Effect of Seed Inoculation with Belgorod Nitragin KM on Yield and Grain Quality of Soybean Varieties in the Central Black Earth Forest-Steppe

Keywords: soybean, Nitragin KM, seed inoculation, variety, yield, protein content, oil content, protein and oil harvest.

Introduction

Currently, soybeans are grown on fields where indigenous soil-dwelling strains of these bacteria, selective toward the host plant, are absent. Therefore, when cultivating soybeans, it is necessary to treat the seeds through inoculation (from Latin inoculate - grafting), i.e., artificial infection with a virulent (pathogenic) active competitive race of nodule bacteria. It is important that these bacteria be capable of symbiosis with soybeans, able to rapidly multiply in root tissue, forming many active (large, pinkish) nodules, which are peculiar tumor-like formations of root tissue [4,5,6].

The symbiotic legume-rhizobial interaction consists in the plant feeding the nodule bacteria with photosynthesis products, which in turn provide the plant with nitrogen, fixing it from the air, converting it from the gaseous form (N2) into a chemically bound state, into a form of ammonium (NH4+) accessible to plants.

The feasibility and high effectiveness of seed inoculation has been demonstrated in a number of studies conducted both in the Central Chernozem Region [1,3,6,7] and in other soybean-growing regions [2,5].

Various inoculants available on the market are used for inoculation. These are biopreparations of live nodule bacteria grown on dry, liquid, or gel-based substrates.

In Russia, several foreign inoculants are currently approved for use: Noctin A (liquid), Nitrobiks-P (dry), Nitrobiks-Zh (liquid), and others, as well as the domestic Rizotorfin-B - a dry peat-based preparation. Russia produces soybean inoculants using the best strains of studied rhizobia, maintained in the departmental collection of useful agricultural microorganisms (St. Petersburg, All-Russian Research Institute of Agricultural Microbiology) and in the All-Russian Collection of Industrial Microorganisms (Moscow).

Internal strains (k-9, M-8, 626a, 634a, 645c) of soybean nodule bacteria were tested on two soybean varieties (Belgorodskaya 48 and Luchezarnaya) in experiments at the Department of Crop Production, Feed Production, and Agricultural Technologies of the Voronezh State Agrarian University. It was established that inoculants increase the number and mass of active nitrogen-fixing nodules on soybean roots, increase its yield and seed protein content. At the same time, the responsiveness of varieties to inoculation with different strains was unequal, but always positive [7,8,9].

In our research, we used Nitragin KM for soybean seed inoculation, developed at LLC "NTC BIO" (Belgorod Region, Shebekino). This inoculant contains a natural, adapted to Chernozem conditions, highly active strain of nodule bacteria Bradyrhizobium japonicum 206.

Methodology

Field experiments and laboratory analyses were conducted in 2015-2016 at the Department of Crop Production, Breeding, and Vegetable Growing of the Belgorod State Agrarian University.

Nitragin KM is a dry granular soybean inoculant containing at least 5 billion nodule bacteria per gram. The preparation was applied through wet seed treatment together with the supplied OMC (organo-mineral complex), which ensures adhesion of the preparation to seeds, its preservation, and additional nutrition for nodule bacteria and plants.

Field experiments were laid out according to generally accepted methodology without mineral fertilizers. Accounting plot area was 4 m2, six replications, systematic plot arrangement.

Soybeans were sown at optimal dates using a C-11 "Alfa" seeder. Seeding rate was 0.9 million/ha of viable seeds, sowing depth 3-4 cm.

Plant height and air-dry mass were determined from an average of 25 plants; nodule number and mass on roots from an average of 10 plants. Leaf area was determined using the "Areas" program (Samara State Agricultural Academy).

Yield was recorded per plot, threshing soybeans with a Sampo SR 2010 combine. Yield was recalculated to 100% purity and 14% moisture.

Weather conditions during the experimental years varied. In 2015, the growing season was quite hot and dry, less favorable than in 2016, when cooler but rainy weather promoted the formation of high soybean yields. The experiment was conducted on typical, medium-thick, heavy-loamy chernozem containing humus in the plow layer of 4.54%, easily hydrolyzable nitrogen - 137.2 mg/kg, available phosphorus - 137 mg/kg, exchangeable potassium - 126.0 mg/kg, pH of salt extract - 6.7.

Results

At the stem elongation stage, nodule numbers increased under the effect of Nitragin KM on average across varieties from 10.9 to 18 per plant, or by 65%, and their mass increased by 57%. As plants grew, this trend continued. Nodule number and mass reached their maximum at the flowering stage. Subsequently, nodules gradually died off, their pinkish coloration changing to greenish (due to the conversion of the pink pigment leghemoglobin to greenish choleglobin). This was clearly visible at the seed-filling stage.

This pattern was exhibited by all varieties, with varietal specificity. At all growth stages, the Belgorodskaya 8 variety had more nodules on its roots, while the Bara variety had somewhat fewer. Thus, at the flowering stage, varieties Belgorodskaya 48, Belgorodskaya 6, and Belgorodskaya 7 had 25.4 to 26.9 nodules per plant, while Belgorodskaya 8 and Bara had 34.0 and 22.5 per plant, respectively (Table 1).

A greater number of active nitrogen-fixing nodules, by improving soybean nitrogen nutrition, strongly influenced plant growth and development. Soybean plants grown from seeds inoculated with Nitragin KM and therefore better supplied with nitrogen nutrition, more rapidly increased in height, mass, and leaf area compared to the control (without seed inoculation).

Plant height increase at the branching, flowering, and pod formation stages averaged 5.7, 5.1, and 4.7 cm across varieties. At the flowering and pod formation stages, the rate of stem height increase diminished. Thus, if during the stem branching period, stem height increased under the effect of inoculation by 23.6%, then at the flowering stage by 10.4%, and at the pod formation stage by only 7.1%. The degree of plant mass increase during these phases significantly increased and amounted to 0.6, 2.7, and 7.3 g, or 23.1, 40.3, and 57.5%.

Leaf area as a result of seed inoculation increased compared to the control at the branching stage by 64 cm2 (19.6%), flowering by 74 cm2 (12.2%), and pod formation by 172 cm2 (21.3%).

It was noted that the inoculant Nitragin KM, by promoting more intensive soybean plant formation, significantly accelerated their maturation. For example, on average over two years, the Belgorodskaya 8 variety in the control variant (without inoculation) had a period from emergence to maturity of 111 days, while with seed inoculation it was 102 days, i.e., the vegetation period was shortened by 9 days. A similar trend was observed in other varieties, but to a lesser extent: Belgorodskaya 6 and Belgorodskaya 48 accelerated maturation by 5 and 4 days, while Belgorodskaya 7 and Bara by 3 days.

Belgorod-bred varieties proved more responsive to seed inoculation, especially Belgorodskaya 8; the Bara variety was less responsive. Thus, by pod formation time, seed inoculation increased plant mass of Belgorod varieties by 58-69%, and the Bara variety by 33.6%. This naturally affected soybean variety yields (Table 2).

The average yield increase across varieties from seed inoculation was 2.3 c/ha or 9.1%. For Belgorod-bred varieties, this increase was greater, from 2.1 to 3.2 c/ha (8.4-11.6%), than for the Bara variety - 1.3 c/ha (5.6%).

The Belgorodskaya 8 variety, as well as Belgorodskaya 7, proved more responsive to seed inoculation. In 2015, their yield increase was 3.2 and 3.3 c/ha (16.5 and 20.8%), and in 2016 - 3.2 and 1.5 c/ha (9 and 4.4%). The Bara variety responded least to seed inoculation: in 2015 the yield increase was 1.5 c/ha (10.8%), in 2016 - 1.1 c/ha (3.4%). Varieties Belgorodskaya 48, Belgorodskaya 6, and Belgorodskaya 7 differed little in yield and responsiveness to seed inoculation.

The yield of all soybean varieties depended more heavily on weather conditions. In the well-moistured 2016, it was almost twice as high as in 2015. The range of variation averaged from 19.1 to 35.8 c/ha (16.7 c/ha) in variants with inoculation and from 16.5 to 33.9 c/ha (17.4 c/ha) in control variants (without inoculation). As a trend, it can be noted that seed inoculation somewhat reduced the range of soybean yield variation by years, i.e., increased its stability. Belgorod-bred soybean varieties (especially Belgorodskaya 8) were more productive than the Bara variety.

The influence of variety on the magnitude and variation of soybean yield within each year was quite significant, especially when comparing the higher-yielding variety (Belgorodskaya 8) with the lower-yielding one (Bara). Thus, in 2015, with seed inoculation, the yield difference between these varieties was 7.2 c/ha, and without inoculation - 5.5 c/ha; in 2016 - 5.6 and 3.5 c/ha, respectively. Seed inoculation with Nitragin KM evidently increased soybean plant resistance to the stressful conditions of 2015.

Inoculation of all soybean varieties with Nitragin KM not only increased their yield but also improved the quality of grain products.

Improvement of soybean nitrogen nutrition through seed inoculation ensured a significant increase in seed protein content. This increase was greatest in the less productive year 2015. On average across varieties, seed protein content increased by 6.8 abs. % (Table 3).

The greatest increase in protein content (by 9.4 abs. %) was observed in 2015 for the Bara soybean variety, while a less significant increase (by 3 abs. %) was noted for the higher-yielding Belgorodskaya 8 variety. Other Belgorod-bred varieties increased seed protein content by 6.3-8.7 abs. %. In 2016, the increase in seed protein content resulting from inoculation was less significant for all soybean varieties (averaging 2.6 abs. %).

Seed oil content also increased as a result of Nitragin KM application, although to a lesser extent than protein content. On average across all varieties, this increase was 2.1 abs. %, with variation across varieties from 0.8 abs. % (Bara) to 2.7 abs. % (Belgorodskaya 6). Seed oil content increased more from inoculation in the dry year 2015 (by an average of 2.8 abs. %), less (by 1.2 abs. %) in the wet year 2016.

Conclusions

1. Soybean plants grown from seeds inoculated with aqueous Nitragin KM suspension formed significantly more active nitrogen-fixing nodules in number and mass on their roots. For example, at the stem elongation stage, the number of nodules on inoculated plant roots compared to the control (without inoculation) increased on average across varieties from 10.9 to 18.0 per plant, or by 65%; at the flowering stage from 17.7 to 27.1 per plant, by 53.1%. More nodules were found on the roots of the Belgorodskaya 8 variety (34 per plant), fewer on the Bara variety (22-25 per plant).
2. Seed inoculation with Nitragin KM resulted (compared to the control) in more intensive increase in plant height, mass, and leafiness, and accelerated maturation of soybean varieties by 3-5 to 9 days. Belgorod-bred varieties proved more responsive to seed inoculation, especially Belgorodskaya 8; the Bara variety was less responsive. For example, by pod formation time, inoculation increased plant mass of Belgorod soybean varieties by 58-69%, and the Bara variety by 33.6%.
3. Soybean variety yields in different (dry and wet) research years were always higher when seed inoculation was used. (The yield increase of all varieties averaged 2.3 c/ha or 9.1% over two years.) Belgorod-bred varieties were more productive (especially Belgorodskaya 8 at 30.7 c/ha) than the Krasnodar variety Bara (24.3 c/ha). Inoculation contributes to both increasing the magnitude and stability of soybean yields across years.
4. Improved nitrogen nutrition through inoculation, along with increased yields, ensured an increase in seed protein content across soybean varieties on average from 36.2% (in control) to 41%, and oil content from 20.5 to 22.6%. Seeds of Belgorodskaya 8 contained more protein and oil (42.5 and 23.4%, respectively); seeds of the Bara variety contained less (39.4 and 21.7%).

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