Estimation of greenhouse gas emissions during the cultivation of industrial hemp ( Cannabis sativa L.) in different temperature regimes

. The emission of carbon dioxide from the soil is an urgent problem, as it not only contributes to climate change, but also affects the reduction of the carbon pool in the soil, which negatively affects its fertility. Cultivation of crops with fast growth, high biomass and atmospheric carbon fixation capacity is effective in terms of reducing soil carbon dioxide emissions and organic carbon storage. The purpose of this work was to evaluate greenhouse gas emissions from the cultivation of industrial hemp ( Cannabis sativa L. , 1753) in different temperature regimes – 15ºС, 20ºС and 30ºС. Soil without plants was used as a control. The respiration activity of the soil microbial community was assessed on the 1 st , 7 th , 14 th , 28 th , 42 nd , 56 th , 70 th , 84 th and 98 th days of incubation. Biomass of plants was measured on the 98 th day. It was established that there were no differences in the respiration activity of the control soil and the soil under industrial hemp ( Cannabis sativa L., 1753) at the beginning of the experiment, however, from the 14th day, the respiration activity was significantly lower in the soil without plants. The influence of the temperature regime on respiration activity was established after 56 days of the experiment. The influence of the temperature regime of vegetation on the morphometric parameters of hemp has been established: the largest biomass of plants was found at a colder incubation temperature (15ºС). It has been established that the emission of greenhouse gases from the soil during the cultivation of industrial hemp ( Cannabis sativa L., 1753) is higher compared to pure soil and depends on the temperature regime.


Introduction
The growing amount of greenhouse gas emissions and, as a result, global climate change, is one of the most important problems of modern mankind [1].According to the Paris Agreement, by 2030 Russia must reduce greenhouse gas emissions by 30% from the 1990 level.The agricultural sector accounts for a significant share of total greenhouse gas emissions [2].
Carbon dioxide is released from the soil through the respiration of soil microorganisms as a result of the decomposition of organic matter.The respiration of soil microorganisms, in turn, is regulated by factors such as ambient temperature and humidity [3].The use of large amounts of organic fertilizers also contributes to carbon dioxide emissions from agricultural soils.In addition, it is necessary to take into account the carbon footprint of machinery as a result of agricultural mechanization [4].In order to reduce the carbon footprint of the agrarian sector, it is proposed to use measures to improve the condition of the soil, optimize the use of fertilizers and manure, increase animal productivity, etc.In addition, it appears to be effective in terms of influencing carbon balance by growing ground cover crops with minimal use of farming techniques [5].
A separate problem associated with greenhouse gas emissions from agricultural soils is the reduction of soil carbon stocks, which adversely affects their structure and fertility.Therefore, reducing carbon dioxide emissions through soil carbon sequestration is of paramount importance [1].Soil carbon sequestration can be divided into two categories: non-biological (due to physical and chemical reactions) and biological (carbon capture and storage through natural processes).Biological carbon sequestration is more economical and is the sequestration of atmospheric carbon through plants and its subsequent conversion into soil organic carbon.Inorganic carbon accumulates in the soil during the decomposition of plant biomass as a result of the formation of carbonates and bicarbonates of calcium, magnesium, potassium and sodium [6].
One of the known plants that contribute to the accumulation of soil carbon is industrial hemp (Cannabis sativa L., 1753).Cultivation of hemp has a positive effect on the concentration of CO 2 in the atmosphere, since it is a ΔCO 2 negative crop, as it accumulates more CO 2 from the environment than it emits.In addition, hemp is a known effective soil carbon sequestering plant due to the large biomass of roots remaining in the soil [6; 16].Many agricultural plants, especially trees, have a high ability to capture atmospheric carbon, but hemp has a faster growth compared to other crops [9].Industrial hemp (Cannabis sativa L., 1753) has a high ability to grow under different climatic conditions, but it is well known that the emission of carbon dioxide from the soil, as well as the functioning of the microbial community that forms soil respiration, are directly dependent on temperature and humidity [2; 14].
The Republic of Tatarstan is one of the leading agricultural regions of Russia with a large area of arable soils, which makes it relevant to study the mechanisms of carbon emission and sequestration in order to reduce the carbon footprint and increase the pool of soil carbon.

Materials and methods
The purpose of this work was to evaluate greenhouse gas emissions during the cultivation of industrial hemp (Cannabis sativa L., 1753) in different temperature conditions -15ºС, 20ºС and 30ºС.For this, the following tasks were set: to analyze the respiration activity of the microbial community of the control soil and the soil sown with industrial hemp (Cannabis sativa L., 1753); to evaluate the biomass of industrial hemp (Cannabis sativa L., 1753); to evaluate the balance of carbon dioxide in the arable layer of soil sown with industrial hemp (Cannabis sativa L., 1753) during the vegetation season.Gray forest soil was used for the experiment.As a carbon sequestering plant, seeds of industrial hemp (Cannabis sativa L., 1753), were sown into the soil, previously germinated to the sprout stage on vermiculite.Two variants were prepared -soil without plants (control) and soil sown with hemp.Each box contained 40 kg of soil and 3 plants of industrial hemp (Cannabis sativa L., 1753).Incubation was carried out for 98 days at different temperature conditions -15ºС, 20ºС and 30ºС.For each temperature regime, 3 repetitions of the control soil and the soil sown with industrial hemp (Cannabis sativa L., 1753).were prepared.As a result, the following variants were obtained -K15, K20, K30 (control soil) and P15, P20, P30 (soil under industrial hemp (Cannabis sativa L., 1753)).
The respiration activity of the soil microbial community was assessed on the 1 st , 7 th , 14 th , 28 th , 42 nd , 56 th , 70 th , 84 th and 98 th days of incubation according to ISO 16072:2002 using gas chromatography [10].Biomass of industrial hemp (Cannabis sativa L., 1753) was measured on the 98 th day.The stem was cut at the point of contact with the soil surface and dried at a temperature of 20°C.After 10 days, the dried biomass was weighed.
All measurements were carried out at least three times.Statistical processing of the obtained results was carried out using Microsoft Office Excel 2010 (USA).All graphical data contain means and standard errors.To assess the significance of differences, the Fisher test was used at α = 0.05.

Results
The results of the assessment of the respiration activity of the soil without plants under different temperature conditions are shown in Figure 1.On the 1 st day, the level of respiration activity was 0.000008, 0.000011 and 0.000010 mg C-CO 2 *g*h -1 in the soil at temperatures of 15ºС, 20ºС and 30ºС, respectively.Further, from the 7 th to 70 th days, in soil samples, respiration activity varied from 0.000004 to 0.000014 mg C-CO 2 *g*h -1 .In general, no effect of temperature on the level of respiration activity in the soil without plants was revealed.On the 84 th and 98 th days of incubation, the level of respiration activity in soil samples decreased and amounted to 0.000001-0.000008mg C-CO 2 *g*h -1 .Further, the emission of carbon dioxide from the soil under industrial hemp (Cannabis sativa L., 1753) was evaluated under different temperature conditions.The results of the assessment of respiration activity are shown in Figure 2. On the 1 st day of incubation, the level of respiration activity in the soil under industrial hemp (Cannabis sativa L., 1753) was 0.000008-0.000011mg C-CO 2 *g*h -1 at temperatures of 15ºС, 20ºС and 30ºС, respectively.Up to the 42 nd day, no effect of temperature on respiration activity was observed.Starting from the 56 th day, there was a significant increase in respiration activity by 2-3.1 times compared with the 42 nd day.Moreover, from the 56 th to 98 th days, differences were found in the level of respiration activity of soil samples incubated at different temperature E3S Web of Conferences 463, 02005 (2023) EESTE2023 https://doi.org/10.1051/e3sconf/202346302005conditions.At higher temperatures (20ºС and 30ºС), respiration activity was 1.1-1.5 times higher than at lower temperatures (15ºС).By the end of incubation, a gradual decrease in the level of respiration activity was observed, and on the 98 th day, its values were 0.000040, 0.000049, and 0.000053 mg C-CO 2 *g*h -1 at temperatures of 15ºC, 20ºC, and 30ºC, respectively.Further, carbon dioxide emission (ΔСО 2 ) from the soil under industrial hemp (Cannabis sativa L., 1753) was calculated for the entire vegetation season (98 days) at various incubation temperature regimes (15ºС, 20ºС and 30ºС).The carbon balance is the difference between the amount of carbon released from the soil (respiration activity, emissions from agricultural practices, emissions from nitrogen fertilization) and the amount of carbon accumulated in the soil in the form of plant biomass and organic fertilizers [1].In this study, the soil was not additionally treated with organic fertilizers, so the following formula was used for calculation (1): ΔСО 2 -carbon balance, RА -soil respiration activity, СО 2 agrotechnics -carbon emissions from agricultural fuel use, С biomass -plant biomass carbon.
Table 1 shows carbon dioxide emissions from various agricultural practices used in the cultivation of industrial hemp (Cannabis sativa L., 1753).Thus, during the vegetation season, the total carbon dioxide emissions during agrotechnical work amount to 399.7 kg*ha -1 (and do not depend on temperature conditions).Source: [11].
To estimate the carbon dioxide emission from the soil for the entire vegetation season (ΔСО 2 ), the area under the curve denoting the respiration activity of the soil was calculated, estimated at the 1 st , 7 th , 14 th , 28 th , 42 nd , 56 th , 70 th , 84 th and 98 th days of incubation.The calculation took into account the depth of the arable soil layer, equal to 10 cm, and the density of the soil, equal to 1.2 g*cm 3 .The results are presented in Table 2.The smallest amount of carbon dioxide was released from the soil sown with industrial hemp (Cannabis sativa L., 1753) at a temperature of 15ºС.At higher temperatures (20ºC and 30ºC) carbon dioxide emissions were higher.Next, the amount of carbon accumulated in the biomass of industrial hemp (Cannabis sativa L., 1753) was calculated.It is known that 1 ton of hemp contains 445 kg of carbon, and the planting density of hemp for the middle lane is 4.8 million seeds per 1 hectare of soil [9].As a result, the carbon content in hemp biomass was 42972.76kg*ha -1 at an E3S Web of Conferences 463, 02005 (2023) EESTE2023 https://doi.org/10.1051/e3sconf/202346302005incubation temperature of 15°C, 37102.32kg*ha -1 at an incubation temperature of 20°C and 31534.48kg*ha -1 at an incubation temperature of 30°C.
Thus, Table 3 presents data on the balance of carbon dioxide in the topsoil layer of soil under industrial hemp (Cannabis sativa L., 1753) for the entire vegetation season (98 days) at various temperature regimes of incubation (15ºС, 20ºС and 30ºС).According to the data obtained, soil planted with industrial hemp (Cannabis sativa L., 1753) releases less carbon dioxide at a colder incubation temperature and contributes more to the conservation of the organic carbon pool due to higher biomass.Thus, the soil planted with industrial hemp (Cannabis sativa L., 1753) had higher values of respiratory activity than the control soil without plants.Different temperature regimes of incubation did not affect the level of respiratory activity of the control soil, while for the soil under industrial hemp (Cannabis sativa L., 1753) an increase in respiratory activity was noted at higher temperatures.Industrial hemp (Cannabis sativa L., 1753) had the highest biomass at a colder incubation temperature.When assessing the balance of carbon dioxide, it was found that in the soil under industrial hemp (Cannabis sativa L., 1753) carbon dioxide emission is lower at a colder incubation temperature.

Discussion
Soil respiration activity, shaped by the microbial community, is highly dependent on soil moisture content, soil temperature, nutrient availability, and pH value [12].An increase in soil temperature leads to an increase in the rate of soil respiration as a result of stimulation of microbial metabolism.Soil moisture also controls microbial activity as it affects the amount of oxygen in soil pores [13].
When assessing the respiration activity in the control soil without plants, no influence of different temperatures was revealed.The detected fluctuations in the level of respiration activity are probably associated with the difference in the life cycles of the microbial community of the soil, depending on the temperature regime [5].The decrease in respiration activity in soil samples by the end of incubation is most likely due to the fact that the content of nutrients for microorganisms gradually decreased in the soil, which resulted in a decrease in their metabolic activity [14].
The respiration activity in the soil planted with industrial hemp (Cannabis sativa L.,1753) was higher than in the control soil.This is due to the fact that plant roots secrete exudates into the soil, which are a source of nutrients for soil microorganisms, which is expressed in an increase in the metabolic activity of the microbial community and an increase in the release of carbon dioxide from the soil [13,15].In its composition, root exudates contain sugars, protein and organic amino acids [17].The noted decrease in respiration activity by the end of the vegetation season (98 th day) is most likely due to the fact that over time, soil microorganisms consumed the available amount of nutrients released by the roots [14].
When assessing the influence of different temperature regimes, an increase in the biomass of industrial hemp (Cannabis sativa L.,1753) plants grown at a colder temperature E3S Web of Conferences 463, 02005 (2023) EESTE2023 https://doi.org/10.1051/e3sconf/202346302005 was noted.It is likely that higher incubation temperatures caused an increase in carbon dioxide concentration, which in turn could affect plant physiology and biomass due to a decrease in the concentration of mineral nutrients in plant tissues [3,17,19].It is reported that the decrease in the concentration of nutrient elements (N, P, K, S, Fe, Mg, Zn) can range from 5 to 25% depending on the element itself, carbon dioxide concentration and time [20].This decrease in nutrient content in plants can have a negative impact on the stability of soil organic matter and soil biogeochemical processes [21].
An assessment of the carbon dioxide balance showed that when growing industrial hemp (Cannabis sativa L.,1753) at a colder temperature, less carbon dioxide is released from the soil.In addition, higher plant biomass brings more carbon into the soil.Thus, in our study, it was shown that the use of industrial hemp (Cannabis sativa L.,1753) is effective in terms of reducing the contribution of carbon dioxide emissions from agricultural soils to the greenhouse effect, as well as its positive effect on soil carbon accumulation.

Conclusion
In this work, an assessment of greenhouse gas emissions was carried out during the cultivation of industrial hemp (Cannabis sativa L., 1753) in different temperature conditions (15ºС, 20ºС and 30ºС).According to the tasks, it was obtained that compared to the control soil without plants, the soil planted with industrial hemp (Cannabis sativa L., 1753) has a higher level of respiration activity due to the positive effect of root exudates on the metabolic activity of the soil microbial community.The influence of different temperature regimes on the respiration activity of the soil under industrial hemp (Cannabis sativa L., 1753) was found, namely, its level was higher at a higher incubation temperature.The biomass of industrial hemp plants (Cannabis sativa L., 1753) was characterized by higher values at colder incubation temperatures.When assessing the balance of carbon dioxide, it was shown that at colder temperatures industrial hemp (Cannabis sativa L., 1753) is an effective sequestering plant, allowing to reduce the amount of carbon dioxide emissions from the soil and increase the content of organic carbon in the soil due to higher biomass.The results obtained are of practical importance, since they describe the dependence of the balance of carbon dioxide in the soil under industrial hemp (Cannabis sativa L., 1753) on the respiratory activity and biomass of plants under different temperature conditions and can be used in agriculture to reduce the carbon footprint and restore the soil carbon pool.In the future, it is planned to supplement this study with results on the effect on carbon dioxide emission from the soil under industrial hemp (Cannabis sativa L., 1753) under various factors (pesticide application, soil contamination with metals and antibiotics).

Fig. 2 .
Fig. 2. Respiration activity of the soil microbial community under industrial hemp (Cannabis sativa L., 1753) under different temperature conditions (K15 -soil under industrial hemp (Cannabis sativa L., 1753) incubated at 15ºС, K20 -soil under industrial hemp (Cannabis sativa L., 1753) incubated at 20ºС, K30 -soil under industrial hemp (Cannabis sativa L., 1753), incubated at 30°C).Source: Compiled by the authors.At the next stage, an assessment was made of the effect of different temperature regimes on the biomass of industrial hemp (Cannabis sativa L., 1753), measured at the end of the vegetation season (98 th day).The evaluation results are shown in Figure3.The highest biomass (20.1 g) was found for industrial hemp (Cannabis sativa L., 1753) grown at a lower temperature of 15ºС.At higher incubation temperatures (20°C and 30°C), the biomass of industrial hemp (Cannabis sativa L., 1753) was lower than at 15°C and amounted to 17.4 and 14.8 g, respectively.

Table 1 .
CO 2 emissions from various types of agrotechnical activities.

Table 2 .
Emission of carbon dioxide from the topsoil during the vegetation season.

Emission of carbon dioxide from the soil during the vegetation season, mg CO 2* g -1 Emission of carbon dioxide from the topsoil during the vegetation season, kg
Source: Compiled by the authors.

Table 3 .
Balance of carbon dioxide in the topsoil layer of soil sown with industrial hemp (Cannabis sativa L., 1753) during the vegetation season.
Source: Compiled by the authors.