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Abstract ［Objectives］ This study was conducted to investigate the effects on agronomic traits, yield and benefit of one-year triple cropping "maize-soybean" strip intercropping system under reduced fertilizer application.
［Methods］ A single factor randomized block design was used, and wide and narrow strip intercropping was adopted. Three different fertilization treatments, namely fertilizers for conventional field fertilization (A1), 80% A1 (A2) and 60% A1 (A3) were applied to the field production. The fertilizing sites were about 25 cm away from maize and repeated three times.
［Results］ Compared with A1, the change laws of the agronomic traits and yield in the A2 and A3 intercropping systems were described as below. Agronomic traits: The plant height became shorter gradually for the spring soybean, but increased gradually for the summer soybean, the bottom pod height gradually reduced, and the node number, pods per plant and seeds per plant increased; and the plant height, ear length, rows per ear, seeds per row and 1 000-seed weight of the spring maize decreased. Yield: The yield of maize as the main crop decreased but not significant, by 3.20% and 3.99%, respectively in A2 and A3, both smaller than 5.00%; the yield of the spring soybean significantly increased by 9.70% and 11.84%, respectively; the yield of the summer soybean increased by 5.18% and 8.98%, respectively; and the total yield increased by 0.20% and 0.92%, respectively. Benefit: The total output value increased by 2.97 % and 4.91 %, respectively, and the total benefit increased by 6.39% and 11.22%, respectively.
［Conclusions］ Under a 40% reduction in fertilizer applied in the field, the one-year triple cropping "maize-soybean" strip intercropping system still met the multi-target requirements of increasing grain production and economic benefits, and thus can be promoted and applied in Guangxi.
Key words Guangxi; Maize-soybean; Intercropping; Fertilization; Yield; Benefit
Received: January 6, 2020Accepted: March 8, 2020
Supported by Earmarked Fund for China Agriculture Research System (CARS-04-CES30); Key Research and Development Project of Guangxi (GK AB16380097, GK AB18221057); Fundamental Scientific Research Fund of Guangxi Academy of Agricultural Sciences (GNK 2020YM118).
Fuyue TANG (1984-), male, P. R. China, associate researcher, master, devoted to research about soybean breeding and cultivation.
*Corresponding author. E-mail: chenyuan500@126.com. In recent years, a new one-year triple cropping "maize-soybean" strip intercropping model developed in Guangxi in recent years has been promoted and applied in Guangxi, because it solves small machinery operation problems in traditional "spring maize/summer soybean" planting system and has the advantages of high land output and resource utilization efficiency［1］. It is necessary to carry out research on the effects of reduced fertilizer application on the agronomic traits, yield and benefit of crops under this model, so as to improve fertilizer utilization rate and production efficiency and provide a scientific theoretical basis for efficient and reasonable fertilizer application in this model. Fertilizers have a contribution rate reaching more than 50% to the output and yield of grain crops, playing a very important role in food production. However, excessive fertilization not only reduces the efficiency of fertilizer use, leads to imbalance of soil nutrients, and thus brings about eutrophication of water bodies. And these problems are getting worse［2-5］. In recent years, many studies have been conducted on the fertilizer utilization mechanism under soybean intercropping. Especially, the two-year triple cropping "wheat/maize/soybean" intercropping system in the Southwest region has been shown to be more conducive to the accumulation of soybean grain protein, thereby increasing soybean protein content［6］. The intercropping system can facilitate the transport of soybean stem and pod nitrogen, phosphorus and potassium to grains, increase the total biomass of the aboveground plant, grain yields and nitrogen absorption for maize and soybean, and greatly improve the use efficiency of nitrogen and phosphorus fertilizers［7-9］. It can increase the activity of maize and soybean rhizosphere soil fungi, actinomycete soil protease and maize urease, and the amount of soybean nitrogen-fixing bacteria. The application of appropriate amount of nitrogen can help maize/soybean to absorb nitrogen in the soil, and low phosphorus is more beneficial to the activation and absorption of Al-P and Fe-P in soil by maize/soybean［10-11］. By these measures, the clear advantages of intercropping are reflected in terms of quality, yield, output value, and nutrient absorption and utilization efficiency. The one-year double cropping "soybean/potato" intercropping system in the northwest region can increase soil organic matter content, promote soil microbial activity and enzyme activity, and effectively improve soil microbial structure. Compared with single cropping, it can significantly improve crops efficient use of N, P and K, the total nutrient absorption of which increased by 47%, 28%, and 41%, respectively. N and P have the greatest impact on the population yield of the system. With the N level increasing, the population yield first increases and then decreases, and with the increase of P level, the population yield increases［12-14］. Chen et al. ［15］ found through four years of experiments on five planting methods that the "soybean/potato" continuous cropping, compared with other four planting methods, had relatively stable population yield and showed significant productivity. For the one-year triple cropping "maize-soybean" strip intercropping pattern in South China, Tang and Chen et al. ［16-17］ studied the effects of different row ratio configurations of maize and soybean and different maize plant types. It was found that the when planting semi-compact maize and spring soybean in wide and narrow rows (160 cm for wide row, 40 cm for narrow row, planting 2-3 rows of spring soybean in the wide rows of maize), the total group output, total LER and IE were the largest, and the social, ecological and economic benefits were the best. Tang et al. ［1］ investigated two intercropping models ("spring maize‖spring soybean/summer soybean", "spring maize/summer soybean") and two rotational planting models ("spring maize/summer soybean", "spring soybean/ summer soybean"). The results showed that the "spring maize‖spring soybean/summer soybean" system had a maximum LER of 2.49 and total yield and output value significantly higher than other planting models. In recent years, Guangxi has developed a new one-year triple cropping "maize-soybean" strip intercropping model, which is to change the traditional "spring maize/summer soybean" to wide and narrow row planting by increasing one season of spring soybean in the wide rows of spring maize in spring and interplanting one season of summer soybean in the wide rows of maize in summer, forming a new "spring maizell spring soybean/summer soybean" intercropping model. This intercorpping model has been promoted and applied because of solving the problems of mechanized operation and increasing one season of grain harvest. At present, the previous studies mainly focus on one-year single cropping and one-year double cropping systems, as well as two-year triple cropping "wheat/maize/soybean" system in Sichuan. There has been no research on the fertilizer use and the agronomic traits, yield and benefit of the one-year triple cropping "spring maize‖spring soybean/summer soybean" strip intercropping model. In this study, with the new one-year triple cropping "spring maize‖spring soybean/summer soybean" strip intercropping model as an object, the fertilization measures with overall better grain production, economic benefit and ecological benefit were explored under the premise of reduced fertilization through the investigation on the effects of reducing fertilizer application on the agronomic traits, yield, and benefits of this model, providing a scientific theoretical basis for efficient and reasonable fertilization and rapid popularization of this model. Materials and Methods
Test location
The test was carried out at Mingyang Base (108°25′E, 22°61′N ) of Guangxi Academy of Agricultural Sciences in Nanning, Guangxi in 2017. The soil is red soil. The pH of the mixed soil sample in the plow layer (0-20 cm) before sowing was 6.8, and other properties were as follows: organic matter content 11.9 g/kg, total nitrogen content 0.053%, total phosphorus content 0.106%, total potassium content 0.170%, alkaline nitrogen content 51.0 mg/kg, available phosphorus content 6.4 mg/kg, and rapidly available potassium content 114.0 mg/kg.
Test materials
For the test soybean variety, independently selected spring soybean variety Guichun 16 and summer soybean variety Guixia 7 were selected. For the spring maize variety, the semi-compact variety Yidan 629 bred by Xiangyang Guanzhilin Technology Co., Ltd. and approved by Guangxi was selected. The test compound fertilizer (15∶15∶15), urea (containing 46% of N), potassium chloride (containing 60% of K) and calcium magnesium phosphate (containing 18% of P) were purchased from the local agricultural market.
Planting mode and specifications
This experiment used a "maize-soybean" ("spring maize‖spring soybean/summer soybean") strip intercropping pattern, in which the wide and narrow maize rows were 160 and 40 cm wide, respectively, and each wide maize row was interplanted with three rows of spring soybean. After the harvest of the spring soybean, about 15-20 d before the harvest of spring maize, three rows of summer soybean were interplanted in each wide row of spring maize. The distance between soybean and maize was 40 cm, and the row spacing of soybean was 40 cm; the maize was planted with a hole spacing of 20 cm, one plant per hole; and the soybean was planted with a hole spacing of 20 cm, three plants per hole.
A single-factor random block design was adopted with 3 fertilization treatments in 3 repetitions. The fertilizing sites of "spring maize‖spring soybean" were 25 cm away from the maize. The "spring maize/summer soybean" in the later period was not fertilized. The plot area was 30 m2 (5 × 6 m). Specifically, treatment A1 was consistent with the fertilization of field production. It was applied with 300 kg/hm2 of compound fertilizer, 150 kg/hm2 of potassium chloride and 150 kg/hm2 of calcium magnesium phosphate as the base fertilizer, with 150 kg/hm2 of urea and 75 kg/hm2 of compound fertilizer for maize seedlings, and with 375 kg/hm2 of compound fertilizer (15∶15∶15) at the flare opening stage. A2 was the same as A1 in fertilizing time, but the fertilizing amount was reduced by 20% compared with A1. A3 was also the same as A1 in fertilizing time, but the fertilizing amount was reduced by 40% compared with A1 (Table 1). Spring maize and spring soybean were planted simultaneously on March 10, 2017. The spring soybean was harvested on June 16, and the spring maize was harvested on July 11. The summer soybean was planted on June 27 and harvested on October 18. Other cultivation and management were the same as in field production.
Fuyue TANG et al. Effects on Agronomic Traits, Yield and Benefit of One-year Triple Cropping "Maize-soybean" Strip Intercropping System Under Reduced Fertilizer Application
Investigation and determination items and methods
Growth period
The growth periods of the spring maize, spring soybean and summer soybean in the field were recorded.
Sampling and measurement methods
When the crops were mature, the maize and soybean grain yields were determined in a middle wide row and narrow row of 10 m2 in each plot, and converted to hectare yields; seven consecutive maize plants were investigated for the ear length, ear diameter, number of rows per ear, seeds per row and 1 000-seed weight; and 10 consecutive soybean plants were tested in laboratory, for soybean plant height, bottom pod height, node number of main stem, branch number, number of effective pods per plant, number of seeds per plant and 100-grain weight.
Economic benefit
The economic benefit was calculated according to Economic benefit=Total output value-Total input cost, where the total input cost includes seed, fertilizer and labor costs. All inputs and benefits were calculated based on local current levels. The seed processing fee was 1 650 yuan/hm2 (900 yuan/hm2 for spring maize, 375 yuan/hm2 for spring soybean, and 375 yuan/hm2 for summer soybean), and the labor required was 15 000 yuan/hm2 (150 d/hm2×100 yuan/d). The prices of fertilizers were as follows: compound fertilizer (15∶15∶15) 2.5 yuan/kg, potassium chloride (containing K 60%) 2.6 yuan/kg, calcium magnesium phosphate fertilizer (containing P 18%) 1.8 yuan/kg, and urea (containing N 46%) 2.0 yuan/kg. The soybean purchase price was 6.0 yuan/kg, and the maize purchase price was 1.8 yuan/kg.
Data analysis
Statistical analysis was performed using Excel 2010 and SPSS 19.0 software.
Results and Analysis
Effects of different fertilization measures on the growth period of the "maize-soybean" strip intercropping system
The growth periods of spring maize, spring soybean, and summer soybean were 117, 87 and 97 d, respectively, in treatments A2, A3 and A1, without differences. It indicated that the reduced fertilizer application had no effect on the growth period of the "maize-soybean" strip intercropping system. Effects of different fertilization measures on the field agronomic traits and yield components in the "maize-soybean" strip intercropping system
Effects of different fertilization measures on the field agronomic traits of the spring soybean and its yield components
From the perspective of plant morphology, the plant heights and bottom pod heights of the spring soybean ranked as A1>A2>A3, the node numbers of main stem followed A3>A2=A1, and the numbers of effective branches showed an order of A1>A3>A2, but the differences were not significant. From the perspective of yield trait, pods per plant, seeds per plant and yields of the spring soybean ranked as A3>A2>A1, of which A1 was significantly lower than A2 and A3, and A2 and A3 were not significantly different, and the 100-seed weights were in order of A3>A1>A2, which were not significant (Table 2). It showed that under the reduced fertilizer application, the plant height of the spring soybean decreased, the bottom pod height became shorter, the node number of main stem increased, the number of pods per plant and the number of seeds per plant increased, and the yield increased significantly.
Effects of different fertilization measures on the field agronomic traits of the spring maize and its yield components
From the perspective of plant morphology, the plant heights and ear lengths of the spring maize were in order of A1≥A2>A3, and the ear diameters ranked as A3>A2≥A1, but the differences were not significant. For plant yield traits, the rows per ear, seeds per row, 1 000-seed weights and yields of the spring maize showed an order of A1>A2>A3, but the differences were not significant (Table 3). It showed that under the reduced fertilizer application, plant height, ear length, ear rows, seeds per row, 1 000-seed weight and yield of the spring maize gradually decreased. A2 and A3 reduced the yield of the spring maize compared with A1 by 3.20% and 3.99%, respectively, both less than 5.00%, so the effect on maize as the main crop was less.
Effects of different fertilization measures on the field agronomic traits of the summer soybean and its yield components
From the perspective of plant morphology, the plant heights and node numbers of main stem of the summer soybean ranked as A1>A2>A3, the bottom pod heights were in order of A3> A2> A1, and the number of effective branches followed A2>A3>A1. The number of effective branches in A2 was significantly larger than that in A1, while the remaining differences were not significant. In terms of yield traits, pods per plant, seeds per plant and yields of the summer soybean all followed A3>A2>A1, of which A1 was significantly lower than A2 and A3, while A2 and A3 were not significantly different, and the 100-seed weights were in order of A3≥A1>A2, without significant differences (Table 3). It showed that when the fertilizer application was reduced, the plant height of the summer soybean increased, the bottom pod height became shorter, the node number of main stem increased, pods per plant and seeds per plant increased, and the yield increased significantly. Comparison of population yield and output benefit among different fertilization measures
From the perspective of yield (Table 5), the "maize-soybean" strip interplanting system showed yields of spring maize ranking as A1>A2>A3, which were not significantly different. Specifically, A2 and A3 decreased the yield of the spring maize by 3.20% and 3.99%, respectively compared with A1. The unit yields of the spring soybean were in order of A3>A2> A1, of which A2 and A3 were significantly higher than A1, 9.70% and 11.84% higher than A1, respectively. The yields of the summer soybean showed an order of A3>A2>A1, of which A3 was significantly higher than A1, leaving remaining differences being not significant, and A2 and A3 increased the yield by 5.18% and 8.98% compared with A1, respectively. The total population yields ranked as A3>A2>A1, but the differences were not significant. A2 (13 600.8 kg/hm2) and A3 (13 504.3 kg/hm2) increased the population yield by 0.20% and 0.92%, respectively, compared with A1 (13 477.0 kg/hm2).
From the perspective of output benefits (Table 5), the total output value followed A3>A2>A1, of which A3 was significantly higher than A1, leaving remaining differences being not significant, and A2 and A3 increased the output value by 2.97% and 4.91% , respectively, compared with A1. The economic benefits were in order of A3>A2＞A1, of which A2 (31 501 yuan/hm2) and A3 (32 932 yuan/hm2) significantly increased the economic benefit by 6.39% and 11.22%, respectively, compared with A1 (29 610 yuan/hm2).
The results showed that after the fertilizing amount was reduced by 20% and 40% in production, the yield of the main crop maize decreased, but the yield reduction was not obvious; and the yields of the spring soybean and summer soybean increased, which not only made up for the drop of the maize yield, but also made the population yield increased. As a result, the economic benefit was significantly increased by reducing the cost of fertilizers and increasing the total output value of the crops. After the fertilizer application of the "maize-soybean" strip intercropping system was reduced by 20% and 40%, the yield of the main crop maize decreased, but increased total grain output and good ecological and economic benefits were achieved.
Discussion
This study showed that in the one-year triple cropping "maize-soybean" strip intercropping system, the spring soybean showed shorter plants and significantly increased yield and maize exhibited decreased yield during the "spring maize‖spring soybean" period, which accord with the results in previous studies that in the "spring maize‖spring soybean" system, the plant height of spring soybean was negatively correlated with its yield, and maize is stronger than soybean in the interspecific competition between maize and soybean［1,7,17-18］. The intercropping of soybean shows obvious advantages over nutrient uptake and utilization of single cropping. Chen et al. ［9］ showed that the total nutrient absorption of N, P and K in "soybean/potato" intercropping increased by 47%, 28% and 41%, respectively. Li et al. ［19］ deemed that when soybean was intercropped with gramineous maize, soybean promotes nitrogen absorption by maize through nitrogen fixation and nitrogen transfer by root nodules and maize stimulates and improves the nitrogen fixation efficiency of soybean nodules through nitrogen absorption, thereby showing intercropping advantage. The "wheat/maize/soybean" intercropping system has higher nitrogen absorption and use efficiency and agronomic use efficiency of maize than the single cropping system, 16.4% and 77.8% higher, respectively; N, P, and K in soybean stems and pods are better transported to the grains, which greatly improves the use efficiency of N and P; there is a strong interspecific competition between maize and soybean, and appropriate nitrogen application can increase the absorption of nitrogen in the soil by "maize/soybean" ; and maize yield is positively related to soil phosphorus content, while soybean is prone to excessive growth, and its yield increases first and then decreases under a higher soil phosphorus content［8-9, 20-24］. This study showed that for the one-year triple cropping "maize-soybean" strip intercropping system, based on a 20% and 40% reduction in the fertilizing amount for soybean (the same as the fertilizing amount for single maize planting), maize yield decreased but not significantly, and the yield reduction was less than 5.00%; and the yields of spring soybean and summer soybean increased, especially after the 40% reduction, the yields of spring and summer soybean increased significantly by 11.84% and 8.98%, respectively. These results are consistent with previous studies. For the one-year triple cropping "maize-soybean" strip composite system, it is necessary to carry out in-depth research on the interspecific transfer of soybean root nodule-fixed N element and nitrate N in soil between soybean and maize, corresponding operating mechanism, balanced utilization of P fertilizers and the mechanism of spring maize straw and intercropping fertilizer in increasing yield and efficiency of soybean, so as to provide better scientific theoretical support for the efficient fertilization of this model. Hong et al. ［25］ studied the effects of different fertilization measures on nitrogen and phosphorus balance and ecological and economic benefits of red soil paddy fields. The results showed that the multiple objectives of ecological environment construction, food production increase and farmers income increase could not be met simultaneously. This study showed that after reducing fertilization, the yield of spring maize as the main crop was reduced, but multi-target requirements such as increase in total food production, eco-friendliness and increase in farmers economic income were realized relying on the increase of one season of spring soybean and one season of summer soybean.
Conclusions
When the amount of fertilizers applied in field production was reduced by 40%, the "maize-soybean" strip intercropping system still met the multi-target requirements of increasing grain production and improving economic benefits, and thus can be promoted and applied in Guangxi.
References
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Editor: Yingzhi GUANGProofreader: Xinxiu ZHU
［Methods］ A single factor randomized block design was used, and wide and narrow strip intercropping was adopted. Three different fertilization treatments, namely fertilizers for conventional field fertilization (A1), 80% A1 (A2) and 60% A1 (A3) were applied to the field production. The fertilizing sites were about 25 cm away from maize and repeated three times.
［Results］ Compared with A1, the change laws of the agronomic traits and yield in the A2 and A3 intercropping systems were described as below. Agronomic traits: The plant height became shorter gradually for the spring soybean, but increased gradually for the summer soybean, the bottom pod height gradually reduced, and the node number, pods per plant and seeds per plant increased; and the plant height, ear length, rows per ear, seeds per row and 1 000-seed weight of the spring maize decreased. Yield: The yield of maize as the main crop decreased but not significant, by 3.20% and 3.99%, respectively in A2 and A3, both smaller than 5.00%; the yield of the spring soybean significantly increased by 9.70% and 11.84%, respectively; the yield of the summer soybean increased by 5.18% and 8.98%, respectively; and the total yield increased by 0.20% and 0.92%, respectively. Benefit: The total output value increased by 2.97 % and 4.91 %, respectively, and the total benefit increased by 6.39% and 11.22%, respectively.
［Conclusions］ Under a 40% reduction in fertilizer applied in the field, the one-year triple cropping "maize-soybean" strip intercropping system still met the multi-target requirements of increasing grain production and economic benefits, and thus can be promoted and applied in Guangxi.
Key words Guangxi; Maize-soybean; Intercropping; Fertilization; Yield; Benefit
Received: January 6, 2020Accepted: March 8, 2020
Supported by Earmarked Fund for China Agriculture Research System (CARS-04-CES30); Key Research and Development Project of Guangxi (GK AB16380097, GK AB18221057); Fundamental Scientific Research Fund of Guangxi Academy of Agricultural Sciences (GNK 2020YM118).
Fuyue TANG (1984-), male, P. R. China, associate researcher, master, devoted to research about soybean breeding and cultivation.
*Corresponding author. E-mail: chenyuan500@126.com. In recent years, a new one-year triple cropping "maize-soybean" strip intercropping model developed in Guangxi in recent years has been promoted and applied in Guangxi, because it solves small machinery operation problems in traditional "spring maize/summer soybean" planting system and has the advantages of high land output and resource utilization efficiency［1］. It is necessary to carry out research on the effects of reduced fertilizer application on the agronomic traits, yield and benefit of crops under this model, so as to improve fertilizer utilization rate and production efficiency and provide a scientific theoretical basis for efficient and reasonable fertilizer application in this model. Fertilizers have a contribution rate reaching more than 50% to the output and yield of grain crops, playing a very important role in food production. However, excessive fertilization not only reduces the efficiency of fertilizer use, leads to imbalance of soil nutrients, and thus brings about eutrophication of water bodies. And these problems are getting worse［2-5］. In recent years, many studies have been conducted on the fertilizer utilization mechanism under soybean intercropping. Especially, the two-year triple cropping "wheat/maize/soybean" intercropping system in the Southwest region has been shown to be more conducive to the accumulation of soybean grain protein, thereby increasing soybean protein content［6］. The intercropping system can facilitate the transport of soybean stem and pod nitrogen, phosphorus and potassium to grains, increase the total biomass of the aboveground plant, grain yields and nitrogen absorption for maize and soybean, and greatly improve the use efficiency of nitrogen and phosphorus fertilizers［7-9］. It can increase the activity of maize and soybean rhizosphere soil fungi, actinomycete soil protease and maize urease, and the amount of soybean nitrogen-fixing bacteria. The application of appropriate amount of nitrogen can help maize/soybean to absorb nitrogen in the soil, and low phosphorus is more beneficial to the activation and absorption of Al-P and Fe-P in soil by maize/soybean［10-11］. By these measures, the clear advantages of intercropping are reflected in terms of quality, yield, output value, and nutrient absorption and utilization efficiency. The one-year double cropping "soybean/potato" intercropping system in the northwest region can increase soil organic matter content, promote soil microbial activity and enzyme activity, and effectively improve soil microbial structure. Compared with single cropping, it can significantly improve crops efficient use of N, P and K, the total nutrient absorption of which increased by 47%, 28%, and 41%, respectively. N and P have the greatest impact on the population yield of the system. With the N level increasing, the population yield first increases and then decreases, and with the increase of P level, the population yield increases［12-14］. Chen et al. ［15］ found through four years of experiments on five planting methods that the "soybean/potato" continuous cropping, compared with other four planting methods, had relatively stable population yield and showed significant productivity. For the one-year triple cropping "maize-soybean" strip intercropping pattern in South China, Tang and Chen et al. ［16-17］ studied the effects of different row ratio configurations of maize and soybean and different maize plant types. It was found that the when planting semi-compact maize and spring soybean in wide and narrow rows (160 cm for wide row, 40 cm for narrow row, planting 2-3 rows of spring soybean in the wide rows of maize), the total group output, total LER and IE were the largest, and the social, ecological and economic benefits were the best. Tang et al. ［1］ investigated two intercropping models ("spring maize‖spring soybean/summer soybean", "spring maize/summer soybean") and two rotational planting models ("spring maize/summer soybean", "spring soybean/ summer soybean"). The results showed that the "spring maize‖spring soybean/summer soybean" system had a maximum LER of 2.49 and total yield and output value significantly higher than other planting models. In recent years, Guangxi has developed a new one-year triple cropping "maize-soybean" strip intercropping model, which is to change the traditional "spring maize/summer soybean" to wide and narrow row planting by increasing one season of spring soybean in the wide rows of spring maize in spring and interplanting one season of summer soybean in the wide rows of maize in summer, forming a new "spring maizell spring soybean/summer soybean" intercropping model. This intercorpping model has been promoted and applied because of solving the problems of mechanized operation and increasing one season of grain harvest. At present, the previous studies mainly focus on one-year single cropping and one-year double cropping systems, as well as two-year triple cropping "wheat/maize/soybean" system in Sichuan. There has been no research on the fertilizer use and the agronomic traits, yield and benefit of the one-year triple cropping "spring maize‖spring soybean/summer soybean" strip intercropping model. In this study, with the new one-year triple cropping "spring maize‖spring soybean/summer soybean" strip intercropping model as an object, the fertilization measures with overall better grain production, economic benefit and ecological benefit were explored under the premise of reduced fertilization through the investigation on the effects of reducing fertilizer application on the agronomic traits, yield, and benefits of this model, providing a scientific theoretical basis for efficient and reasonable fertilization and rapid popularization of this model. Materials and Methods
Test location
The test was carried out at Mingyang Base (108°25′E, 22°61′N ) of Guangxi Academy of Agricultural Sciences in Nanning, Guangxi in 2017. The soil is red soil. The pH of the mixed soil sample in the plow layer (0-20 cm) before sowing was 6.8, and other properties were as follows: organic matter content 11.9 g/kg, total nitrogen content 0.053%, total phosphorus content 0.106%, total potassium content 0.170%, alkaline nitrogen content 51.0 mg/kg, available phosphorus content 6.4 mg/kg, and rapidly available potassium content 114.0 mg/kg.
Test materials
For the test soybean variety, independently selected spring soybean variety Guichun 16 and summer soybean variety Guixia 7 were selected. For the spring maize variety, the semi-compact variety Yidan 629 bred by Xiangyang Guanzhilin Technology Co., Ltd. and approved by Guangxi was selected. The test compound fertilizer (15∶15∶15), urea (containing 46% of N), potassium chloride (containing 60% of K) and calcium magnesium phosphate (containing 18% of P) were purchased from the local agricultural market.
Planting mode and specifications
This experiment used a "maize-soybean" ("spring maize‖spring soybean/summer soybean") strip intercropping pattern, in which the wide and narrow maize rows were 160 and 40 cm wide, respectively, and each wide maize row was interplanted with three rows of spring soybean. After the harvest of the spring soybean, about 15-20 d before the harvest of spring maize, three rows of summer soybean were interplanted in each wide row of spring maize. The distance between soybean and maize was 40 cm, and the row spacing of soybean was 40 cm; the maize was planted with a hole spacing of 20 cm, one plant per hole; and the soybean was planted with a hole spacing of 20 cm, three plants per hole.
A single-factor random block design was adopted with 3 fertilization treatments in 3 repetitions. The fertilizing sites of "spring maize‖spring soybean" were 25 cm away from the maize. The "spring maize/summer soybean" in the later period was not fertilized. The plot area was 30 m2 (5 × 6 m). Specifically, treatment A1 was consistent with the fertilization of field production. It was applied with 300 kg/hm2 of compound fertilizer, 150 kg/hm2 of potassium chloride and 150 kg/hm2 of calcium magnesium phosphate as the base fertilizer, with 150 kg/hm2 of urea and 75 kg/hm2 of compound fertilizer for maize seedlings, and with 375 kg/hm2 of compound fertilizer (15∶15∶15) at the flare opening stage. A2 was the same as A1 in fertilizing time, but the fertilizing amount was reduced by 20% compared with A1. A3 was also the same as A1 in fertilizing time, but the fertilizing amount was reduced by 40% compared with A1 (Table 1). Spring maize and spring soybean were planted simultaneously on March 10, 2017. The spring soybean was harvested on June 16, and the spring maize was harvested on July 11. The summer soybean was planted on June 27 and harvested on October 18. Other cultivation and management were the same as in field production.
Fuyue TANG et al. Effects on Agronomic Traits, Yield and Benefit of One-year Triple Cropping "Maize-soybean" Strip Intercropping System Under Reduced Fertilizer Application
Investigation and determination items and methods
Growth period
The growth periods of the spring maize, spring soybean and summer soybean in the field were recorded.
Sampling and measurement methods
When the crops were mature, the maize and soybean grain yields were determined in a middle wide row and narrow row of 10 m2 in each plot, and converted to hectare yields; seven consecutive maize plants were investigated for the ear length, ear diameter, number of rows per ear, seeds per row and 1 000-seed weight; and 10 consecutive soybean plants were tested in laboratory, for soybean plant height, bottom pod height, node number of main stem, branch number, number of effective pods per plant, number of seeds per plant and 100-grain weight.
Economic benefit
The economic benefit was calculated according to Economic benefit=Total output value-Total input cost, where the total input cost includes seed, fertilizer and labor costs. All inputs and benefits were calculated based on local current levels. The seed processing fee was 1 650 yuan/hm2 (900 yuan/hm2 for spring maize, 375 yuan/hm2 for spring soybean, and 375 yuan/hm2 for summer soybean), and the labor required was 15 000 yuan/hm2 (150 d/hm2×100 yuan/d). The prices of fertilizers were as follows: compound fertilizer (15∶15∶15) 2.5 yuan/kg, potassium chloride (containing K 60%) 2.6 yuan/kg, calcium magnesium phosphate fertilizer (containing P 18%) 1.8 yuan/kg, and urea (containing N 46%) 2.0 yuan/kg. The soybean purchase price was 6.0 yuan/kg, and the maize purchase price was 1.8 yuan/kg.
Data analysis
Statistical analysis was performed using Excel 2010 and SPSS 19.0 software.
Results and Analysis
Effects of different fertilization measures on the growth period of the "maize-soybean" strip intercropping system
The growth periods of spring maize, spring soybean, and summer soybean were 117, 87 and 97 d, respectively, in treatments A2, A3 and A1, without differences. It indicated that the reduced fertilizer application had no effect on the growth period of the "maize-soybean" strip intercropping system. Effects of different fertilization measures on the field agronomic traits and yield components in the "maize-soybean" strip intercropping system
Effects of different fertilization measures on the field agronomic traits of the spring soybean and its yield components
From the perspective of plant morphology, the plant heights and bottom pod heights of the spring soybean ranked as A1>A2>A3, the node numbers of main stem followed A3>A2=A1, and the numbers of effective branches showed an order of A1>A3>A2, but the differences were not significant. From the perspective of yield trait, pods per plant, seeds per plant and yields of the spring soybean ranked as A3>A2>A1, of which A1 was significantly lower than A2 and A3, and A2 and A3 were not significantly different, and the 100-seed weights were in order of A3>A1>A2, which were not significant (Table 2). It showed that under the reduced fertilizer application, the plant height of the spring soybean decreased, the bottom pod height became shorter, the node number of main stem increased, the number of pods per plant and the number of seeds per plant increased, and the yield increased significantly.
Effects of different fertilization measures on the field agronomic traits of the spring maize and its yield components
From the perspective of plant morphology, the plant heights and ear lengths of the spring maize were in order of A1≥A2>A3, and the ear diameters ranked as A3>A2≥A1, but the differences were not significant. For plant yield traits, the rows per ear, seeds per row, 1 000-seed weights and yields of the spring maize showed an order of A1>A2>A3, but the differences were not significant (Table 3). It showed that under the reduced fertilizer application, plant height, ear length, ear rows, seeds per row, 1 000-seed weight and yield of the spring maize gradually decreased. A2 and A3 reduced the yield of the spring maize compared with A1 by 3.20% and 3.99%, respectively, both less than 5.00%, so the effect on maize as the main crop was less.
Effects of different fertilization measures on the field agronomic traits of the summer soybean and its yield components
From the perspective of plant morphology, the plant heights and node numbers of main stem of the summer soybean ranked as A1>A2>A3, the bottom pod heights were in order of A3> A2> A1, and the number of effective branches followed A2>A3>A1. The number of effective branches in A2 was significantly larger than that in A1, while the remaining differences were not significant. In terms of yield traits, pods per plant, seeds per plant and yields of the summer soybean all followed A3>A2>A1, of which A1 was significantly lower than A2 and A3, while A2 and A3 were not significantly different, and the 100-seed weights were in order of A3≥A1>A2, without significant differences (Table 3). It showed that when the fertilizer application was reduced, the plant height of the summer soybean increased, the bottom pod height became shorter, the node number of main stem increased, pods per plant and seeds per plant increased, and the yield increased significantly. Comparison of population yield and output benefit among different fertilization measures
From the perspective of yield (Table 5), the "maize-soybean" strip interplanting system showed yields of spring maize ranking as A1>A2>A3, which were not significantly different. Specifically, A2 and A3 decreased the yield of the spring maize by 3.20% and 3.99%, respectively compared with A1. The unit yields of the spring soybean were in order of A3>A2> A1, of which A2 and A3 were significantly higher than A1, 9.70% and 11.84% higher than A1, respectively. The yields of the summer soybean showed an order of A3>A2>A1, of which A3 was significantly higher than A1, leaving remaining differences being not significant, and A2 and A3 increased the yield by 5.18% and 8.98% compared with A1, respectively. The total population yields ranked as A3>A2>A1, but the differences were not significant. A2 (13 600.8 kg/hm2) and A3 (13 504.3 kg/hm2) increased the population yield by 0.20% and 0.92%, respectively, compared with A1 (13 477.0 kg/hm2).
From the perspective of output benefits (Table 5), the total output value followed A3>A2>A1, of which A3 was significantly higher than A1, leaving remaining differences being not significant, and A2 and A3 increased the output value by 2.97% and 4.91% , respectively, compared with A1. The economic benefits were in order of A3>A2＞A1, of which A2 (31 501 yuan/hm2) and A3 (32 932 yuan/hm2) significantly increased the economic benefit by 6.39% and 11.22%, respectively, compared with A1 (29 610 yuan/hm2).
The results showed that after the fertilizing amount was reduced by 20% and 40% in production, the yield of the main crop maize decreased, but the yield reduction was not obvious; and the yields of the spring soybean and summer soybean increased, which not only made up for the drop of the maize yield, but also made the population yield increased. As a result, the economic benefit was significantly increased by reducing the cost of fertilizers and increasing the total output value of the crops. After the fertilizer application of the "maize-soybean" strip intercropping system was reduced by 20% and 40%, the yield of the main crop maize decreased, but increased total grain output and good ecological and economic benefits were achieved.
Discussion
This study showed that in the one-year triple cropping "maize-soybean" strip intercropping system, the spring soybean showed shorter plants and significantly increased yield and maize exhibited decreased yield during the "spring maize‖spring soybean" period, which accord with the results in previous studies that in the "spring maize‖spring soybean" system, the plant height of spring soybean was negatively correlated with its yield, and maize is stronger than soybean in the interspecific competition between maize and soybean［1,7,17-18］. The intercropping of soybean shows obvious advantages over nutrient uptake and utilization of single cropping. Chen et al. ［9］ showed that the total nutrient absorption of N, P and K in "soybean/potato" intercropping increased by 47%, 28% and 41%, respectively. Li et al. ［19］ deemed that when soybean was intercropped with gramineous maize, soybean promotes nitrogen absorption by maize through nitrogen fixation and nitrogen transfer by root nodules and maize stimulates and improves the nitrogen fixation efficiency of soybean nodules through nitrogen absorption, thereby showing intercropping advantage. The "wheat/maize/soybean" intercropping system has higher nitrogen absorption and use efficiency and agronomic use efficiency of maize than the single cropping system, 16.4% and 77.8% higher, respectively; N, P, and K in soybean stems and pods are better transported to the grains, which greatly improves the use efficiency of N and P; there is a strong interspecific competition between maize and soybean, and appropriate nitrogen application can increase the absorption of nitrogen in the soil by "maize/soybean" ; and maize yield is positively related to soil phosphorus content, while soybean is prone to excessive growth, and its yield increases first and then decreases under a higher soil phosphorus content［8-9, 20-24］. This study showed that for the one-year triple cropping "maize-soybean" strip intercropping system, based on a 20% and 40% reduction in the fertilizing amount for soybean (the same as the fertilizing amount for single maize planting), maize yield decreased but not significantly, and the yield reduction was less than 5.00%; and the yields of spring soybean and summer soybean increased, especially after the 40% reduction, the yields of spring and summer soybean increased significantly by 11.84% and 8.98%, respectively. These results are consistent with previous studies. For the one-year triple cropping "maize-soybean" strip composite system, it is necessary to carry out in-depth research on the interspecific transfer of soybean root nodule-fixed N element and nitrate N in soil between soybean and maize, corresponding operating mechanism, balanced utilization of P fertilizers and the mechanism of spring maize straw and intercropping fertilizer in increasing yield and efficiency of soybean, so as to provide better scientific theoretical support for the efficient fertilization of this model. Hong et al. ［25］ studied the effects of different fertilization measures on nitrogen and phosphorus balance and ecological and economic benefits of red soil paddy fields. The results showed that the multiple objectives of ecological environment construction, food production increase and farmers income increase could not be met simultaneously. This study showed that after reducing fertilization, the yield of spring maize as the main crop was reduced, but multi-target requirements such as increase in total food production, eco-friendliness and increase in farmers economic income were realized relying on the increase of one season of spring soybean and one season of summer soybean.
Conclusions
When the amount of fertilizers applied in field production was reduced by 40%, the "maize-soybean" strip intercropping system still met the multi-target requirements of increasing grain production and improving economic benefits, and thus can be promoted and applied in Guangxi.
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