​The Impact of Growing Legume Plants under Conditions of Biologization and Soil Cultivation on Chernozem Fertility and Productivity of Rotation Crops

The long-term use of land in monoculture farming typically has negative consequences. Even so, the practice of growing monocultures is conducive to high harvest efficiency and productivity, especially when mineral and organic fertilizers are applied (Semedo 2017). The result of monoculture farming is agrocenosis formation. Agrocenosis is an unstable ecosystem that comes under attack from pests that can partially or completely destroy crops in one or two seasons. The ease with which pests spread across the agrocenosis results from its monocultural composition where conditions are favorable to pest development (Ponisio et al., 2014). Fertilizers fail to create a long-term positive effect, since at least 50 per cent of nutrients applied to the soil is leached out and the amount that does reach the plant may be harmful. At the same time, the intensive land use can lead to the exhaustion and degradation of land, cause the loss of organisms on different trophic levels and facilitate the depletion of soil microflora (Akhtar et al., 2016). Considering that soil is a basic medium for plant growth, soil degradation can entail disturbances within phytocenosis and zoocenosis.
       
Farmers across the world are seeking to avoid the above problems. For this, they decided to abandon the traditional ways of treating the land that involve mechanized tillage and the use of chemicals. The alternative is organic farming, which advocates for the mechanical treatment of soil without using pesticides and other toxic substances (Salleh et al., 2018). This practice is conducive to agriculture biologization. Organic farming is widely practiced in many developed and developing countries such as the USA, Canada, Australia, Kenya, Malaysia, etc. Thus, transitioning from conventional to organic production takes place on a global scale (Symanczik et al., 2017).
       
The main idea of organic farming is to improve soil fertility without causing harm to the environment. In organic production, the anthropogenic influence on ecosystems is minimal. One of the advantages of organic agriculture is the production of safe products. Thus, organic farming has multifaceted positive effects on human health and ecosystem (de Tombeur et al., 2018a). Organic practices include, but are not limited to: using compost and organic manure; encouraging beneficial insects to control pests; using natural soil amendments; and reducing tillage (Hirschfeld and Van Acker 2019). On average, organic products are 20 to 30 percent more expensive, but sticking to organic practices will allow keeping the soil fertile (Vitari and David 2017).
       
Biologization of agricultural systems involves grass cultivation, polyculture adoption and the use of break crops. It is expected that at least one-quarter of arable land will be covered with grasses. At the same time, crop residues are not removed from the field. Deep cultivation is prohibited in order to keep the soil microstructure intact (de Tombeur et al., 2018b). Sometimes, organic farming avoids or minimizes soil disturbance. Even the native plants may be kept. This enables the creation of a stable agroecosystem that is neither a natural phytocenosis, nor a typical monoculture system, but something in the between. Normally, an agroecosystem contains both a crop and a native weed species (Udomkun et al., 2019). A sustainable agroecosystem design depends upon several important factors such as soil microrelief, soil microclimate and more. Crop species are incorporated into a natural setting that is stable and where trophic relationships are strong. The enrichment of soil is carried out without application of fertilizers. The disadvantage of this approach is that it is not suitable for vast areas, at least for now (Sharma 2017). Furthermore, permaculture systems that receive non-chemical enrichments will not provide yields that are as high as those obtained through traditional methods. Nevertheless, permacultures have obvious advantages over traditional system designs. For instance, they provide a deeper assortment of products. Permaculture designs are complex. Although they allow a vast range of crops, the harvests are relatively small. This type of organic practice is ideal for small farms (Hathaway 2016) that seek to meet mass demand. The bottom line is that the quality of organic products will be higher than that of products obtained through conventional farming.
       
In the shortage of financial and material-technical resources and because of price imbalances and climate anomalies observed in recent years, expanding the cultivation of perennial legumes and cereals became one of the major directions taken in agriculture to improve land fertility (Didur et al., 2020; Patil et al., 2020). Growing more grain legumes to harvest more protein-rich seeds is among the crucial strategies used by farmers to enhance the production of vegetable protein (Wobeng et al., 2020). Cereals are not only an important source of vegetable protein, but also an essential component of alternative farming systems. Perennial grasses serve as the basis for forage production, while perennial legumes with a well-developed root system facilitate soil fertility by leaving behind a large amount of organic matter, which improves soil structure and enriches it with nitrogen (Yadav et al., 2019). The phytosanitary role of perennial grasses is also tremendous: they create unfavorable conditions for many types of weeds, enabling the control of weed seeds and cause many pests and pathogenic microflora to die. That aside, perennial grasses are an effective means of protecting soil from water and wind erosion (Xiong et al., 2012).  When decomposing in the presence of oxygen, perennial grass residues form humus, the fraction of the soil medium that improves the solubility of soil minerals. The process of organic matter mineralization takes place. Plant residues provide beneficial soil microorganisms with energy necessary for vital activity (Wang et al., 2014).
       
Using a mixture of perennial legumes and cereals is better than planting them separately. The advantage lies in the fact that the root system of perennial cereals develops largely in the arable horizon and after decaying, facilitates the development of aerobic bacteria. Perennial legumes, on the other hand, root deeply into the soil and transfer nutrients (phosphorus and calcium) to the upper horizons (Verde et al., 2013). The value of the perennial layer is that their roots, which contain a substantial amount of proteins, carbohydrates and other organic substances, are plowed into the soil. Roots of some legumes, including, but not limited to, clover and alfalfa, contain a lot of nitrogen and calcium (Tuyen et al., 2016).
       
One of the organic farming practices is legume cultivation. Legumes are used as nitrogen suppliers because of their ability to fix atmospheric nitrogen in symbiotic associations with soil bacteria (Kaushik et al., 2018). They also can make up a substantial part of the human diet, yet they fail to fully replace meat. Moreover, legumes contain anti-nutritional compounds such as tannins and vicin that can degrade the taste of food. For this reason, legumes such as clovers and alfalfa are commonly used as fodder crops (Smith and Lampkin 2019).
       
Another promising method is the use of brake crops. This technique has long been known to farmers, yet it was not widely used until now. In modern agroecology, cereal and bean fodder mixtures are most often considered. The idea of brake crop cultivation is to enrich soil after it has been exhausted from growing soil-depleting crops. The literature provides evidence on the effectiveness of rotations such as winter wheat/winter vetch, buckwheat/millet, lupine/spring wheat or barley and sunflower/winter wheat + legumes (de Tombeur et al., 2018a, 2018b; Roberts 2017).
       
This study examines the binary mixtures of legumes with cereals and sunflower crops. Our study is dedicated to the study of binary sunflower crops with legumes. The relevance of the study lies in the absence of similar studies that use biologization techniques (sideration and fertilization with straw of cereal crops) when growing sunflower. The purpose of this study is to investigate how biologization and the use of different soil cultivation techniques in legume cropping affects chernozem soil fertility and productivity of rotation crops.
       
The results may be used in farms with a similar soil and similar microclimatic conditions. The joint and simultaneous coexistence of several agricultural crops will significantly complicate the structure of the plant community, causing a shift to an equilibrium.

K = R×10/Σt

The experiments show that the use of legumes such as alfalfa and clover in crop rotation allows preserving and improving soil fertility, as evidenced by data from biological soil analysis and crop yield observations.
       
Data in Table 1 show a decrease in soil water content in the 1-meter soil layer during the growing season. The best moisture conditions were observed after winter wheat with alfalfa in the third year. During the experiment with wheat alone, the quantity of water contained in soil was lower (p = 0.05).
 

       
In the two-crop experiments with alfalfa, the structural change of soil was detected. Fig 1 shows an upward trend in the number of soil aggregates in the second year, with the annual growth ranging between 13 and 14 per cent. In the first year, the difference was not significant, but in the second year, the amount of soil aggregates doubled, reaching 28 per cent. The third year saw a 5 per cent increase after winter wheat with alfalfa. Over the three-year period, the overall degree of soil aggregation was improved by 8 per cent. In the control experiment, the soil aggregation parameter decreased by 47 per cent, by contract (p = 0.001).
 

       
In addition to the structural effect, the use of legumes had a significant impact on water resistance (Fig 2). In the experiment without legumes, the water resistance of soil aggregates dropped 3 per cent, whilst after the use of companion crop (alfalfa), this parameter demonstrated 2.4-percent improvement (p = 0.05). This trend continued into the second year: in the two-crop experiment with alfalfa, the water resistance parameter increased by 5 per cent; in the one-crop experiment without legumes, it decreased by 4 per cent. In the third year, the water resistance parameter after winter wheat with alfalfa was found to increase by 4.3 per cent. Overall, the total increase of water resistance was 9.7 per cent.
 

       
The experiments revealed a change of detritus content (Fig 3). The amount of detritus in the control plot dropped by 40.2 per cent over the study period. The use of pioneer plants resulted in up to 71 per cent increase of detritus (p=0.001). Depending on the crop mixture, the following results were obtained. The amount of detritus after sunflower with alfalfa was 1.6 times higher compared to control. Other crop mixes were less effective. Alfalfa alone provided 1.3 times more dead biomass than the control, whilst winter wheat with alfalfa was conducive to 1.2 times higher amount of detritus.
 

       
The best mineral content of soil was recorded after two-crop experiments with alfalfa. The change of phosphorus ranged between 6 and 8 per cent depending on the type of soil treatment. The change of mobile potassium varied between 6 and 19 per cent. In the control experiment, phosphorus and potassium content decreased by 10 and 5 percent, respectively. The difference between fallow and binary crop mixture is significant (p=0.001).
       
The above factors had a very significant impact on crop yields (Table 2). Sunflower yields were found to be excessive in two-crop experiments with alfalfa regardless of the type of soil treatment. Yields were higher by 3 to 8 per cent than those from the control experiments (p=0.05). The winter wheat yields in two-crop experiments, however, were lower compared to the control (p=0.05). The barley yields were higher when planted after winter wheat with alfalfa.
 

       
At the present stage of agricultural development, chernozem soils experience degradation caused by the insufficient use of expensive mineral and organic fertilizers (Tadele 2020). Therefore, it is necessary improve the cropping systems to concentrate primarily at preserving and increasing soil fertility. In this light, there is an urgent need to develop recommendations for the use of biologization techniques in crop cultivation to increase the content of organic matter in the soil. The present work closes this gap. Binary sowings of crops with perennial legumes and the combined use of stubble crops and grain straw for fertilization can be effective when enhancing chernozem fertility (Sun et al., 2020). At the same time, the rate of residue decomposition is a crucial aspect of impact crop residues have on soil fertility (Verde et al., 2013).
       
The traditional farming practices are designed to achieve maximum efficiency of food production. This approach dominated until the end of the last century. There is no doubt that traditional farming has negative health implications due to the use of chemicals (Nieder et al., 2018). The consequences are the increase in the number of diseases linked to obesity and gastrointestinal tract, such as gastritis, poisoning and cancer.
       
A fundamentally new approach to organic farming is based on biologization (Średnicka-Tober et al., 2016). Agricultural landscapes change when the number of monocultural fields decreases. Biologization encourages the introduction of new plant species into the ecosystem and thus it is conducive to biodiversity. In contrast to traditional farming, organic agriculture does not offer immediate strong results (Pinela et al., 2018). However, it sustains soil fertility and protect the agroecosystem. In recent decades, there has been a constant decrease in arable land due to soil dehumification, desertification, global climate changes and mineralization (Sparvoli et al., 2016). Redefining agriculture with the concept of biologization is an effective solution against soil degradation (De Schutter 2017).
 
The cultivation scheme examined in this study allows for a gradual transition from traditional to exclusively organic farming. Of conventional practices, only mechanical tillage was decided to be included. Even so, this solution is suitable for a smooth transition. Existing works confirm that organic farming improve a range of soil properties, such as fertility, structure and resistance (de Tombeur et al., 2018a, 2018b; Gustavsson et al., 2011; The Rockefeller Foundation 2013). To transition from monoculture to permaculture requires gradually improving soil fertility and soil structure. Otherwise, farmers can expect the onset of mineralization and desertification, especially in areas with dry climates.

Binary mixtures of legumes with cereals or sunflowers with the use of biologization techniques help normalize soil moisture, micronutrient content, moisture resistance of soil aggregates and the amount of detritus. The present study revealed a steady tendency towards an increase of dead biomass (1.5 times), soil aggregates (one-third) and water resistance of those aggregates (9 per cent). In these conditions, crops had 3 to 8 per cent increase in yields.