Introduction

Cultivating rice-alternative crops in paddy fields serves as a solution to the problem of rice overproduction, while also alleviating the issue of self-sufficiency in major crops. Thus, paddy field crop diversification involves growing forage crops, soybeans (Glycine max (L.) Merrill), or corn (Zea mays L.) instead of rice (Oryza sativa L.) to control rice production. Recently, the cultivation of rice varieties bred for different purposes has increased, such as powder rice and forage rice, which have cultivation methods similar to that of conventional rice. The cultivation of these crops addresses food and feed self-sufficiency issues and also explores the potential use of harvest residues for biofuel production. Additionally, repeated cultivation of the same crop in the same soil over successive years can result in declining yields and deteriorating quality owing to continuous cropping disorders. Repeated cropping disorders arise from a combination of factors, including soil acidification, secondary salinization, changes in the physical and chemical properties of the soil, an increase in soil-borne plant diseases, and the accumulation of toxic substances. To mitigate these issues, repeated monoculture should be avoided and/or instead sustainable cropping systems such as crop rotation, double cropping, or multiple cropping should be adopted (Bedoussac et al., 2015). Owing to the monsoon climate, Korea has primarily developed a double-cropping system, where rice is cultivated in the summer, and barley, wheat, or forage crops are grown in the winter. This method allows for the efficient use of limited arable land. Multiple cropping, which involves harvesting more than twice a year, enhances agricultural productivity and diversifies cropping systems. Higher diversity of cropping systems is expected to improve the sustainability of crop production, enhance pest control, increase resilience to climate change, and/or reduce fertilizer usage (Ogazie et al., 2022).

Recently, domestic consumption of wheat (Triticum aestivum L.) has been steadily increasing in Korea, necessitating efforts to enhance wheat self-sufficiency. However, since arable land suitable for crop cultivation is limited, exploring strategies for winter crop cultivation using fallow paddy fields after harvest of rice or cash crops is essential. Wheat varieties cultivated in Korea include ‘Jokyoung’, ‘Geumgang’, ‘Sae-Geumgang’, ‘Hwanggeumal’, ‘Baekjung’, and ‘Yeonbaek’, varying based on regional preferences or cultivation conditions (Choi et al., 2015; Kang et al., 2014; Kim et al., 2013). In particular, ‘Jokyoung’ and ‘Geumgang’ wheat can be grown nationwide except in high-altitude mountainous areas and are widely used for baking and multipurpose applications owing to their high protein content. A previous study conducted by our research team reported that the grain yield of ‘Jokyoung’ wheat increased by more than 100 kg per 10a when cultivated in soil previously used for rice-alternative crops (black soybeans, sesame, and perilla) compared with soil previously used for rice monoculture (Oh et al., 2022). Moreover, the study confirmed improvements in wheat quality. The cultivation of rice-alternative crops not only enhances the physical and chemical properties of paddy soil, but also contributes to the production of high-quality wheat more suitable for the baking process. In some regions, studies are being conducted on paddy-based double-cropping systems, where wheat is grown following upland crops such as corn and soybeans as alternatives to rice (Choi et al., 2023; Seo et al., 2021).

This study aimed to propose an optimal cropping system to increase wheat yield and mitigate rice monoculture disorders by examining the growth characteristics and yields of wheat under multiple-cropping systems using paddy fields in the southern lowland regions.

Materials and Methods

Plant Materials and Cultivation Conditions

This study was conducted in a paddy field at the Southern Crop Research Division of the National Institute of Crop Science in Miryang, Gyeongsangnam-do, Korea (N35°49’342’’, E128°74’224’’, 8 m above sea level), where rice had been repeatedly cultivated for several years. The experiment was designed with cropping systems comprising solely upland crops (Perilla-IRG-Sesame-Wheat and Soybean-IRG-Sesame-Wheat) and including rice (Rice-Wheat-Rice-Wheat and Rice-IRG-Sesame-Wheat) (Fig. 1). Each experimental plot was divided into sections of 60 m² (3 m × 20 m) per replication, with four replications per cropping system. The plant materials used in this study focused on wheat as the final crop in each cropping system. The wheat (Triticum aestivum L.) varieties used were ‘Jokyoung’ and ‘Geumgang’. As preceding crops, summer crops included rice (Oryza sativa L. cv. ‘Daebo’), perilla (Perilla frutescens (L.) Britton cv. ‘Deulsaem’), and soybean (Glycine max (L.) Merr. cv. ‘Daewon’) in the first year, and rice and sesame (Sesamum indicum L. cv. ‘Goenbaek’) in the second year. For winter crops, wheat (T. aestivum L.) and Italian ryegrass (Lolium multiflorum Lam. cv. ‘Greenfarm’) were cultivated in the first year, while only wheat was grown in the second year. The cultivation of crops in each cropping system followed the standard agricultural practices outlined in the Rural Development Administration’s standard farming manuals (RDA, 2000; 2001; 2021a; 2021b; 2022). Crop residues from the preceding crops were removed before plowing but the remaining stubbles were incorporated into the soil through plowing. The summer preceding crops in the second year, rice and sesame, were harvested before mid-October. After harvest, the soil was plowed and leveled 10 days before sowing wheat. Fertilizer was applied at a rate of 4.0-8.3-4.1 kg/10a of N-P₂O₅-K₂O before sowing wheat. Experimental plots for each wheat variety were divided into 30 m² sections (3 m × 10 m) per replication, with four replications. The final crop, wheat, was sown at a rate of 16 kg/10a on October 30, 2020, using a ridge-broadcast sowing method. No irrigation was provided during the cultivation period. After winter dormancy, additional nitrogen fertilizer was applied at a rate of 2.7 kg/10a immediately after tillering and again 20 days later. Other cultivation practices followed the RDA’s standard wheat cultivation guidelines (RDA, 2001). Harvesting was conducted in early June.

Fig. 1.

Pictures illustrating the growth of crops grown under long-term multi-cropping systems in the paddy fields during the experimental period (June 2019 - May 2021). The treatment names are formed using the first letter of each crop in the order that they were grown. R (rice; Oryza sativa L.), W (wheat; Triticum aestivum L.), I (Italian ryegrass; Lolium multiflorum Lam.), S (sesame, Sesamum indicum L.), P (perilla; Perilla frutescens L.), Sb (soybean; Glycine max L.).

Analysis of Meteorological Conditions

Atmospheric meteorological conditions, including air temperature, relative humidity, precipitation, wind speed, and solar radiation, were analyzed using data collected during the study period (May 2019–June 2021) from the Miryang weather station (N35°49’147’’, E128°74’412’’, 8 m above sea level) located near the experimental site (Fig. 2). Special attention was placed on the atmospheric environmental factors during the cultivation period (October 30, 2020－June 9, 2021) of wheat, which was the final crop in each cropping system.

Fig. 2.

Changes in daily air temperature (A), daily mean relative humidity (B), daily precipitation (C), daily sunshine durations (D), and daily mean wind speed (E) in the paddy field during the experimental period (May 2019－June 2021). The upper arrows P and H indicate the days when seed sowing/seedling transplanting and crop harvesting were conducted in situ, respectively.

Analysis of Physicochemical Properties of Soil

Soil samples were collected from each cropping system before and after the experiment by slightly removing the surface layer, and obtaining topsoil samples (0–15 cm) from each cropping system using a soil auger (ø 35 mm; Single Edelman Auger, Eijkelkamp, Zeveenar, the Netherlands), with four replications. The samples were air-dried and used for physicochemical analysis. For analyzing the soil pH and electrical conductivity (EC), the soil samples were mixed with distilled water at a 1:5 ratio, and shaken for an hour, and the suspension was analyzed using a pH meter (Five Easy Plus FP20, Mettler Toledo, Greifensee, Switzerland) and an electrical conductivity meter (InoLab Cond Level 1, Wissenschaftlich Technische Werkstätten, Weilheim, Germany). Soil organic matter (OM), total nitrogen (T-N), and available phosphate (Av. P₂O₅) were analyzed using the Tyurin, Kjeldahl, and Lancaster methods, respectively. Exchangeable cations (K⁺, Ca²⁺, Mg²⁺, Na⁺) were extracted using 1M ammonium acetate (pH 7.0) and analyzed with an inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin-Elmer Optima 3300DV, Waltham, MA, USA). These soil properties were compared with the optimal soil conditions for wheat cultivation (NAAS, 2010).

Growth Characteristics and Grain Yield of Wheat

The growth characteristics of wheat were assessed based on the Research Survey and Analysis Standards of the Rural Development Administration (RDA, 2012). The heading and maturity dates were recorded, and 55 days after heading, twenty plants per variety (five plants per plot, replicated four times) were selected for measuring traits such as plant height, spike length, number of spikes per unit area, and number of grains per spike. The heading date was defined as the day when 40% of the total tillers had emerged, and the maturity date was determined as the day when more than 80% of the spikes had turned yellow. Plant height was measured from the ground surface to the base of the spike, whereas spike length was measured from the base to the tip of the spike. The number of spikes per unit area was calculated by counting the total number of spikes within a 40 × 40 cm area at four randomly selected locations per plot and converting the value to the number of spikes per square meter. The number of grains per spike was determined by randomly collecting 20 spikes from four locations per plot, threshing them, and counting the grains. Liter weight was measured using a grain volume tester, excluding seeds that passed through a 2.0 mm sieve. The thousand-grain weight (TGW) was determined with four replications by weighing 1,000 fully matured grains after removing underdeveloped ones. Grain yield was determined by harvesting spikes from a 3 m² area (1 m × 3 m) in the central part of each plot. The harvested spikes were threshed, dried (to a moisture content of 14%), and weighed, excluding immature grains. The final yield was converted to a per 10a basis.

Analysis of Crude Protein Content in Wheat Flour

Selected wheat grains were tempered and then ground using a laboratory mill (Bühler MLU-202, Bühler AG, Uzwil, Switzerland) for analysis with four replications. Crude protein content was determined by measuring the total nitrogen content in wheat flour samples using a nitrogen/protein analyzer (Kjeltec 8400, Foss, Hillerod, Denmark). The nitrogen content was multiplied by a factor of 5.75 to calculate the protein content, and the results were corrected based on a moisture content of 14%.

Statistical Analysis

All analyses were conducted using the SPSS statistical package 18.0 (SPSS, Chicago, IL, USA). To assess the differences among cropping systems or between varieties, a one-way analysis of variance (ANOVA) was performed, and the significance of the differences between means was tested using Duncan’s multiple range test (p < 0.05). Additionally, a two-way analysis of variance (ANOVA) was conducted to evaluate the interaction effects between wheat varieties and the cropping systems.

Results and Discussion

Atmospheric Environmental Factors during Wheat Cultivation Period

The average daily temperature during the cultivation period (from October 30, 2020, to June 9, 2021) of the last crop ‘wheat’ of each multiple cropping system was 22.7°C (minimum temperature 18.7°C, maximum temperature 27.7°C), with an average daily relative humidity of 75.4%, cumulative rainfall of 932.3 mm, average wind speed of 1.0 m/sec, and cumulative sunshine hours of 689.1 hr (Fig. 2).

The optimal temperature for wheat growth is between 17-23°C (Porter and Gawith, 1999). The wheat varieties used in this study, ‘Jokyoung’, and ‘Geumgang’, are cold-resistant and can overwinter in areas with a minimum temperature of -8°C in January, allowing cultivation across most of the country, except mountainous high-altitude regions. In the southern regions, although the minimum temperature in January occasionally drops below -8°C, cold spells are usually short-lived, and winter temperatures are relatively mild, and therefore, both ‘Jokyoung’ and ‘Geumgang’ are expected to overwinter without issues. However, prolonged exposure to low temperatures during the winter (from December to February) can lead to reduced germination rates and tillering (RDA, 2001), and both low temperatures and dryness may worsen the damage. Furthermore, during the wheat growth regeneration and tillering stages (February to March) in the southern regions, adequate rainfall and relative humidity maintained above 60% are expected to prevent any frost damage. However, in May, during the grain-filling stage, frequent rainfall results in insufficient sunshine, which may lead to a decrease in spike number or cause damage from pests, diseases, and pre-harvest sprouting. Therefore, it is necessary to explore appropriate pest control measures. Additionally, from mid-April (the heading stage) to late May (the grain-filling stage), when the average wind speed exceeds 1.5 m/sec for 10 days, there is a risk of lodging. The damage from lodging and pre-harvest sprouting can increase particularly when strong wind is accompanied by rainfall during the grain-filling stage. However, in this study, there is no day with both rainfall exceeding 20 mm and average wind speeds over 1.5 m/sec during the grain-filling stage, and lodging damage was observed in neither of the two wheat varieties.

Chemical Properties of the Soil

The initial paddy soil in the experimental site, where rice had been cultivated repeatedly for many years, had organic matter, K⁺, and Ca²⁺ levels within the standard range for wheat cultivation. However, the soil pH, along with the available phosphate and Mg²⁺ content, were lower than the optimal standards. Compared with the rice-wheat-rice-wheat cropping system, the soil pH increased and the contents of organic matter, available phosphate, K⁺, Ca²⁺, and Mg²⁺ also increased in the multi-cropping systems that included rice-alternative crops. In particular, while there was no statistically significant difference in pH between the rice-wheat-rice-wheat and rice-IRG-sesame-wheat systems, the soil gradually became alkaline in the multi-cropping systems comprising solely field crops such as perilla-IRG-sesame-wheat and soybean-IRG-sesame-wheat. Additionally, the contents of K⁺, Ca²⁺, and Mg²⁺ were significantly higher in the multi-cropping systems comprising only field crops (perilla-IRG-sesame-wheat, soybean-IRG-sesame-wheat) and in the rice-IRG-sesame-wheat system, compared with the rice-wheat-rice-wheat system (Table 1). Soil pH is an important factor that affects various chemical and biological properties of the soil. In acidic soils, nutrient absorption is hindered, and the activity of soil microorganisms is also affected. Therefore, reducing the acidity through proper fertilization is necessary. The Mg²⁺ content in the soil before wheat cultivation was somewhat lower than the optimal standard in all cropping systems, with exhibiting even lower levels in the rice-inclusive systems. Nevertheless, as rice was substituted with field crops, the paddy soil gradually improved to become more suitable for wheat cultivation, and the agricultural soil environment was optimized for increasing the potential productivity, thereby enhancing the yield. For example, when soybean and corn are cultivated in paddy soil, soil bulk density decreases, and porosity increases, improving the soil’s physical properties compared with that of soil during rice cultivation (Seo et al., 2021). Additionally, when rice-alternative crops such as black soybean, sesame, and perilla are cultivated in the paddy soil as preceding crops, the available phosphate, exchangeable Ca²⁺, and Mg²⁺ contents in the soil increase, improving the chemical properties of the soil and providing agricultural benefits (Oh et al., 2022). Such changes have also been observed in two-cropping systems with rice-alternative crops (Choi et al., 2023; Oh et al., 2022; Seo et al., 2021). The decomposition of organic residues, such as stumps from preceding crops, appears to enrich the soil by providing suitable nutrients for the crops. Furthermore, the decomposition of organic residues and nutrient transport to the crops are related to soil physical properties, therefore, adjusting the cropping system in a manner that improves the aeration of the paddy soil is necessary.

Wheat Growth Characteristics

In the southern region, wheat heading occurred between April 17 and 19, and maturity occurred between May 28 and June 1. The heading and maturity of ‘Geumgang’ wheat were 2－3 days earlier than that of ‘Jokyoung’ wheat. Among the cropping systems, for ‘Jokyoung’ wheat, no difference was observed in heading and maturity; however, for ‘Geumgang’ wheat, in the rice-wheat-rice-wheat system, heading was 1 day later and the maturity was 1 day earlier. Regarding the growth characteristics of wheat, the culm length of both wheat varieties was significantly higher in the perilla-IRG-sesame-wheat, soybean-IRG-sesame-wheat, and rice-IRG-sesame-wheat systems than that in the rice-wheat-rice-wheat system. The spike length did not indicate any significant differences among cropping systems. The number of spikes per unit area for both wheat varieties was higher in the perilla-IRG-sesame-wheat, soybean-IRG-sesame-wheat, and rice-IRG-sesame-wheat systems compared with that in the rice-wheat-rice-wheat system. The number of grains per spike demonstrated no significant differences between cropping systems for ‘Jokyoung’ wheat. However, it was particularly higher for ‘Geumgang’ wheat in the soybean-IRG-sesame-wheat system (Table 2). These results indicate that in multiple cropping systems, tilling of previous crop stubble improves soil physical properties, and the decomposition of organic residues releases available nutrients that are absorbed by the roots of subsequent crops, thereby increasing growth and yield.

The heading and maturity period of wheat is significant for securing safe growth of subsequent crops, such as rice or field crops, during double cropping. Harvesting must be completed before the monsoon season. Moreover, to cultivate field crops in the paddy field, the field needs to be prepared before it rains. Both the ‘Jokyoung’ and ‘Geumgang’ wheat varieties can be harvested in early Junes, therefore, wheat cultivation in the southern region will not pose a problem, and the crop rotation system can be operated smoothly. Moreover, legumes, such as soybeans, are extremely beneficial as preceding crops for cultivating other crops such as wheat, barley, rye, and oats (Ingver et al., 2019), and can be applied in various niches as part of crop rotation. Furthermore, green manure crops, forage crops, and other plants are suitable as niche crops in crop rotations where it is undesirable to maintain leguminous species for more than a year (Ross et al., 2009). This study confirmed that multi-cropping systems including leguminous plants or alternative crops to rice affected the physical and chemical properties of the soil and ultimately had a direct impact on wheat growth. However, during the ripening period in May, frequent rainfall led to the occasional occurrence of smut disease in some fields, and there were concerns about the risk of pre-harvest sprouting (data not provided). Among the growth characteristics of wheat, the stem length, spike length, spike number, and grain number per spike were generally favorable in the soil where black soybean was cultivated as the previous crop, which is consistent with reports by Oh et al. (2022). Grain yield is influenced by factors such as spike number per unit area, grain number per spike, liter weight, thousand grain weight, and ripening rate, and increases in these factors lead to higher yields (Oh et al., 2021, 2022; RDA, 2001). Crop yield is determined by the crop’s genetic characteristics and external environmental factors such as the weather and soil conditions during the growing period. However, in this study, as wheat was grown in the same climatic region, it can be inferred that the yield was more influenced by the variety and the subtle changes in soil environment owing to the cropping system rather than by weather factors.

Changes in Wheat Productivity and Quality

The seed yield of ‘Jokyoung’ wheat was higher by more than 80 kg/10a than that of ‘Geumgang’ wheat. Among the cropping systems, both varieties had significantly higher yields in the perilla-IRG-sesame-wheat, soybean-IRG-sesame-wheat, and rice-IRG-sesame-wheat systems compared with that in the rice-wheat-rice-wheat system. In particular, the cropping system that included perilla demonstrated higher productivity. This can be attributed to the fact that the paddy soil, which had been compacted by repeated rice cultivation, was improved in physical properties when field crops were continuously cultivated. Owing to the decomposition of previous crop residues, the content of soluble nutrients such as K⁺, Ca²⁺, and Mg²⁺ increased, as observed in Table 1, as did the number of soil pores, leading to improved soil quality (Nascente and Stone, 2018; Zhou et al., 2014). The moisture content of the seeds did not demonstrate significant differences among the cropping systems; however, both the liter weight and 1000-grain weight were higher in the cropping systems composed of field crops compared with that in the rice-wheat-rice-wheat system. The protein content was lower in ‘Jokyoung’ wheat at less than 10%, whereas ‘Geumgang’ wheat had higher protein content, exceeding 10% (Table 3).

A higher protein content is required for wheat used in baking (Jung and Eun, 2003; Kim et al., 1997). Among wheat varieties bred in Korea, ‘Hwanggeumal’ has a high protein content of 14%, whereas the ‘Jokyoung’ and ‘Geumgang’ varieties used in this study have a protein content within 12% (RDA NICS, 2024). Similar to grain yield, the protein content is also influenced by the crop’s genetic factors, agricultural practices, and weather conditions. The quality of crops can vary depending on the growing region, sowing time, and environmental conditions during the growing period. Even for the same wheat variety, the protein content is higher when grown in the fall rather than in the spring (Son et al., 2022). Among the cropping systems, both varieties demonstrated higher protein content in the soybean-IRG-sesame-wheat system. This is because the decomposition of previous crop residues increased the total soil nitrogen, organic matter, and available phosphorus content in the topsoil (Table 1). It appears that, compared with rice, field crops such as soybeans hold more nutrients in their residues, and the decomposition of these residues results in the observed increase. In northern China, when wheat and corn were grown in a double-cropping system, returning the straw of previous crops to the soil or applying nitrogen fertilizers had been shown to improve soil fertility and crop growth, which in turn increased grain yield and protein content (Li et al., 2023). Additionally, the increase in nitrogen supplied as top dressing during wheat cultivation leads to an increase in protein content in the grains, thereby improving quality (Kim et al., 2018; Mei et al., 2019). Therefore, strategies such as applying nitrogen fertilizers as top dressing or cultivating legume crops as previous crops could be considered during the cultivation process.

Conclusion

The average temperature 0.4°C in January in the Milyang, with the average minimum temperature -5.3°C, was suitable for growth and development of winter wheat. Additionally, when the paddy fields were converted to multiple cropping systems with field crops such as perilla, soybean, and sesame, the physical and chemical properties of the soil were rapidly improved to be suitable for wheat cultivation. However, the Mg²⁺ content is slightly lower than the optimal standard, necessitating proper fertilization before sowing. The seed yields were significantly higher in the perilla-IRG-sesame-wheat, soybean-IRG-sesame-wheat, and rice-IRG-sesame-wheat systems compared with that in the rice-wheat-rice-wheat system. Therefore, to increase wheat yields in paddy fields, multiple cropping systems composed of field crops such as perilla, soybean, and sesame could be advantageous, and cultivating the ‘Jokyoung’ variety is recommended. However, although the ‘Geumgang’ variety has lower productivity than ‘Jokyoung’ variety, its higher protein content makes it worth considering cultivation for the purpose of bakering.