Abstract
Continuous cropping obstacle is prevalent for protected cultivated cucumber in China. Intercropping garlic may effectively relieve this obstacle due to its allelopathic and antimicrobial effects. A two-growing season investigation was carried out during autumn 2009 to spring 2010 in plastic tunnel to determine the effects of intercropped garlic and green garlic on the overall growth of cucumber and soil biological properties. Results showed that green garlic exhibited the inhibitory effect on the growth of cucumber in spring cultivation 2010. Garlic–cucumber intercropping system increased yield of cucumber differently depending on garlic cultivars, with only cv. G005 showing significant increase (13.4%). Intercropping systems were evaluated as a greater net benefit system as compared to monoculture cultivation. Populations of soil bacteria and actinomyces were stimulated, while fungi were inhibited under intercropping system. Activities of soil invertase, urease, and alkaline phosphatase were encouraged under intercropping system in spring cultivation 2010 compared with monoculture. The promotion effect of intercropped garlic on urease and alkaline phosphatase maintained till garlic harvest. These results suggest that intercropping system can improve soil biology environment and alleviate continuous cropping obstacle of cucumber at different levels.
Introduction
Cucumber (Cucumis sativus L.) is a popular vegetable, widely cultivated around the world. Plastic-greenhouse cultivation is becoming a prevailing agricultural practice in China, because of its controllable environment and advantage of yield and income compared with open-field cultivation (Yao et al., Citation2008). However, the continuous cropping obstacle in protected cultivation happens as results in the reduction of crop yield and quality and unbalance of soil ecology (Ye et al., Citation2004; Yao et al., Citation2008), which has been considered as a major threat to sustainable agriculture production. Therefore, it becomes critical to improve the soil ecology and increase the yield of continuous cropping cucumber under protected cultivation. Crop rotation is an effective practice to overcome the continuous cropping obstacles. However, because of the land use efficiency and cultivation habits of farmers, it is difficult to carry out this practice under protected cultivation. Mixed cultivation of different crop species on same piece of land can help to fix this problem. Some researches have demonstrated that intercropping could relieve soil sickness by improving soil quality (Latif et al., Citation1992; Li et al., Citation1999), ecological microclimate (Olasantan et al., Citation1996) and increasing crop productivity (Li et al., Citation1999; Zhou et al., Citation2011). Therefore, intercropping is more applicable in comparison to crop rotation practice in order to decrease continuous cropping obstacles. The selection of best companion crops is a critical subject.
Garlic is one of the important allelopathic and antimicrobial crops. Our previous studies found that garlic (shoots, roots, bulbs) showed promotion effect on seed germination and seedling growth of some vegetable crops at low concentration and suppression effect at high concentration by bioassay (Zhou et al., Citation2007a, Citationb; Wang et al., Citation2009; Cheng et al., Citation2011). Moreover, root exudates of garlic had significant inhibition effect on Phytophthora capsici, and it varied depending on the concentrations and cultivars (Khan & Cheng, Citation2010; Khan et al., Citation2011). Allicin in garlic juice inhibited cucumber downy mildew caused by Pseudoperonospora cubensis both in vitro and in vivo on the leaf surface (Portz et al., Citation2008). Thus, garlic may effectively be incorporated into an intercropping system as companion crop. Some studies have demonstrated that intercropping garlic can discourage infestation of the tested insects in potato (Mogahed, Citation2003), attack of green peach aphids in tobacco (Lai et al., Citation2011), and weed invasion (Mueller et al., Citation1998).Yield advantages from intercropping with garlic have been reported in autumn planted sugarcane/garlic (Rathore et al., Citation1999; Singh et al., Citation1999), Chinese cabbages–garlic (Unlu et al., Citation2010), cucumber–garlic (Zhou et al., Citation2011) intercropping systems. However, so far no report has confirmed whether different amounts and cultivars of garlic intercropped demonstrate different effects on the growth and yield of cucumber in cucumber–garlic intercropping system.
Soil microbial and enzyme activities regulate soil quality and functioning because of their crucial involvement in many ecosystem processes and soil management practices (Yao et al., Citation2008; Lagomarsin et al., Citation2011). Soil enzyme activities could provide indications of changes in metabolic capacity and nutrient cycling due to management practices (Saha et al., Citation2008). Compared to monoculture systems, multi-cropping systems have increased soil microbial biomass and enzyme activities (Klose et al., Citation1999; Acosta-Martínez et al., Citation2003). More studies have showed that intercropping systems have significant impact on the microbial community and population structure in soil rhizosphere through the production of root exudates (Ren et al., Citation2008). In cucumber–garlic intercropping systems, recent research reported that soil enzyme activities, soil microbial communities were affected and this effect was sustained in later growing seasons (Zhou et al., Citation2011). However, it still needs investigations on how the soil microbial population and soil enzyme activities at different growing stages are changed and what is the impact of different cultivars of garlic in different combinations grown along with cucumber.
The objective of this study is to evaluate the effect of intercropping garlic of different cultivars and green garlic on growth and yield of continuous cropping cucumber, as well as on the dynamic changes of soil microbial population and enzyme activities during both co-growth and monoculture stages in the intercropping system.
Materials and methods
Sites description
The experiment was conducted in autumn 2009 and spring 2010 in a plastic tunnel at the experimental station of College of Horticulture, Northwest A & F University, in Yangling, Shaanxi Province (Northwest China, N 34° 16′, E 108° 4′). The area under experiment had been previously planted with cucumber crop for six cultivation seasons (three years). The chemical characteristics of the soil were as follows: electrolytic conductivity 228 µS·cm−1; pH (1:1 water) 7.79; organic matter 17.22 g·kg−1; available N 96.95 mg·kg−1; total P 1.14 g·kg−1; available K 467.68 mg·kg−1 and available P 271.92 mg·kg−1.
Experimental design and management
The treatments included (1) cucumber monoculture (M), (2) cucumber intercropping with garlic cv. G005 (IG005), (3) cucumber intercropping with garlic cv. G024 (IG024), (4) cucumber intercropping with garlic cv. G087 (IG087), and (5) cucumber intercropping with green garlic (IG). Three plots (replications) of 3.5 m×1.2 m were setup for each treatment and arranged in a complete randomized block design. The cucumber cultivar 09-F1 and Jinyou No.38 were used in autumn 2009 and spring 2010, respectively. Cucumber was planted at a spacing of 60×25 cm, two rows in each plot. Then garlic cloves (to harvest bulbs) were intercropped with three rows in 15×6 cm apart between two cucumber rows. On the other hand, garlic bulbs (to harvest green garlic) were planted between the cucumber rows one after another (). In autumn 2009, cucumber seedlings were transplanted on 1 August and harvested on 8 November. Garlic was intercropped one month after cucumber transplantation (1 September 2009) and harvested at the end of April in spring 2010. During spring 2010, Cucumber was transplanted on 17 March and finished on 20 July. In total, co-growth duration of the two crops came to more than three months.
Figure 1. Diagram showing the arrangement of the rows of cucumber (
), garlic bulbs (
) in the plastic tunnel experiment: (a) cucumber monoculture (b) cucumber/garlic intercropping (c) cucumber/green garlic intercropping.
Before the cucumber transplanting, experimental plots were uniformly tilled and used 104 m3 of the decomposed cattle manure, 595 kg ammonium hydrogen carbonate and 595 kg ha−1 calcium superphosphate as base fertilizer. All plots were well irrigated manually and hand-weeded during crop growth. At fruit-setting stage, 397 kg ha−1 potassium nitrate was fertilized with irrigation about 8 times in autumn 2009 and 10 times in spring 2010. The standard agronomic practices were equally performed in all the treatments.
Plant growth and yield
In autumn 2009, cucumber plant height and stem diameter were measured by randomly selecting five plants per plot prior to intercropping garlic, and subsequently recorded at intervals of 20 days till three weeks before the cucumber was harvested. After the garlic harvesting, the growth of cucumber was observed every 20 days in spring 2010. Plant height was measured from the ground to the shoot apex. The stem diameter was noted at height of 5 cm from ground level for each selected plant.
Cucumber fruits were harvested between 27 September and 6 November in autumn 2009 and then between 21 April and 19 July in spring 2010. At each harvest, the total number of fruits and weight of fruits were recorded. During the entire period of growth, the green garlic was cut and weighted four times on 22 September, 10 October, 1 and 19 November, respectively. In spring 2010, garlic scapes were harvested at the first week of April. After half of month, garlic bulbs were harvested. The number and weight of bulbs of three cultivars were recorded separately. Economic returns were analyzed following the procedure adopted by Iqbal et al. (Citation2007). Net benefit was calculated by subtracting the total variable cost from the gross benefits for each treatment. Variable costs (ha−1) of purchased inputs, labor, and sale of products were assessed by the prevailing market prices.
Soil microbial population
Soil samples were manually collected using an auger from points selected 5 cm apart from cucumber plants and at 0–10 cm depth every 20 days during the co-growth stage and then every 30 days during sole cucumber stage. The soil samples were stored in insulated plastic bags and immediately transported to laboratory. Samples were kept at 4°C to analyze microbial densities within three days, including bacteria, actinomyces, and fungi using a plate dilution method (El-Tarabily et al., Citation1996; Ren et al., Citation2008). The data were expressed on an oven-dried (105°C) soil weight basis.
Soil enzyme activities
Soil enzyme studies were performed upon the same samples as used for soil microbial examination. Samples were air-dried and ground and passed through a 1 mm mesh and kept at room temperature. Activities of invertase, urease, and alkaline phosphatase were detected according to Guan and Shen (Citation1986) and Tabatabai (Citation1994). Catalase was assayed using a titration method (Johnson & Temple, Citation1964).
Statistical analysis
The experiment was designed in a complete randomized block design with three replicates. All data were assessed by analysis of variance using the Statistical Analysis System software and Least Significant Difference test at p<0.05 was used for multiple comparisons.
Results and discussion
Intercropping systems could influence soil biological environment due to production of root exudates, most of which as allelopathic substances. Root exudation served as an important energy and carbon source for micro-organism in soil (Qian et al., Citation1997). Root exudates have low molecular compounds such as sugars, amino acids, organic acids, and phenolic compounds and these compounds especially phenolic affect the growth and development of plants and micro-organism. Root exudates also possessed higher molecular weight compounds such as flavonoids, fatty acids, steroids, tannins, vitamins, alkaloids, enzymes, and growth regulators that involved in primary and secondary plant metabolic processes (Fan et al., Citation1997; Uren, Citation2000).
Cucumber plant growth and yield
Intercropping systems exhibited significant effect on cucumber plant growth varying among treatments and cultivation seasons ( and ). In autumn cultivation 2009, there was no significant difference in plant height and stem diameter of cucumber at different growth stages in different cropping systems. However, in spring 2010, intercropped garlic showed negative effects on plant height and stem diameter compared to the monocropping system for a short-time period just after the garlic harvest. However, inhibitory effect reduced gradually with the growth of cucumber. It was observed only in green garlic treatment that cucumber stem diameter was reduced during the entire growth stage of cucumber. Choesin and Boerner (Citation1991) reported that allelochemicals might accumulate and persist at phytotoxic levels and come in contact with the target plant, which would influence growth of plants in field situations. In this experiment, different weight of various garlic cultivars and green garlic was used, with 1698 kg for cv. G005, 2913 kg for cv. G024, 2810 kg for cv. G087 and 38,881 kg ha−1 for green garlic. Obviously, in the treatment with green garlic had more quantity of garlic and might secrete more allelopathic substances. So further research is needed to investigate which quantities of green garlic are most appropriate for green garlic–cucumber intercropping system.
Table I. The dynamic change of cucumber plant height (cm) of different treatments in autumn cultivation 2009 and spring cultivation 2010.
Table II. The dynamic change of stem diameter (mm) of cucumber in different treatments in autumn cultivation 2009 and spring cultivation 2010.
Intercropping with garlic cv. G005 significantly increased cucumber yield by 13.4% compared to sole cucumber in spring 2010 (). However, there was no significant yield difference among the other treatments. In autumn cultivation 2009, no significant yield difference was observed among different treatments. These results indicate that intercropping garlic increased cucumber yield depending on cultivars, and so far few studies have been focused upon this aspect (Zhou et al., Citation2011). The same yield promoting effect had also been observed in other intercropping systems, such as cucumber-okra (Ofosu-Anim & Limbani, Citation2007), vetch-oat (Canan & Orak, Citation2007), and maize-cassava (Olasantan et al., Citation1996) systems. All intercropping systems produced higher net benefit and net returns than monoculture (). The green garlic produced the highest net return up to 27.5% in comparison with the monocropped cucumber.
Table III. Yield and net benefit of cucumber in different treatments in autumn cultivation 2009 and spring cultivation 2010.
Soil microbial populations
The population of soil bacteria, fungi, and actinomyces were affected significantly among intercropping systems compared with monoculture (). Growing season also played an important role in soil microbial spectrum that is the population of bacteria and actinomyces was reduced while that of fungi was amplified in the spring 2010 as compared to autumn 2009.
Figure 2. Total soil microbial populations of (a) culturable bacteria, (b) fungi, and (c) actinomyces in soil samples collected at intervals of 20 days in co-growth stage and 30 days in sole cucumber in autumn cultivation 2009 and spring cultivation 2010. M, cucumber monoculture; IG005, cucumber intercropping with garlic cv. G005; IG024, cucumber intercropping with garlic cv. G024; IG087, cucumber intercropping with garlic cv. G087; IG, cucumber intercropping with green garlic. Data in a column followed by the same letter are not significantly different at p=0.05, analysis of variance with Least Significant Difference test.
From autumn 2009 to spring 2010, intercropping systems significantly increased the number of bacteria at different growth stages of cucumber by different degrees varying among the treatments (a). The fungal population in different intercropping systems was noted to be diversified in autumn 2009. However, all treatments exhibited significant inhibition in comparison with monoculture cucumber just within 20 days of cucumber transplanted in spring 2010 (b). The numbers of actinomyces were significantly stimulated among intercropping systems except for the treatment intercropped with garlic cv. G005 on different sampling dates and cultivation seasons (c). Studies claimed that bacterial population decreased dramatically, but the fungal and actinomyces amounts increased in the continuous cropping system of vegetables, especially under plastic tunnel and greenhouse cultivation (Lin et al., Citation2004). Results of this study indicate that intercropping systems can alleviate the change of microbial environment. These results are in agreement with conclusions of Ren et al. (Citation2008), who worked on aerobic rice–watermelon intercropping system. However, the effect observed in intercropping systems existed only in co-growth stage. This might be due to the root exudates and potential allelochemicals released in soil, which were used by soil microbial species as organic carbon sources (Inderjit & Callaway, Citation2003). In our study, as the growth of cucumber, the inhibitory effect of intercropped garlic on cucumber plant growth gradually disappeared, might be due to soil microbial degradation/transformation of allelochemicals (Inderjit, Citation2005).
Soil enzyme activities
Soil enzyme activity is considered to be a major factor contributing to overall soil microbial activity and soil quality (Eivazi & Bayan, Citation2002). In this study, enzyme activities varied widely among the treatments, growing seasons and sampling dates (). The activities of all the tested soil enzymes were lower in spring cultivation 2010 than autumn 2009.
Figure 3. Soil enzyme activities of (a) invertase, (b) urease, (c) catalase, and (d) alkaline phosphatase in soil samples collected at intervals of 20 days in co-growth stage and 30 days in sole cucumber in autumn 2009 and spring 2010. M, cucumber monoculture; IG005, cucumber intercropping with garlic G005; IG024, cucumber intercropping with garlic G024; IG087, cucumber intercropping with garlic G087; IG, cucumber intercropping with green garlic. Data in a column followed by the same letter are not significantly different at p=0.05, analysis of variance with Least Significant Difference test.
Activities of soil invertase and urease were inhibited in intercropping systems in autumn 2009 but enhanced significantly in spring 2010 compared with monoculture (a, b). No significant difference in soil catalase activity was noted among intercropping systems and monoculture in autumn 2009. Soil catalase activity was higher in intercropping systems than monocropping except on 20 days after transplant of cucumber in spring 2010 (c). Soil alkaline phosphatase activity in intercropping systems was stimulated significantly in both the autumn and the spring cultivations, compared with monoculture (d). The promotion effect on the activities of soil enzymes except for invertase was maintained till the garlic harvest. These results are in coherence with the results of Zhou et al. (Citation2011), who found that impacts of intercropping systems on soil enzyme activities was still retained in subsequent growing seasons. Gu et al. (Citation2009) reported that soil enzyme activities were influenced by allelopathic rice variety through the release of allelochemicals. In this study, soil enzyme activities were stimulated in the second growing season, which might be resulted from the accumulation of allelochemicals of garlic root exudates. So more investigation should be considered to determine which allelochemical influence the soil microbial and enzyme activities by collecting and identifying the allelopathic components from soil.
Acknowledgements
This research was supported by the project of State Natural Science Foundation (No.31171949), the project of State Commonwealth (Agriculture) Scientific Research (No.200903018), and National Undergraduates Innovating Experimentation Project (No.2009-10). I sincerely thank Undergraduate Xue Churan, Yang Weiwei, and Yang Bo for participating in this study. I also thank Abdul Rehman Khan, Muhammad Ali Khan, and Imran Ahmad for reading the manuscript prior to submission.
Related Research Data
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