The global average temperature was highest in 2016, with Korea experiencing the hottest summer (Korea Meteorological Administration, 2017). Meteorological data from 1981 to 2010 and experiments conducted between 2015 and 2017 in the Cheonan Meteorological Center (2017) are shown in Table 1. Global warming affects the flowering of many crops and global ecosystems (Jeong et al., 2011).
Meteorological data between 1981 and 2010 (Normal year) and from 2015 to 2017 (experiment period) were obtained from the Cheonan Meteorological Center (Table 1). The minimum, maximum, and average temperatures were higher from 2015-17 than from 1981–2010 throughout the year. Daily minimum temperatures were higher from 2015-17 than in 1981–2010 (0.5 to 1.2℃ from November to May and not more than 0.2 ℃ from July to September). The precipitation in the experiment period was higher than the Normal year throughout the year, especially from October to December when wheat emerges and tillers. However, the sunshine duration in the experiment period was slightly lower than the Normal year throughout the year.
Since arable land has become limited due to the rapid urbanization in the late 20th century, a double-cropping system could achieve efficient land use. In Cheonan, wheat is planted in October and harvested in June the following year (RDA, 2011) due to the high temperatures in the region during winter (Table 1). Meanwhile, the small red bean is sown in June and harvested in October of the same year (RDA, 2005). Therefore, the double-cropping system of wheat and small red beans could succeed in central Korea, especially utilizing the rice field during reduced rice consumption.
The chemical properties of the soil samples were within the optimum ranges established by Standards for fertilization of soil (NIAST, 2010a) except for the exchangeable potassium (K⁺) and pH level (Table 2). For instance, Organic matter was 22 g/kg, phosphorous was 225 g/kg, and electric conductivity was 1.0 dS/m. However, the exchangeable K⁺ level was 0.66 cmol/kg, slightly higher than the acceptable range, while the pH value was slightly lower (6.0) than the optimal pH between 6.5 and 7.0 (Table 2). Therefore, the soil used was suitable for wheat and small red bean cultivation (RDA. 2012).
2 − 1. Wheat growth
Late October to mid-June: The average, maximum and minimum temperatures increased by 0.8℃, 0.7℃, and 0.6℃, respectively, during the wheat cultivation period between 2015 and 2017, compared with the previous years (1981–2010). Similarly, the IPCC report (3rd and 5th) pointed out that global warming increases the average earth temperature (IPCC, 2001, 2014). The effective accumulated temperature of the days when the daily average temperature is above 0°C increased by 173°C during the wheat cultivation period in 2015–2017, higher than in previous years (1981–2010). The monthly average temperature decreased only in October during the sowing period and increased from November (Table 1) (Fig. 1).
The suitable period for wheat sowing in South Korea is between mid and late October based on the standard agricultural recommendations (RDA, 2011). Wheat was sown on October 26 in the Cheonan area, considering the harvest time of the two crops. Furthermore, the average monthly meteorological factors and changing trends were investigated until June 25, the harvest period of the second year.
In addition, in autumn wheat, the average minimum temperature in January reached − 15℃ to safely over-winter for wheat (Kang et al., 2010), which is lower than − 5.0 and − 7.9°C in mid-to-late October (RDA, 2011). The low temperature was due to increased precipitation (over 212 mm) and reduced sunshine duration (less than 0.3 hours) in the experiment period. The precipitation between October and December is critical for ensuring the increased number of wheat grains per year (Fig. 1(e), Table 1). The precipitation increased by 15, 48, and 41 mm, respectively, between 2015 and 2017, higher than in previous years. However, the precipitation during the harvest period (June) decreased by 1.6 times (35 mm). The average temperature increased from late October to November over the experiment period. Late October was a suitable planting period due to the 23mm precipitation. The average temperature increased in June during the harvest period, while the precipitation gradually decreased, indicating that wheat can be cultivated over the winter in the Cheonan area (Fig. 1, Table 1).
Heading of three wheat varieties occurred between 24 and 27 April (Table 3), coinciding with the nationwide heading date (Kang et al., 2014). However, since the daily average temperature in mid-February 2017 in the central region where Cheonan is located was 0.4°C higher than in 2016, the heading dates of three varieties in 2017 were all two days earlier than in 2016. Therefore, the difference is related to the high temperatures (Nahar et al., 2010). The precipitation and sunshine duration were lower (79.7 mm and 6.7 hours, respectively) in 2017 than in 2016 due to the recent global warming in South Korea (Suh & Kim, 2015).
2–2. The effect of additional nitrogen fertilization on SPAD and plant N content
Photosynthesis peaked in mid-April and gradually decreased 21 days after heading, before significantly declining during the wheat growth period (RDA. 2011). In this study, the effects of different fertilization treatments on the SPAD value and leaf nitrogen content 20 days after heading were investigated. The SPAD was associated with nitrogen content in wheat leaves, consistent with previous results in rice (Oryza sativa L.) (Kim et al., 2002; Hong et al., 2003).
The average SPAD values of ‘Keumgang’, ‘Sooan’ and ‘Goso’ were 54.0, 53.9, and 48.3, respectively, after different nitrogen fertilizer treatments. There SPAD values between ‘Keumgang’ and ‘Sooan’ were not different, while ‘Goso’ had the lowest average SPAD value. However, the average yield was significantly different among the varieties, with ‘Goso’ having the highest (496.7kg/10a) and ‘Keumgang’ the lowest (452.4 kg/10a). ‘Goso’ also had the lowest average leaf nitrogen concentration (3.69%), while ‘Sooan’ had the highest average (4.33%) (Table 4).
Therefore, Nitrogen fertilizer levels significantly affect the yield. Kim et al. (2002) indicated that the leaf color concentration should be determined before wheat cultivation for proper fertilization. The leaf SPAD in each variety was highest under the N3 treatment but significantly lower under the N1 treatment relative to N3. ‘Keumgang’ leaves had the highest nitrogen concentration (4.41%). The yield per 10a was also highest under N3 treatment and lowest under N1 treatment, with ‘Goso’ having the highest yield (521.6 kg/10a) (Table 4).
2–3. Effect of additional nitrogen fertilizers on the growth characteristics of wheat
The Rural Development Administration recommends 9.4 kg of nitrogen fertilizer per 10a as a food quality standard (NIAST, 2010b) for wheat in South Korea. However, most farmers do exceed the recommended quantity. In this experiment, the growth and yield of wheat were analyzed at different nitrogen fertilizer levels.
In the central region of South Korea, nitrogen is traditionally applied once before the elongation of wheat internode to avoid delay in wheat maturity. Therefore, three different nitrogen fertilizer treatments were used according to the existing cultivation method. The wheat growth characteristics, such as culm and spike lengths, were recorded during the nitrogen experiments in a two-year wheat-small red bean double-cropping system in the Cheonan area (Table 5).
The culm length is directly related to stem mechanical properties and lodging resistance (Huang et al., 2016). The average stem length was 69.5, 80.5, and 68.5 cm in ‘Keumgang’, ‘Sooan’, and ‘Goso’, respectively (Table 5).
The average spike lengths of ‘Keumgang’, ‘Sooan’ and ‘Goso’ were 7.5, 7.4, and 8.3 cm, respectively, and did not differ significantly among the nitrogen fertilization levels (N1, N2, N3) (Table 5). Kim et al. (2013) had previously reported similar results, where spike length was not directly associated with nitrogen levels, while culm length increased with higher nitrogen fertilization levels. Although the culm and spike lengths were significantly different among the three varieties in our study, the difference was not significant among the three treatments. The differences could be due to climate and soil conditions during internode elongation (Table 5).
The temperature and sunshine duration comparison between Cheonan and southern regions (Gwangju, Jeon-ju) from March to April during the internode growth period of wheat is shown in Table 6. The average and maximum temperatures were lower by 1.2 to 1.8°C and 0.8 to 1.4°C, respectively, in the Cheonan area compared with the southern area. However, the sunshine duration was similar in both regions.
The internode growth stage was consistent among reports related to meteorological factors, such as the highest temperature, average temperature, and sunshine duration between March and April (Choi et al., 2016).
2–4. The effect of nitrogen fertilizers on wheat yield
Cook & Baten. (1938) reported that nitrogen fertilization significantly increases the number of wheat ears. Nitrogen fertilizer significantly increases the number of ears and yield than 1000-grain weight in barley, thus increasing the yield per unit area (Maadi et al., 2012). Besides the number of ears, other components are also associated with increased yield (Den & Lambert, 1953; Middleton et al., 1964). Kim et al. (1984) also reported that the number of ears and 1000 grain weight substantially affect barley yield. Moreover, increasing fertilization was more effective than increasing sowing seeds in wheat (Larter et al., 1971; Liang et al., 2014). However, the high yields of some high-yielding wheat regions are due to genotype improvement, mechanization and the application of large amounts of nitrogen fertilizer and other pesticides. This intensification level depends largely on fossil fuels and may not be sustainable (Curtis & Halford, 2013).
The yield-related characteristics of wheat, based on the varieties and nitrogen fertilization levels, are shown in Table 7. The average number of grains per ear of the three varieties ‘Keumgang’, ‘Sooan’ and ‘Goso’ were 38.1, 37.2, and 42.3, respectively.
The number of grains per panicle was highest under N3 treatment, contrary to Kim et al. (2013). In their study, the characteristics of the variety significantly affected the number of grains per ear than the fertilizer type and levels.
The average number of ears per m2 for ‘Keumgang’, ‘Sooan’ and ‘Goso’ were 599.7, 646.7, and 627.0, respectively, showing significant differences among the varieties. However, the number of ears per m2 was highest under N3 treatment. Ullah et al. (2018) reported similar results, indicating the influence of nitrogen fertilizer levels.
Furthermore, the average 1000 grain weight of ‘Keumgang’, ‘Sooan’, and ‘Goso’ was 40.3, 40.0, and 38.9 g, respectively, exhibiting no significant difference among the varieties. This finding could be due to the influence of the unique characteristics of varieties, consistent with Kim et al. (2013). Hobbs (1953) reported that nitrogen treatment significantly increases the number of ears per unit area and grains per ear than the 1,000-grain weight, indicating that the characteristics of the crop variety affect the weight of 1000 grains.
The average yields of ‘Keumgang’, Sooan’ and ‘Goso’ were 452.4, 487.3, and 496.7 kg per 10a, respectively. Besides, the yields were significantly different at the three nitrogen fertilizer levels (Table 7), with ‘Goso’ having the highest yield (521.6 kg/10a) under N3 treatment. The yields increased with higher nitrogen fertilizer application. Therefore, nitrogen fertilizer substantially influences wheat yield (Worzella, 1943; Cook et al., 1938; Black et al., 1946). Ayoub et al. (1994), reported that final wheat yield increases with higher nitrogen fertilizer levels, similar to this study.
2–5. Growth characteristics of small red bean
Optimum sowing time of small red bean as a second crop after wheat
Early July to mid-October: The average, maximum and minimum temperatures increased by 0.5℃, 0.3℃, and 0.6℃, respectively, during the small red bean breeding period between 2015 and 2017, higher than the normal year (1981–2010). The temperature increase in winter was greater than that in summer during the red bean growth period compared with the wheat growth period, similar to the previous research result (Kim et al. 2016). The precipitation was 485 mm higher in 2015–2017 than in the normal year. This could be attributed to the torrential rainfall of 233 mm in the Cheonan area on July 16, 2017. However, the overall precipitation and the sunshine duration increased in the experiment period. In the past three years, the sunshine duration has decreased by 0.7 hours compared with the normal year (Table 1) (Fig. 2).
Rural Development Administration (2014) and Kim et al. (1981) reported that the optimum sowing time for small red beans is between mid-June and mid-July in central Korea in single-season sowing. However, the wheat has to be harvested after mid-June if small red beans are planted after wheat. Therefore, it is necessary to explore the possibility of planting in July.
Besides, delayed harvesting affects the small red bean varieties in Korea, including the ‘Chungju’, since over 90% of the small red bean have an intermediate growth type (Yoon et al., 2012). However, the recently developed varieties, such as ‘Hongeon’ and ‘Arari’, have a determinate and semi-determinate growth type (Lee et al., 2011). Therefore, the sowing period can be extended based on the chosen variety.
The number of days from sowing to flowering in ‘Chungju’ was 49 days, 47 days, and 46 days, on July 1, July 10, and July 20, respectively. For ‘Arari’, it took 47 days, 46 days, and 46 days, respectively (Table 8). ‘Hongeon’ flowered earliest (33 to 34 days) among the varieties used in this study. Furthermore, our findings revealed that the unique characteristics of the varieties influenced the number of days to the flowering stage, consistent with the previous studies (Lee et al., 1991).
The growth temperature was suitable up to the flowering period when the three varieties were sown between July 1 and July 20 (Table 8). Besides, the number of days to flowering was also not significantly different between July and June sowing.
2–6. Climate during small red bean growing period (early July to mid-October)
The average monthly meteorological elements from July 1, the first sowing date of small red bean grown after harvesting wheat, to October 21, the final harvesting date, are shown in Table 7.
The average, maximum, and minimum temperatures increased by 0.5 ℃, 0.3 ℃, and 0.6 ℃, respectively, in the experiment period (2015–2017), higher than in the normal year (1981–2010). The temperature increase in winter was higher than in summer during the experiment period, consistent with a previous Korean study (Kim et al., 2016). The precipitation was 484.4 mm in the experiment period was higher than in the normal year (1981–2010), with 232.7 mm of rainfall per day on the Cheonan area on July 16, 2017. However, overall precipitation increased between 2015 and 2017 except in September (2016). Sunshine duration also reduced by 0.7 hours in the last three years compared with the normal year (Fig. 2, Table 1).
In the entire small red bean growth period, the cumulative temperature has been reported to be good for flowering and fruiting at 1,000 ℃ or higher (Tasaki, 1957; Cho et al., 2003). For the July 1, July 10, and July 20 sowing dates, ‘Chungju’ had cumulative temperatures from 1,168.3 ℃ to 1,257.1 ℃, with an average of 47 days to flowering date (Table 8). However, ‘Arari’ had an average of 46 days. In both varieties, the cumulative temperature decreased with a delay of the sowing date.
‘Hongeon’ had an average of 33 days from sowing to flowering date for the three sowing dates of July 1, July 10, and July 20, with cumulative temperatures of 833.9℃, 863.2℃, and 899.3 ℃, respectively. The cumulative temperature required for ‘Hongeon’ growth is also 1,000 ℃ or higher. Therefore, the cumulative temperature does not influence growth and harvest when sowing before July 20 except for the ‘Hongeon’ in Cheonan.
2–7. Effect of sowing date and density on small red bean yield
Several studies on yield characteristics based on sowing date have been reported on soybeans. Cha & Lee. (1979) indicated that culm length increases in the dense planting regardless of the sowing time of the soybean when planted after harvesting barley. Furthermore, the number of branches, pods per plant, and seeds per pod increases in the sparse planting plot.
Park et al. (2015) showed that the stem length increases with an increased number of plants per hill due to plant competition. However, the stem length does not elongate enough to induce lodging since the cultivation period is short.
Besides, Rho et al. (2003), reported 100-seed weight, number of pods per plant, and number of seeds per pod as the yield characteristics of small red beans. However, stem length and flowering date are indirect factors (Yol et al., 2010).
The growth and yield characteristics of the small red bean based on the sowing date and density are shown in Tables 9 and 10. The analysis was conducted for three years (2015–2017) in the wheat-small red bean double-cropping system in the Cheonan area, the central area of South Korea.
The average stem lengths of ‘Chungju’, ‘Hongeon’, and ‘Arari’ were 61.4 cm, 44.2 cm, and 59.3 cm, respectively. The stem lengths were significantly different between sowing dates and sowing density (Table 9), with July 10 and 20 producing the longest and shortest lengths, respectively. Moreover, the stem length was longest at 60 × 15 cm and shortest at 60 × 25 cm. ‘Chungju’ stem length was longest (66.0 cm) at 60 × 15 cm on July 1, ‘Hongeon’ (51.7 cm) at 60 × 25 cm on July 1, and ‘Arari’ (63.1 cm) at 60 × 15 cm on July 10.
However, ‘Chungju’, ‘Hongeon’, and ‘Arari’ stem lengths were shortest (51.7, 39.4, and 52.0 cm, respectively), at 60 × 25 cm on July 20. The stem length increase with higher planting density could be attributed to the competition for nutrients between plants. Similarly, the narrower the interval between plants, the longer the soybean stems, from late sowing with second cultivation after wheat harvesting (Weber, 1966). Other studies also reported similar results, where the culm length decreased with the delayed sowing date of the small red bean (Hong et al., 1989; Cho et al., 2003).
The average number of pods per plant in ‘Chungju’, ‘Hongeon’, and ‘Arari’ were 34.1, 23.6, and 31.4, respectively. The number of pods was significantly different depending on the cultivation method, such as sowing dates and sowing density (Table 9), highest on July 1, and lowest on July 20. The pods were also highest at 60 × 25 cm spacing, while lowest at 60 × 15 cm. The number of pods was highest at 60 × 25 cm, in ‘Chungju’ (46.1 pods) on July 1, Hongeon (28.7 pods) on July 20, and ‘Arari’ (39.3 pods) on July 1.
Therefore, the number of pods per plant increases with decreased sowing density. Furthermore, the number of pods per plant in ‘Chungju’, ‘Hongeon’, and ‘Arari’ were lowest (23.8, 18.7, and 21.9, respectively) at 60 × 15 cm sowed on July 20, indicating that the number of pods increases with increased branches due to the long period of vegetative growth and the wider density between individual plants. Similarly, the number of seeds per plant in soybean increases with early sowing date and broader sowing density (Bastidas et al., 2008).
The average number of seeds per pod was 8.18, 8.62, and 8.57 in ‘Chungju’, ‘Hongeon’ and ‘Arari’, respectively, and showed significant difference with sowing dates (Table 10). However, sowing density did not affect the number of seeds per pod. Overall, the average number of seeds per pod was highest (8.61) on July 1 and the lowest (8.31) on July 20. The total number of seeds per pod was about 8, similar to Hong et al. (1989), indicating that the number of seeds per pod is similar regardless of the sowing date. However, the average number of seeds per pod of ‘Chungju’, ‘Hongeon’ and ‘Arari’ was 6.0, 6.5, and 6.8, respectively, as presented in NICS (2017), the average score of the national adaptation experiment conducted for over three years. In this study, the number of seeds per pod was about 2 seeds higher than the national average.
The 100-grain weight was 15.1, 14.8, and 15.4 g for ‘Chungju’, ‘Hongan’, and ‘Arari’, respectively, significantly different at various sowing dates (Table 10). The 100-grain weight was highest on July 20 and least on July 1. The weight was also highest at 60 × 15 cm and least at 60 × 25 cm. The 100-grain weight was highest in ‘Arari’ (16.7 g) at 60 × 15 cm on July 20 and least in ‘Chungju’ at 60 × 15 cm on July 1. The average 100-grain weight was 14.0 g, and it increased with delayed sowing. Similarly, Kang (1985) indicated that the weight of the seeds increases with delayed sowing of the small red bean.
The ‘Chungju’, ‘Hongeon’, and ‘Arari’ yields were 237.6, 186.7, and 222.9 kg per 10a, respectively, and showed a significant difference between sowing date and sowing density. The yield was highest on July 1 and lowest on July 20. Besides, the yield was highest at 60 × 15 cm and least at 60 × 25 cm. The yield was highest at 60 × 15 cm, in ‘Chungju’ on July 10, ‘Arari’ on July 10, and ‘Hongeon’ on July 1 (Table 10). Similarly, the soybean yield was higher with increased sowing density (Park et al., 2015). Furthermore, the yield was lowest in ‘Hongeon’ at 60 × 20 cm on July 10, in ‘Arari’ at 60 × 25 cm on July 20, and in ‘Chungju’ on July 20 at 60 × 25 cm, (159.6, 179.6, and 188.0 kg per 10a, respectively). ‘Chungju’ and ‘Arari’ yield were higher on July 1 and July 10 than July 20 sowing. However, the ‘Hongeon’ yield was similar on different sowing dates.
Therefore, a high yield was obtained at an earlier sowing date and shorter sowing density. Besides, the three varieties had good yield even when sowed on July 20. For instance, ‘Hongeon’ sowing was delayed due to the bad weather conditions, such as rainfall after harvesting wheat, increasing the sowing period compared to ‘Chungju’ or ‘Arari’, thus high yield.