Technical hemp Cannabis sativa L. growth and functional diversity of soil microbiota in a model cultivation under elevated air temperatures

Global climate change on Earth and the resulting ecological, economic, and social risks are linked to the accumulation of greenhouse gases in the atmosphere, particularly CO2. The use of CO2 super-absorbing plants is considered an effective method for reducing (sequestering) CO2 levels in the air, with technical hemp (Cannabis sativa L.) recently attracting significant attention. This crop is characterized by rapid growth, resilience to environmental factors, low cultivation costs, and versatile biomass use as raw material for various industries. Currently, there are no programs or regulations in Russia for using technical hemp to sequester atmospheric CO2. When developing such programs, it is essential to consider the ability of plants to alter the composition and functional activity of soil microbiota, which can, in turn, affect plant growth rates, soil fertility indicators, and CO2 emissions from the soil cover. Additionally, it is important to determine whether the high efficiency of CO2 sequestration in the plant-soil-microorganism ecosystem will persist under global temperature increases and how soil conditions and its microbial components will change. To address these objectives, we conducted a model experiment in a greenhouse setting to evaluate the impact of temperatures (15 °С is typical for the growing season in central Russia; 20 °С and 30 °С are elevated temperatures) on the growth of technical hemp and chlorophyll content, carbon accumulation in the soil beneath plants, biodiversity, and functional activity of soil microbiota. This included assessing microorganisms' ability to decompose carbon-containing substrates using Biolog EcoPlates™. All measurements, except for soil carbon content, morphometric characteristics of hemp plants, and chlorophyll content, were carried out dynamically on 7th, 28th, 56th, and 98th days (average duration of the growing season in central Russia). Soil carbon content was determined on 1st and 98th days; chlorophyll content on 28th, 56th, and 98th days; and morphometric measurements were made on 98th day (end of the experiment). The research was conducted from March 14 to June 19 in a greenhouse at Kazan Federal University in 2023. The results showed that technical hemp exhibited maximum growth for both aboveground and underground parts at 30 °С, with plant height reaching 64.57 cm and root length 25.27 cm. In comparison to 20 °С where minimum values were observed (36.70 cm and 14.36 cm, respectively) growth at 30 °С was 76 % higher (p th day for all temperatures, with maximum values observed only on 56th day of the experiment. By the end of the experiment, chlorophyll levels decreased across all variants without statistically significant differences (p > 0.05). At the same temperature, samples with soil without hemp did not significantly differ from those with hemp concerning soil microbial biomass: at 15 °С, values were respectively 0.92-0.64 mg/kg and 0.96-0.62 mg/kg; at 20 °С 1.92-0.84 mg/kg and 1.74-1.03 mg/kg; at 30 °С 3.58-2.06 mg/kg and 3.08-2.24 mg/kg (p > 0.05). Increased temperatures led to higher microbial biomass both in the absence and presence of plants (3.58 mg/kg and 3.08 mg/kg on 7th day, respectively; p th day for samples without plants (0.49; 0.66; 0.54) and with plants (0.29; 0.64; 0.30), as well as on 98th day for samples without plants (0.13; 0.36; 0.14) and with plants (0.55; 1.17; 0.54), which did not show significant differences (p > 0.05). However, on 7th day an increase in microbial activity was noted in samples with plants, especially at temperatures of 20 °С and 30 °С (1.23; 0.89; p th day were established when assessing the alpha diversity (Shannon index) of soil microbial communities: the highest diversity was noted at 15 °С, 20 °С, and 30 °С both in samples without plants (2.31; 1.90; and 1.60) and with plants (2.58; 2.46; and 2.32; p th and 28th days. Samples incubated under plants at all three temperatures on 98th day formed a separate group, indicating greater uniformity in microbial communities. The contents of total organic carbon (TOC) and soil organic carbon (SOC) in the soil remained within error margins and did not depend on the presence of plants or temperature (p > 0.05). Thus, it has been shown that in hemp, with an increase in temperature during the growing season, the length of the above-ground and underground parts of the plant increases, which leads to an increase in the efficiency of atmospheric carbon sequestration. The amount of microbial biomass in the soil changes, and the metabolic activity of microorganisms increases already on the 7th day of the experiment. The presented model system can be used to assess the efficiency of sequestration and the state of functional groups of the soil microbial community when developing programs for growing industrial hemp (C . sativa L. ) as a superabsorbent plant, including at elevated temperatures, when it is important to maintain the diversity of soil microbial community.