Monitoring carbon dioxide fluxes during fallow land conversion in the Subtaiga zone

. The work is devoted to the study of the temporal variability of CO 2 fluxes on fallow lands during their introduction into agricultural turnover. The aim was to determine the CO 2 emissions of soil from fallow lands during their introduction into agricultural turnover by different technologies under the conditions of the subtaiga natural-climatic zone of the southern part of western Siberia. In 2022, scientific research was carried out on key plots of fallow lands in the subtaiga zone of the Omsk region. The soil of the fallow land plot is thick grey forest loamy (Luvic Greyzemic Phaeozems). It was established that technologies of fallow lands introduction into turnover have a significant impact on CO 2 emission. Compared to the control, it has increased by 1.44 times in the subtaiga zone under agrotechnological technology (soil treatment) and by 1.27 times under combined technology (soil treatment + herbicide treatment). The materials are the basis for information-methodological support for greenhouse gas monitoring in the fallow lands of the territories of the south of western Siberia of Russia.


Introduction
Currently, one of the main environmental problems is the emission of greenhouse gases into the atmosphere from various human activities. Fallow land is a significant reservoir for the accumulation of greenhouse gases. But when they are ploughed up (put into agricultural rotation), the soil emission of CO2 increases according to numerous data, which requires a detailed study. Considering the high degree of uncertainty of estimates of biogenic greenhouse gas emissions [1][2][3][4][5], a monitoring system should also be developed to evaluate various fallow lands depending on the natural zone, soil cover and relief, as well as technologies for their introduction into circulation. This will make it possible to refine the overall estimates of the contribution of fallow lands to carbon deposition and balance.
A monitoring system developed to assess different fallow lands and technologies for their conversion into different natural zones will make it possible to make decisions on the management of carbon sequestration and balance.
The purpose of the research is to determine soil CO2 emissions from fallow lands when they are brought into agricultural turnover by different technologies in conditions of the subtaiga natural-climatic zone of the south of Western Siberia.

Materials and methods
To study the impact of fallow land introduction technologies on greenhouse gas emissions, field works were carried out in Tarsky district of the Omsk region. Soil of the fallow land plot is gray forest thick loamy (Luvic Greyzemic Phaeozems). Field experiments on study of different technologies of fallow lands introduction into turnover were conducted, works on technologies implementation were carried out. Experiments were laid on the plot with fallow lands age of 5-10 years. The first technology provides only agrotechnical treatments (agrotechnical technology), the second -with chemical treatments with herbicide (combined technology).
The agronomic technology in each experimental plot is adapted to the study area. The area of the experimental plots was 200 square metres.
Technology 2. The main tillage: ploughing with plough PLN 3-35 at a depth of 20-22 cm, followed by discing with BDT-3 harrow in two tracks at a depth of 10 -12 cm + chemical treatment with herbicide "Glyphosate" at a dose of 2 litres per hectare by preparation + the third treatment: chemical treatment with herbicide "Glyphosate" at a dose of 2 litres per hectare by preparation.
The first treatment was carried out on 9.06, the second on 11.07 and the third on 21.09.2022.
Carbon dioxide emissions were determined monthly at 10-15-day intervals from May to October. Plastic devices-insulators consisting of a base 10 cm high and a cylindrical vessel with a hole for soil air intake 20 cm high and 23.5 cm in diameter were used as chambers.
Repeatability was three times. The bases were incised into the soil to a depth of 6 cm before determining respiration rate. The plants were cut at soil level. Thus, the carbon flux determined was the total respiration of soil microflora, roots, etc. excluding the respiration of the above-ground tier of phytomass.
Gas sampling for chamber CO2 content was carried out with a soil chamber air sampler in hermetically sealed, vaccumulated glass vials (30 ml volume) overnight (24 hours) at three-hour intervals. Exposure time was 5, 10, 30 minutes with control samples taken after 5, 10, 30 minutes. Gas samples were analysed using a Crystal 5000.2 gas chromatograph.
Soil temperature, soil moisture (every 10 cm up to 100 cm), air temperature and nitrogen, phosphorus and potassium content (0-20 cm) were determined at the sites selected for CO2 emission studies during the entire measurement period in parallel with gas sampling. Soil temperature was determined with soil thermometers, soil moisture -by weight method, content of macronutrients by standard methods appropriate for soil type.
Soil CO2 release rate was calculated according to the formula (Panikov and Gorbenko, 1992; Larionova et al, 1998) [6][7]: here VI CO2 is СО2 emission, mg C/m 2 /hour; C2 and C1 are the final and initial СО2 concentrations inside the insulator, mg C/m 3 ; H -height of the insulator above the soil surface, m; t -exposure time, hour.
The result of the total respiration measurement must characterise the respiration of the soil (roots and micro-organisms), whereby the contribution of the above-ground phytomass, which also breathes under dark chamber conditions, must be minimised. The phytomass inside the base was therefore cut off with scissors to a level of 3 cm each time just before starting the measurement. No more than one hour passed between cutting the phytomass and measuring the flow.

Results
СО2 soil emission is a dynamic indicator that varies greatly depending on the climatic zone, time of year (season) of the day, agricultural technologies [8][9][10][11][12].
Daily dynamics of СО2 soil flows. Daily dynamics of soil CO2 emission was measured in each fallow plot where an inoculation technology (tillage or chemical herbicide treatment) was applied. The age of these fallow lands was 5-10 years.
The most pronounced daily dynamics of soil CO2 emission was in the fallow land at the introduction into the turnover with the use of agricultural technology in July and varied from 132.7 g to 175.9 g CO2/m -2 • day -1 ), in Junefrom 83.1 g to 115.5 g СО2/m -2 • day -1 ); on fallow lands at application of combined technology: in July from 107.1 to 155.5 g and in June from 83.0 to 104.7 g СО2/m -2 • day -1 at highly contrasting night and day indicators of air and soil temperatures with higher intensity of soil CO2 fluxes in morning and day hours and lower -in evening and night hours. The range of soil CO2 fluxes during the day narrowed to some extent in August and September (Figure 1). The reason for this is the seasonal variability of air temperature, temperature and soil moisture [13][14][15][16][17][18][19][20]. The agro-technical technology of putting fallow lands into circulation contributed to greater CO2 emission than the combined one. At the same time, the differences in fluxes intensity b y September become significantly less.

Conclusion
Technologies of introduction of fallow lands into turnover have a significant impact on CO2 emission. Compared to the control, it increased in agrotechnological technology (soil tillage) from 70.6 g to 101.5 g CO2 / m -2 -day -1 , in combined technology (soil tillage + herbicide treatment) to 89.8 CO2 / m -2 -day -1 (in average for the period after the beginning of the tillage). At the same time, the maximum increase in flux was observed in the first weeks after the first treatment. Compared to the control, it increased by 1.44 times in the subtaiga zone under agrotechnical technology (tillage) and by 1.27 times under combined (tillage + herbicide treatment).

Compliance with ethical standards
Sources of funding for each author and the whole team of authors: The work was supported by the Russian Science Foundation (Project No. 22-17-20049).
Conflict of Interest: The authors declare that they have no conflict of interest. This article does not contain any research involving the use of animals as subjects. This article does not contain any research involving humans as research subjects.