% higher (averaging 35%) than continuous wheat cropping. For accumulated temperature, the relative resource use of the seven rotations ranged from 75 to 94%, with an average of 82%, being 21–37% higher (averaging 28%) than continuous wheat cropping. For solar radiation, the relative resource use of the seven rotations ranged from 56 to 77%, with an average of 66%, being 4–30% higher (averaging 18%) than continuous wheat

106

F.-R. Li et al. / Agriculture, Ecosystems and Environment 91 (2002) 101–111

Fig. 2. Monthly rainfall distribution at the study site during the 3 years (1989–1991) of the experiment compared with the 40-year mean.

cropping (Fig. 3). This suggests that adoption of the alternative rotation practices made more efficient use of environmental resources compared with continuous wheat cropping. In one study of the 2-year winter wheat–fallow (WF) rotation and the 3-year winter wheat–sorghum–fallow (WSF) rotation in the Great Plains (US), Nilson et al. (1985) reported that WSF had a better production performance (grain yield) than WF, largely because the former had better water availability than the latter resulting from efficient storage and use of precipitation (Unger, 1984; Norwood, 1994; Peterson et al., 1996).

3.3. Production performance and energy efficiency Despite the fact that only two of the seven rotations produced more grain yields compared with continuous wheat cropping (P < 0.05), all rotations were significantly higher (P < 0.05) in total above-ground biomass production (grain yield plus crop residue) than continuous wheat cropping (Fig. 4). Further, the total root mass in A–W–C, W/V–C–W, W–S–W and W/B–C–P was also higher (P < 0.05) than that found in continuous wheat cropping. Integrating nitrogen fixing legumes into wheat monoculture had a marked

Fig. 3. A comparison of relative resource use (defining as a percentage of growing season rainfall, accumulated temperature above 5 ◦ C and solar radiation relative to annual values) between different alternative rotations and continuous wheat cropping. R1 = A–W–C; R2 = W/V–C–W; R3 = W/B–C–P; R4 = W–S–W; R5 = W–R/Sy–W; R6 = W/V–M–L; R7 = W–R/V–W; and CWC = W–W–W. See Fig. 1 for each rotation.

F.-R. Li et al. / Agriculture, Ecosystems and Environment 91 (2002) 101–111

107

Fig. 4. A comparison of production performance as indicated by the total grain yield, total above-ground biomass (grain yield plus crop residue) and total root mass (0–30 cm) between different alternative rotations and continuous wheat cropping. For each rotation, total grain yield is the sum of all crops grown in the 3-year cycle. This is the same case for total above-ground biomass and total root mass. R1 = A–W–C; R2 = W/V–C–W; R3 = W/B–C–P; R4 = W–S–W; R5 = W–R/Sy–W; R6 = W/V–M–L; R7 = W–R/V–W; and CWC = W–W–W. See Fig. 1 for each rotation. Significance of differences at P < 0.05 compared with continuous wheat cropping.

positive effect on improving subsequent crop yields. A good example is A–W–C and W–S–W. In both rotations, despite the fact that the grain sowing area was reduced by one-third, the total grain yields were 11.1 (A–W–C) and 12.2 Mg ha−1 (W–S–W), respectively, which was similar or slightly higher than that (11.3 Mg ha−1 ) of continuous wheat cropping. This result may be explained solely by a higher likelihood of the inclusion of leguminous species, which suggests that the availability of N for the subsequent crop may be improved by the nitrogen fixing legumes present in these two rotations resulting from more nitrogen supply. Studies have demonstrated that sweetclover can fix approximately 30–50 kg N ha−1 per year (Wang and Fan, 1987), and that growing crops like corn following leguminous crop produce an average of 8% more grain yields than those planted after other crops (Liu, 1988). In one study conducted at the same area, Gao et al. (1994) reported that alfalfa can fix about 60–90 kg N ha−1 per year. For the seven rotations, total energy inputs ranged from 17.6 × 103 (A–W–C) to 27.0 × 103 MJ ha−1 (W/V–C–W) with an average of 23.3 × 103 MJ ha−1 , being slightly less than that (24.3 × 103 MJ ha−1 ) of continuous wheat cropping (Table 2). However, all rotations except W–S–W were higher in total energy outputs than continuous wheat cropping. In particular, the most productive rotations were W/V–C–W, W/B–C–P and W–R/Sy–W with the total energy

outputs of 99.9 × 103 , 83.0 × 103 and 81.8 × 103 MJ ha−1 , being significantly higher (P < 0.05) than that (64.8 × 103 MJ ha−1 ) of continuous wheat cropping. Similarly, all rotations were higher in energy efficiency (expressing as an output/input ratio) Table 2 Total energy inputs and outputs (×103 MJ ha−1 ) and output/ input ratios for different alternative rotations and continuous wheat croppinga Total energy input

Total energy output

Output/input ratio

Continuous wheat cropping W–W–W 24.28

64.84

2.67

Alternative rotations A–W–C 17.61 W–S–W 21.11 W/V–C–W 26.98 W–R/Sy–W 26.74 W–R/V–W 22.92 W/B–C–P 24.26 W/V–M–L 23.74

69.01 60.57 99.87 81.83 70.53 82.97 71.94

3.92b 2.87 3.70b 3.06b 3.08b 3.42b 3.03

Mean

76.67

3.28

LSD0.05 a

23.34

0.39

See Fig. 1 for each rotation. W: winter wheat; R: winter rapeseed; C: corn; P: potato; M: pearl millet; L: linseed; A: alfalfa; S: sweetclover; Sy: soybean; B: broomcorn millet; V: vetch; /: followed by summer fallow crops. b Significance of differences at P < 0.05 in comparison with continuous wheat cropping.

108

F.-R. Li et al. / Agriculture, Ecosystems and Environment 91 (2002) 101–111

but the most efficient rotations were A–W–C, W/V–C–W, W/B–C–P, W–R/V–W and W–R/Sy–W with the output/input ratios of 3.9, 3.7, 3.4, 3.1 and 3.1, which was significantly higher (P < 0.05) than that (2.7) of continuous wheat cropping. 3.4. Changes in soil physical and chemical properties After a 3-year cultivation, the rotations did not affect soil bulk density, despite the fact that soil bulk densities in some rotations (W–S–W, W/V–C–W and W/B–C–P) were slightly reduced compared with the initial value measured before the experiment. No significant differences in this variable were found between the rotations and continuous wheat cropping (Table 3). Unlike soil bulk density, however, the rotations had a positive impact on increasing soil water-stable aggregates. Among the seven rotations, the percentage of soil water-stable aggregates as dry soil weight was 10.2, 6.8, 6.3 and 4.8% for A–W–C, W/B–C–P, W/V–M–L and W–R/Sy–W, which was significantly higher (P < 0.05) than the initial measurement (2.8%) and continuous wheat cropping (3.0%). The increased percentage of soil water-stable aggregates under the above rotations was probably

attributed to the overall effect of crop rotations, while more root mass left to the ploughed layer and more nitrogen supply resulting from N fixation by leguminous species in these rotations were underlying causes (Li and Gu, 1992). A considerable variation in total N, total P, available N and available P among rotations indicates that rotation systems differed significantly in response to the soil N and P. In this study, all rotations except W/V–C–W tended to be higher in total N content than the initial measurement and continuous wheat cropping, but significant differences were noted only in W–S–W, A–W–C, W/B–C–P and W–R/V–W as compared to the initial measurement and in W–S–W, A–W–C and W/B–C–P as compared to continuous wheat cropping. This is the same case for available N, where the available N in A–W–C, W–S–W, W/B–C–P and W–R/Sy–W was significantly higher (P < 0.05) than the initial measurement and continuous wheat cropping (Table 3). These results demonstrate that the rotations, particularly those based on the inclusion of legumes had a beneficial effect on increasing the availability of N. This was consistent with the study of Gao (1994), who found from a 4-year study conducted at the same site that integrating forage legumes into winter wheat monoculture significantly increased N

Table 3 Changes in soil physical and chemical properties (0–30 cm) after a 3-year cycle for different alternative rotations and continuous wheat cropping compared with the initial measurementsa Soil bulk density (Mg m−3 ) Initial value

1.21

Continuous wheat cropping W–W–W 1.23 Alternative rotations A–W–C 1.25 W–S–W 1.15 W/V–C–W 1.18 W–R/Sy–W 1.24 W–R/V–W 1.22 W/B–C–P 1.13 W/V–M–L 1.29 Mean LSD0.05

1.21

Aggregate as dry soil weight (%)

Soil organic matter (g kg−1 )

Total N (g kg−1 )

Total P (g kg−1 )

Available N (mg kg−1 )

Available P (mg kg−1 )

2.82

10.0

0.54

1.78

55.8

4.1

3.04

10.2

0.55

1.65

53.9

3.8

10.15b 3.05 3.21 4.83b 3.25 6.82b 6.33b

11.9b 10.8 10.7 10.4 10.5 9.9 9.5

0.73b 0.76b 0.53 0.63b 0.64b 0.71b 0.59

1.36b 1.42b 1.68 1.79 1.83 1.85 1.69

73.8b 72.1b 61.7 64.8b 60.7 67.3b 58.9

2.9b 3.9 4.0 4.6 4.3 4.7b 3.7

5.38

10.5

0.66

1.66

65.6

4.3

0.09

0.21

1.46

1.68

8.87

0.59

a See Fig. 1 for each rotation. W: winter wheat; R: winter rapeseed; C: corn; P: potato; M: pearl millet; L: linseed; A: alfalfa; S: sweetclover; Sy: soybean; B: broomcorn millet; V: vetch; /: followed by summer fallow crops. b Significance of differences at P < 0.05 in comparison with the initial measurements.

F.-R. Li et al. / Agriculture, Ecosystems and Environment 91 (2002) 101–111

109

Fig. 5. A comparison of soil conservation effectiveness between different alternative rotations and continuous wheat cropping using the values of WSCEI. R1 = A–W–C; R2 = W/V–C–W; R3 = W/B–C–P; R4 = W–S–W; R5 = W–R/Sy–W; R6 = W/V–M–L; R7 = W–R/V–W; and CWC = W–W–W. See Fig. 1 for each rotation.

content compared with continuous wheat cropping. However, the total P in the two rotations with legumes and the available P in the rotation with alfalfa were found to be significantly lower than both the initial measurement and continuous wheat cropping (Table 3), reflecting that these two rotations increased the availability of N but reduced P content, largely due to greater depletion of soil P by leguminous species (Gao, 1994). Shang and Liu (1988) also reported that continuous soybean cropping increased nitrate N content but decreased P and K contents. This study found no significant differences in soil physical and chemical parameters between continuous wheat cropping and the initial measurements (Table 3). 3.5. The effectiveness of soil conservation practices The effectiveness of soil conservation for different rotation systems can be assessed by comparing their values of WSCEI. The larger the WSCEI value for a cropping system, the better the soil conservation effectiveness for that cropping system. The WSCEI values of the seven rotations ranged from 0.67 (W–R/V–W) to 0.91 (A–W–C), being significantly higher than that (0.34) of continuous wheat cropping (Fig. 5). This suggests that the rotations performed much better in conserving soil and water resources than did continuous wheat cropping. From a soil conservation perspective, it is feasible to cultivate winter wheat followed by a 3-month fallow crop that covers the ground during the erosion period, thus ensuring a year-round plant

coverage of the ground. However, the productivity of this system often fluctuates, largely because of a poor match between water supply and demand for winter wheat under summer monsoon conditions (Liu, 1999; Li et al., 2000b). It is possible in terms of soil conservation, fertility sustainability and productivity improvement to grow winter wheat followed by a 3-month legume fallow crop (vetch or soybean) in year 1 and a summer crop (corn, potato and pearl millet) cultivation in the next. This system allows the soil to be covered during both erosion-prone rainy periods while leaving the soil bare for about seven months (October–April) every 2 years. Another alternative is to grow winter wheat followed by a 15-month forage legume (alfalfa or sweetclover) cultivation in years 1 and 2 and then a summer crop cultivation in year 3. This system provides a soil cover during three erosion-prone rainy periods while leaving the soil bare for about 7 months every 3 years. As most of this 7-month period is winter with low rainfall (snow) and temperatures below 0 ◦ C, soil evaporation is very low and the risk of erosion is also low.

4. Conclusions The results of this study led us to the following conclusions: (1) practices of the alternative rotations had a greater potential use of environmental resources. On average, the relative resource use of the seven rotations was 35% (rainfall), 28% (accumulated temperature) and 18% (solar radiation) higher than continuous

110

F.-R. Li et al. / Agriculture, Ecosystems and Environment 91 (2002) 101–111

wheat cropping; (2) despite showing no clear advantage in grain yields, all rotations were significantly higher in total above-ground biomass production and more efficient in energy transformation compared with continuous wheat cropping. Among the seven rotations, W/V–C–W, W/B–C–P and W–R/Sy–W were most productive with significant higher energy outputs, and A–W–C, W/V–C–W, W/B–C–P, W–R/V–W and W–R/Sy–W were most efficient with significant higher energy output/input ratios; (3) overall, the rotations did not affect soil bulk density but some rotations significantly increased soil water-stable aggregates compared with the initial measurement and continuous wheat cropping. Although a marked improvement in the availability of N was noted in the rotations with legumes, the total P in these rotations was significantly lower than the initial measurement and continuous wheat cropping. No significant differences in soil physical and chemical properties were found between the initial measurement and continuous wheat cropping; (4) the values of WSCEI for the seven rotations were much higher than that of continuous wheat cropping, suggesting that the rotations performed much better in conserving soil and water resources than continuous wheat cropping; (5) as a result of this study, it is strongly recommended to utilize winter wheat followed by a 3-month legume fallow crop in year 1 and then a summer crop in the next. This system provides a soil cover during both erosion-prone rainy periods while leaving the soil bare for about 7 months (October–April) every 2 years. Another alternative is to cultivate winter wheat followed by a 15-month legume crop cultivation in years 1 and 2 and then a summer crop cultivation in year 3. This system allows the soil to be covered during three rainy periods while leaving the soil bare for about 7 months (October–April) every 3 years. As most of this 7-month period is winter with low rainfall (snow) and temperatures below 0 ◦ C, not only is soil evaporation very low but the risk of erosion is also low.

Acknowledgements The authors thank Xiao-Hu Zhang and Jun-Cheng Li for their assistance in conducting the field experiment. Thanks are also due to the two anonymous reviewers for their helpful comments on the

manuscript. This research was supported by the National 973 Project “The Desertification Process and Its Prevention in north China” (G2000048704), and a special President’s fund from the Chinese Academy of Sciences.

References Alegre, J.C., Cassel, D.K., 1996. Dynamics of soil physical properties under alternative systems to slash-and-burn. Agric. Ecosyst. Environ. 58, 39–48. Chen, W.N., Dong, Z.B., Li, Z.S., Yang, Z.T., 1996. Wind tunnel test of the influence of moisture on the erodibility of Loessial sandy loam soils by wind. J. Arid Environ. 34, 391–402. Chinese Soil Taxonomy Cooperative Research Group, Institute of Soil Science, Academic Sinica, 1995. Chinese Soil Taxonomy (revised proposal). Chinese Agricultural Science and Technology Press, Beijing, China. Freedman, S.M., 1982. Human labour as an energy source for rice production in the developing world. Agroecosystems 8, 125– 136. Gao, C.-Y., 1994. Practical significance of rotation. In: Ren, J.Z. (Ed.), China–Australia Gansu Grassland Agricultural Systems Research and Development Project. Gansu Science and Technology Press, Lanzhou, pp. 56–61. Gao, C.-Y., Liu, Z.H., Zhang, X.H., Li, J.-C., Jiang, L.D., 1994. Evaluation for role of legumes in rotation systems in the Loess Plateau. In: Ren, J.Z. (Ed.), China–Australia Gansu Grassland Agricultural Systems Research and Development Project. Gansu Science and Technology Press, Lanzhou, pp. 40–47. Hamazaki, T., Paningbatan Jr., E.P., 1988. Procedures for Soil Analysis. College of Agriculture, University of the Philippines at Los Banos and Tropical Agricultural Research Center, 94 pp. Hurst, C., Rogers, P., 1983. Animal energetics: a proposed model of cattle and buffalo in the Indian subcontinent. Biomass 3, 136–149. Li, S.K., 1992. On Harnessing Sloping Lands in Priority Areas of Soil Erosion at the Middle Reaches of the Yellow River. Gansu Science and Technology Press, Lanzhou. Li, F.-R., 1998. Studies on Arid Agricultural Ecosystems. Shaanxi Science and Technology Press, Xian. Li, F.-R., Gao, C.-Y., 1994. An overall evaluation of the effectiveness of soil conservation for some rotation systems with fallow crops in a semi-arid Loess region of eastern Gansu. Chin. J. Ecol. 3, 51–55. Li, S.Q., Gu, X.Y., 1992. Effect of combined application of organic and inorganic fertilizers on soil structure. In: Fu, J.P., Wang, Z.Q. (Eds.), Research on Agricultural Environmental Ecology and Amelioration of Soils. China Science Press, Beijing, pp. 32–38. Li, F.-R., Cook, S., Geballe, G.T., Burch Jr., W.R., 2000a. Rainwater harvesting agriculture: an integrated system for water management on rainfed land in China’s semi-arid areas. Ambio 29 (8), 477–483.

F.-R. Li et al. / Agriculture, Ecosystems and Environment 91 (2002) 101–111 Li, F.-R., Zhao, S.L., Geballe, G.T., 2000b. Water use patterns and agronomic performance for some cropping systems with and without fallow crops in a semi-arid environment of northwest China. Agric. Ecosyst. Environ. 79, 129–142. Liu, J., 1988. Some analysis of the inter-cropping of corns with sweetclovers. Liaoning Agric. Sci. 123, 48–50. Liu, G., 1999. Soil conservation and sustainable agriculture on the Loess Plateau: challenges and prospects. Ambio 28, 663–668. Mitchell, R., 1979. The Analysis of Indian Agroecosystems. Interprint, New Delhi, 180 pp. Nilson, E.B., Stahlman, P.W., Thompson, C.A., Gwin, R.E., Norwood, C.A., Lamm, F.R., Mikesell, M.E., 1985. Wheat-grain sorghum–fallow using reduced tillage and herbicides, Kans. Agric. Exp. Stn. Rep. Prog. 482. Norwood, C.A., 1994. Profile water distribution and grain yield as affected by cropping system and tillage. Agro. J. 86, 558–563. Olsen, S.R., Cole, C.V., Watanabe, F.S., Dean, L.A., 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonate. US Department of Agricultural Circulars, 939 pp. Peterson, G.A., Schlegel, A.J., Tanaka, D.L., Jones, O.R., 1996. Precipitation-use efficiency as affected by cropping and tillage systems. J. Prod. Agric. 9, 180–186. Pimentel, D., Hurd, L.E., Bellotti, A.C., Forster, M.J., Oka, I.N., Sholes, O.D., Whitman, R.J., 1973. Food production and the energy crisis. Science 182, 443–449. Ren, J.Z., 1992. Ecological productivity of grassland farming system on the Loess Plateau of China. In: Ren, J.Z. (Ed.), Proceedings of the International Conference on Agroecosystems

111

in the Loess Plateau. Gansu Science and Technology Press, Lanzhou, pp. 3–7. Shan, L., 1993. The Theory and Practice of Dryland Agriculture in the Loess Plateau of China. Science Press, Beijing, China. Shang, S., Liu, H., 1988. The influences of continuous cropping on soybean production. Heilongjiang Agric. Sci. 60, 37–40. Tang, K., 1992. Soil erosion and soil conservation in China. International Training Course on Soil and Water Conservation, Yangling, Shaanxi, China, pp. 6–7. Unger, P.W., 1984. Tillage and residue effects on wheat, sorghum, and sunflower grown rotation. Soil Sci. Soc. Am. J. 48, 885– 891. Wang, D., Fan, F., 1987. Study of the index of continuous cropping. Liaoning Agric. Sci. 114, 9–11. Wen, D., Pimentel, D., 1992. Ecological resource management to achieve a productive, sustainable agricultural system in northeast China. Agric. Ecosyst. Environ. 41, 215–230. Zhang, Z.H., 1992. Grassland farming and its technologies in the Loess Plateau. In: Ren, J.Z. (Ed.), Proceedings of the International Conference on Agroecosystems in the Loess Plateau. Gansu Science and Technology Press, Lanzhou, pp. 97–101. Zhang, Q.W., Shang, Z.B., Xu, J.Q., 1989. Evaluation of the national productivity potential of dryland farming on the Loess Plateau and discussion of the outlet of dryland farming. J. Natural Res. 4 (1), 4–11. Zhu, Z.X., Stewart, B.A., Fu, X.J., 1994. Double cropping wheat and corn in a sub-humid region of China. Field Crops Res. 36, 175–183.