The effect of cover crops and inoculants on the content of easily hydrolyzable nitrogen in the soil during the cultivation of sainfoin

Goal. To study the effect of cover crops and inoculants on the nitrogen regime of the soil during the cultivation of sainfoin under arid conditions in the Eastern part of the Northern Steppe of Ukraine. Methods. Field (observation of plant growth and development, yield formation), laboratory (study of quantitative and qualitative soil properties), measurement and weighing (yield determination), and computational-comparative (assessment of the effectiveness of cultivation techniques). Statistical analysis of the research results was performed using Microsoft Excel 2024. Results. The study was conducted in 2024–2025 at the experimental field of the Donetsk State Agricultural Research Station of the National Academy of Agrarian Sciences of Ukraine (DSARS of NAAS). They studied the no-till method of sowing the Rezonans variety of vetch-leaved sainfoin (4.0 million seeds/ha) and in combination with cover crops: the Avers variety of spring barley (3.5 million seeds/ha), the Orfei variety of oilseed flax (4.0 million seeds/ha), and the sunflower hybrid Biloba (50,000 seeds/ha). The biological preparations Ryzobofit (1 L/t) and Ryzohumin (2 L/t) were used to inoculate sainfoin seeds. The soil was typical low-humus heavy-loam chornozem. The use of the microbiological preparations Ryzobofit and Ryzohumin for inoculating the seeds of sainfoin under a cover crop of spring barley contributed to an increase in soil nitrogen content by 4.0 and 7.0 kg/ha, respectively. Soil nitrogen levels in the cover crop treatments (oilseed flax) were consistent with the data obtained for non-cover crop cultivation, except for the values in the treatment where sainfoin seeds were inoculated with the Ryzobofit preparation (nitrogen content was 1.0 kg/ha lower). In the treatments where sunflower served as the cover crop, the lowest soil nitrogen levels were observed. When inoculants were applied to the seeds of sainfoin, nitrogen fixation increased: Ryzohumin contributed to a 0.3 kg/ha increase in nitrogen compared to the initial level, while Ryzobofit contributed to a 0.4 kg/ha increase. In the second year, the plants of the sainfoin actively accumulated biological nitrogen in a soil layer 0–20 cm thick. The highest content of easily hydrolyzable nitrogen after seed harvest was recorded in the oilseed flax treatments where the sainfoin seeds were inoculated with the Ryzobofit preparation (87.4 kg/ha). Conclusions. It was demonstrated that the content of easily hydrolyzable nitrogen in the soil depended on the cover crop and the use of inoculants. The highest rates of nitrogen accumulation in the soil were observed when oilseed flax was used as a cover crop.