Decomposition of sorghum (Sorghum bicolor L.) crop residues in agrocenoses of the Right-Bank Forest Steppe of Ukraine

Abstract

Aim. To quantify and analyse the amount of sorghum plant residues returned to the soil, their accumulation rate, carbon sequestration potential, and decomposition, as well as the intensity of their degradation following the application of a biodestructor. Methods. The field experiment was conducted in 2023–2025 on a typical low-humus chernozem soil at the experimental field of the Institute of Bioenergy Crops and Sugar Beet NAAS (Ksaverivka Druha, Kyiv Region, Ukraine). The experimental design included two treatments: UAN-32 at 30 l/ha (control) and UAN-32 at 30 l/ha + biodestructor E.K.O. Stim Premium at 3 l/ha. After panicle removal, the leaf-and-stem biomass was chopped, treated with the respective products, and incorporated into the 8–15 cm soil layer using disc tillage implements. The extent of residue decomposition was determined using the litterbag method after 6, 8, and 10 months. The mass, dry matter content, and chemical composition of aboveground and root residues were determined using gravimetric, physiological, biochemical, and agrochemical methods. The major groups of phytochemical compounds were assessed by direct biotesting, while carbon sequestration was estimated by calculation. Results. After grain harvesting, leaf-and-stem biomass ranged from 35.2 to 43.6 t/ha, dry matter from 22.7 to 26.7 t/ha, and lignin from 3.9 to 4.5 kg/ha, which resulted in a low rate of residue decomposition. The root mass sampled from the rhizosphere soil to a depth of 60 cm ranged from 2.24 to 3.50 kg. Estimated carbon sequestration by plant residues reached 80.7 kg CO₂-eq/ha at a total biomass yield, including the root system, of 47.1 t/ha and 66.1 kg CO₂-eq/ha at a biomass yield of 28.7 t/ha. The application of the biodestructor effectively accelerated the decomposition of sorghum crop residues in the soil. After 6, 8, and 10 months, the extent of residue decomposition under UAN-32 application was 27.3, 39.5, and 61.0%, respectively, whereas the additional application of E.K.O. Stim Premium increased these values to 45.9, 64.2, and 76.5%, respectively. Various phytochemical compounds were detected during the decomposition of sorghum residues in the soil, irrespective of biodestructor application or decomposition period. Tannins occurred at the highest concentrations: under UAN-32 application, their content was 6.8% after 6 months, 6.3% after 8 months, and 5.9% after 10 months of decomposition. After 6 months, the contents of glycosides and hydroxycinnamic acids were 4.7 and 2.9%, respectively. The application of E.K.O. Stim Premium reduced the content of phytochemical compounds in the organic residues by an average of 0.4–2.5% at the respective sampling times. Conclusions. The factors limiting the widespread use of sorghum harvest residues as an organic fertiliser include their slow decomposition under conditions of low soil biological activity and their potential phytotoxicity. The application of the biodestructor accelerated the decomposition of sorghum harvest residues by an average of 23.2%. The incorporation of sorghum crop residues resulted in carbon sequestration equivalent to 66.1–80.7 kg CO₂-eq/ha. The decomposition of organic sorghum residues was accompanied by the accumulation of various phytochemical compounds in the soil, irrespective of biodestructor application or decomposition period; this should be considered when planning crop rotations.