Use of superabsorbent plants for urban greening as a tool to sequester atmosphere carbon

. The purpose of this work was to evaluate the possibility of increasing the CO 2 sequestration of hemp in the conditions of urban growth in central Russia by introducing non-traditional fertilizers from agricultural waste. For this, an experiment was carried out on the cultivation of industrial hemp, including the application of fertilizers (biochar, compost, a mixture of biochar and compost). Morphometric indicators, biomass, and chlorophyll content were measured. The absence of a significant effect of fertilizers on morphological (the length of the ground part, the length of the root, biomass) and physiological (chlorophyll content) characteristics was established. Based on the data of the “Register of Green Spaces”, the area of green spaces in Kazan was determined, which can be occupied by hemp (landscaping along roads, sanitary protection zones) – 630.8 hectares. It has been established that it is most expedient to plant hemp using biochar, since this method allows additional carbon sequestration and reduces CO 2 emissions from transport by up to 4% per growing season.CO 2 is a greenhouse gas with a minimal Global Warming Potential (GWP), however, its contribution to climate change is high due to the large mass of emissions. At the same time, at the moment there are technologies for capturing CO 2 . However, there is not enough information in the literature about the use of supersequestrator plants such as industrial hemp ( Cannabis sativa L., 1753) in large cities to reduce the carbon footprint, and there is practically no data on how sequestration can be increased through the use of various fertilizers, for example biochar.


Introduction
Soil is one of the largest carbon storage pools, with about 3 times the carbon stock of vegetation and 2 times the atmosphere [1].Therefore, even small changes in soil carbon stocks can have a significant impact on the content of greenhouse gases (carbon dioxide, methane, nitrous oxide) in the atmosphere and, as a result, make a significant contribution to climate change.The main source of carbon in the atmosphere is carbon dioxide (CO 2 ), the concentration of which is constantly increasing.Over the past 20 years, CO 2 emissions have grown 1.44 times, reaching their all-time high of 36.3Gt (Gigatonnes) in 2021 [2].
Climate change causes a lengthening of the period of droughts in some regions, an increase in the amount of heavy rainfall and floods in others, which in turn leads to a global reduction in crops by several times [3].Production of many foodstuffs will be reduced according to yield forecasts [4].Russia, according to the British Petroleum (London, Great Britain) statistical review of world energy 2022, ranks fourth in terms of CO 2 emissions in the world, accounting for 5.6% of the global volume [5].
Atmospheric carbon in the form of CO 2 can be captured by various methods: physicochemical (carbonization of minerals, reduction of CO 2 leakage during industrial processes, etc.) and biological (soil sequestration, phytosequestration) [6].The simplest and most accessible biotechnology for fixing atmospheric carbon is fixation by plants due to carbon fixation during photosynthesis and its accumulation in the form of biomass of shoots, roots, soil exudates, which leads to an increase in soil organic matter [4].The amount of CO 2 absorbed by plants depends on the phase of their growth; therefore, the essence of the technology is reduced to the regulation of planting density, time of sowing and harvesting of plants after the end of the active growth of their biomass, accompanied by carbon fixation.Fast-growing plants that actively grow biomass are able to fix more carbon than others.Such plants are classified as so-called.super absorbers.Examples include Acacia berlandieri Benth., 1842, Forestiera angustifolia Torr., 1858, Spanish moss L., 1762, C. sativa L., 1753 [7].The latter seems to be the most suitable species for landscaping, as it is undemanding to the type of soil, characterized by a rapid increase in biomass, and is used for the production of non-food products (hemp, fabrics, building blocks, paper, ropes, etc.).One hectare of industrial hemp per growing season can absorb up to 22 tons of CO 2 .A ton of technical hemp stalks contains 0.445 tons of carbon absorbed from the atmosphere, which corresponds to 1.63 tons of CO 2 .At the same time, in the roots of plants that remain in the soil and are transferred by the soil microbiome into soil organic matter, it retains about 0.084 tons of carbon per ton of harvested crop.Additionally, it is worth considering the prevention of CO 2 emissions by replacing traditional raw materials with hemp products.So, according to UK Hempcrete (Chesterfield, Great Britain) company statistics, hemp building blocks can fix about 110 kg of CO 2 per m 3 of wall, for comparison, the carbon footprint of concrete blocks during the construction of the same m 3 of wall is 200 kg of CO 2 .The construction sector accounts for 10% of total CO 2 emissions [8].Audi (Ingolstadt, Germany) introduced plastic instead of plastic in the manufacture of side panels in the A3 car model and reduced the weight of the car and fuel consumption, and also allowed carbon fixation [9].The developed root system of technical hemp helps protect slopes from erosion, which is important for roadside ravines and slopes.Hemp is also able to grow on soils contaminated with various toxic components, that enables it to grow in the roadside soils containing components for melting snow [10; 11; 12].
Taking into account the advantages of industrial hemp described above, the potential of its use for carbon dioxide absorption in specific areas should be assessed taking into account the growth of its biomass in given climatic and edaphic conditions.The potential assessment should include data on the volumes of anthropogenic carbon dioxide emissions in these areas.In the scientific literature, we have not found publications on the possibilities of using technical hemp to absorb carbon dioxide emissions in the cities of central Russia.

Materials and methods
The purpose of this work was to evaluate the efficiency of CO 2 uptake by industrial hemp plants, which can potentially be used for landscaping roadside lanes in the city of Kazan, located in central Russia.
To achieve the goal, the following tasks were set:  Conduct a growing experiment using the carbon dioxide superabsorbent plant C. sativa; E3S Web of Conferences 463, 02008 (2023) EESTE2023 https://doi.org/10.1051/e3sconf/202346302008 To reveal the influence of various types of fertilizers on morphometric parameters (stem length, root length), plant biomass and chlorophyll content;  Evaluate the possibilities of using C. sativa plants to reduce the carbon footprint in the conditions of a million-city.1) Hemp; For the study, we used hemp seeds of the Rodnik variety, included in the state register of breeding achievements approved for use in the territory of the Russian Federation [13].This variety is monoecious, the full ripening of seeds is early, it is used both for obtaining hemp fiber and seeds.Declared characteristics of the variety: stem of medium length, green, stem yield 110 kg/ha, technical maturity occurs in 90 days [14].
2) Biochar and compost; Bedding chicken manure with a sawdust content of at least 25% from the poultry farm LLC Chelninsky Broiler (Naberezhnye Chelny, Russia) served as a raw material for obtaining fertilizers.Biochar was obtained during the process of slow pyrolysis at the peak temperature 400 °С and retention time -3 hours.The total carbon content in the final biochar product was 56%.Compost was prepared from the chicken manure during 120 days.The second variant of the compost was prepared in a similar way with the addition of 10% biochar by weight when preparing the compost mixture.
3) Vegetation experiment; For the vegetation experiment, urbanozem was selected from the roadside area.Soil characteristic: total carbon content was 0.550 mg/kg, total nitrogen content was 0.01 mg/kg, phosphorus content was 701 mg/kg and potassium content -239 mg/kg.The vegetation experiment was carried out in a greenhouse at a constant temperature of 22±2 °С, soil moisture 60±5% for 90 days.Hemp plants were grown in 60-liter plastic containers, in which 40 kg of soil were placed.The soil was treated as follows: K -control soil without additives, B -soil with biochar in the amount of 10% by weight, C -soil with the addition of compost in the amount of 10% by weight, BC -soil with compost with 10% biochar by weight.
On the 7th, 14th, 28th and 35th, 60th day, the chlorophyll content was measured.Chlorophyll content was assessed using a portable Force-A chlorophyll meter (Dualex, France), based on the measurement of UV absorption by the leaf epidermis using double excitation of chlorophyll fluorescence [15].After the end of the vegetation experiment on a day 90, morphometric parameters (stem length, root length) and plant biomass were analyzed.

4) Statistics;
Each of the variants of the experiment was laid in three agility.The tables and figures show the mean values and standard deviations.

Results
A vegetative experiment was carried out to calculate carbon uptake by industrial hemp plants grown under different tillage methods.The content of chlorophyll in the leaves of industrial hemp was 18.7-32.7 µg/cm 2 (Figure 1).The content of chlorophyll is closely related to the rate of photosynthesis and the accumulation of organic matter [16].Thus, the more chlorophyll is contained in the leaf blade, the more efficient the process of carbon absorption from the atmosphere at the moment.At the end of the vegetation experiment, the morphometric parameters of hemp were evaluated.It was found that by the 90th day of vegetation, the height of the shoots was 189-207 cm, which corresponds to the characteristics of the "Rodnik" variety (Figure 2).The root length was 56-81 cm, the biomass of 6 plants in one variant was 223-305 g, the dry matter content was 40% (Figure 3).A significant difference in the length of the root, shoot and biomass in plants grown on poor soil -urbanozem (variant K), on urbanozem with fertilization (options C, B and CB) was not established.The results obtained correspond to the data presented in the literature on the undemandingness of hemp to a high supply of soil with micro-and macroelements, respectively, hemp can be grown on urban soils without adding additional micro-and macroelements to achieve high biomass [10; 17].At the next stage, the potential for the absorption of exhaust gases by the hemp plant was calculated.To do this, at the first stage, the area of green spaces along the roads (538.8 hectares, taking into account the length of the road-street network of 196 km and the width of the roadside part of 1.5 meters) and lawn areas of the city of Kazan (92 hectares) in the central part of Russia with the population 1.2 million people who could be planted with hemp instead of the usual landscaping option with a lawn [18].Thus, the area suitable for growing technical hemp in total is 630.8 hectares.The hemp plant is planted with a distance of 40-60 cm between plants.If 0.25 m 2 is planted per plant, then 40,000 plants can be sown per 1 ha.When taking into account these indicators and the plant biomass obtained in our experiment, it is possible to identify the amount of biomass that will be obtained using compost and biochar (Table 1) when sowing hemp in urban soil.The yield of hemp in terms of dry biomass on inhospitable areas of Kazan for different variants of fertilizer treatment will be 2,246, 2,763, 2,483 and 3080 tons of dry hemp biomass for options K, C, B and CB, respectively.To recalculate the amount of carbon dioxide that a hemp plant can fix during the growing season, the content of cellulose, hemicellulose and lignin in the plant was used.So one ton of dry hemp biomass fixes 1.5302 tons of CO 2 equivalent [19].Having the yield indicators of various hemp variants, it is possible to calculate how much CO 2 can be fixed in the territory of Kazan during the growing season.The results are presented in Table 1.Option K without fertilization allows to fix 3,662 t CO 2 equivalent during the growing season, and the highest value, 1.4 times more than the control, is characterized by the variant with the addition of compost and biochar CB and is 5,022 t CO 2 equivalent.
It has been established that from 1 hectare of urban soils, the hemp plant C. sativa of the Rodnik variety is able to fix 5.8, 7.1, 6.4 and 8.0 tons of carbon dioxide during the growing season with soil fertilization options K, C, B, CB, respectively.It is of interest to evaluate the potential of hemp in terms of a plant for landscaping grown in a city with a population of one million in central Russia.Table 1 shows that 3,662-5,022 tons of CO 2 will be captured when planting areas along the roads and in the lawn areas of the territory of Kazan in the amount of 630,8 hectares.The established levels of carbon capture values are comparable to those from the carbon footprint of urban road transport.So, as of January 2021, there were more than 545.8 thousand cars in Kazan [18] [20].In 12 months, a passenger car travels a total of 17,500 kilometers [21].Thus, a motorist drives an average of 51 km per day, while emitting 6.7 kg of CO 2 .Based on this, we can conclude that for 1 day in Kazan, emissions from cars amount to 3402 tons of CO 2 .
To reduce the carbon footprint of vehicles in the city, carbon can be sequestered using hemp plants.To do this, in options with the use of fertilizers, it is also necessary to take into account the carbon footprint of compost and biochar.According to Intergovernmental Panel on Climate Change (IPCC) 2006 Guidelines for National Greenhouse Gas Inventories Chapter 10, the carbon footprint of compost is zero [22].The carbon footprint of biochar consists of two components: production and burial of biochar in the soil, where it can fix carbon for up to 100 years.It is possible to produce biochar in cycles, using the heat released during the combustion of raw materials, so gas emissions are excluded.The carbon content of chicken manure biochar is 52%, which corresponds to 520 kg of carbon per 1 tonne of biochar.According to literature data, 68% of stable carbon remains in the soil for 100 years [23].Thus, when 1 ton of biochar is introduced into the soil, 535.5 kg of carbon will be sequestered.In our experiment, 2 variants with biochar are proposed: B -fertilizer with biochar in the amount of 10%, CB -fertilizer with compost, with the addition of biochar 10% by weight of the compost volume.The results of all calculations are presented in Table 2. has higher fertilizing properties and contains a large amount of trace elements.
It was found that it is most effective to use hemp with biochar fertilizer in the amount of 10%, since this method of growing hemp allows to increase the efficiency of carbon sequestration up to 4%.Thus, the hemp plant is an excellent alternative for landscaping large cities.This will make it possible to use infertile plots of land and reduce the city's carbon footprint.

Discussion
For hemp grown without fertilizers, the following quantitative indicators were obtainedroot and stem length, biomass, chlorophyll content.The values obtained are, in general, lower than those declared in the patent of this hemp variety and amount to 81-89%, with the exception of variants C and CB for which the value was 100-111% of the optimal [14].This is probably due to differences in growing conditions, since in our experiment urban depleted soil was used, and the optimal data for the patent were obtained on agricultural soils.It is shown that the content of chlorophyll in the leaves varies at different stages of plant development and does not differ statically for different types of fertilizers.So the maximum amount of chlorophyll is set at day 35 for all tillage options.No significant differences were found between the variants.Chlorophyll plays a critical role in photosynthesis due to its function in the transmission, distribution and conversion of light energy.Similar patterns of growth in the chlorophyll content at the flowering stage of plants are also given in the literature [24].The introduction of non-traditional fertilizers (biochar, compost and compost with biochar) led to an increase in crop yield by 11-37%.The greatest impact was exerted by the introduction of compost containing biochar, the smallest application of pure biochar.This is probably due to the fact that biochar compost has higher fertilizing properties and contains a large amount of trace elements.
To increase the biomass of hemp and due to this, the fixation of carbon dioxide associated with it, according to the literature, is similarly tried using different methods: irrigation, loosening and fertilization.Thus, the application of nitrogen fertilizers allows increasing the biomass of technical hemp by 37%, and fertilizer with compost from animal waste by 29% [25].
However, we found that the most effective to use hemp with biochar fertilizer in the amount of 10%, since this method of growing hemp allows to increase the efficiency of carbon sequestration up to 4%.
In Kazan, 630.8 ha of areas are available for planting hemp instead of lawns, on which, using various types of fertilizers, 2,246, 2,763, 2,483 and 3,080 tons of dry hemp biomass can be grown for options K, C, B and CB, respectively.However, when recalculating the potential use of hemp, it is also worth considering the contribution of fertilizer to the carbon footprint.It was revealed that the fertilizer option with biochar in the amount of 10% allows to increase the % of carbon dioxide fixation by 0.7-2.5%.The results obtained in the present study confirm James Vosper's conclusions that industrial hemp can be considered as a crop that will contribute to the country's achievement in reducing global carbon dioxide content in the atmosphere [19].However, in this paper, the author considers poor soils in Australia, and our study revealed that hemp is also suitable for central Russia and that it is possible to increase the proportion of carbon sequestration due to fertilizer.

Conclusion
This study shows that on urban soils in the conditions of central Russia, it is possible to cultivate hemp technical (which is a superabsorbent plant).The yield in this cultivation is 81-111% of the optimal, obtained in the conditions of agricultural production.The use of hemp for the purposes of planting greenery in the city (on the example of Kazan) will allow capturing 3662-5022 tons of CO 2 equivalent during the growing season.Additional sequestration can be achieved by using biochar as a fertilizer, in which case the total capture will be 14182 t CO 2 at an application rate of 30 t/ha.This will neutralize up to 4% of the carbon footprint of urban transport (on the example of Kazan) during the growing season.The prospects for further research within the framework of this work are to assess the use of hemp for the territory of the more northern regions of the country.It is also possible to re-evaluate crop yields within a city rather than a greenhouse experiment.

E3SFig. 1 .
Fig. 1.The content of chlorophyll in hemp plants when applying various types of fertilizers to the soil.Source: Compiled by the authors.

Fig. 2 .
Fig. 2. The effect of applying various types of fertilizers to the soil on the characteristics of a hemp plant.Source: Compiled by the authors.

Fig. 3 .
Fig. 3.The effect of applying various types of fertilizers to the soil on the biomass of a hemp plant.Source: Compiled by the authors.

Table 1 .
Indicators of yield, dry biomass and CO 2 fixation in the territory of poor soils in the city of Kazan.
Source: Compiled by the authors.
№443/2009 of the European parliament and of the council of 23 April 2009 , the carbon dioxide emission from the fuel used of one car is about 130 g per kilometer

Table 2 .
Indicators of emissions and fixation of CO 2 for the growing season of hemp in 90 days for the city of Kazan.