CO₂ emissions and their changes with H₂O emissions, soil moisture, and temperature during the wetting–drying process of the soil mixed with different biochar materials

Agriculture is responsible for a quarter of global warming (Bennetzen et al. 2016). Soil can act as a reserve for organic carbon as well as a source of CO₂ emissions from the soil. Under faulty soil management conditions, the soil loses organic carbon and CO₂ is thus emitted into the atmosphere (Yerli et al. 2019). In this case, the increased CO₂ in the atmosphere absorbs the heat and causes global warming. Climate change, which is the effect of global warming, is regarded as the most worrying issue today (WMO 2019). In this context, studies investigating the effects of CO₂ emissions and thus global warming on agricultural activities are noteworthy.

Biochar can be defined as a charred organic improver as a result of the pyrolysis of biomass under oxygen-limited conditions at elevated temperatures. With its low bulk density, high porosity and surface area properties, biochar increases water retention in the soil and enables more profitable management of irrigation in today's conditions, in which effective management of water resources is very important (Tufenkci et al. 2022). Biochar both facilitates the crop's access to water and contributes to irrigation water savings by keeping soil moisture easier and preserving it for a long time, not only inside the pores but also between the particles due to its micropores (Cakmakci & Sahin 2022).

Biochar increases soil and crop productivity due to its low decomposition rate and long-term persistence in the soil (Tufenkci et al. 2022). The bulk density of biochar applied soil may decrease, and the soil porosity and soil hydraulic properties may improve (Ahmad Bhat et al. 2022). Thus, more effective irrigation management can be achieved. The biochar applied to the soil is effective in developing and improving the physical (Mukherjee & Lal 2013; Blanco-Canqui 2017; Zhang et al. 2020), chemical (Chintala et al. 2014; Rechberger et al. 2017; Jing et al. 2020), and biological (Luo et al. 2013; Dong et al. 2015; Somerville et al. 2020) properties of soil. In this context, it also provides positive reflections on the increase in the yield and quality of the crop (Zoghi et al. 2019). These contributions of biochar allow more economical and sustainable agricultural management by decreasing the need for synthetic fertilizers. Laghari et al. (2015) found that biochar increased phosphorus up to 70% and potassium up to 11% in the soil. Similarly, Apori et al. (2021) stated that nitrogen increased by 36% and phosphorus increased by 17% with biochar application. In addition, biochar plays an active role in biotic and abiotic stress conditions, increasing the resistance of the crop against stress conditions (Kul et al. 2021).

In addition to these positive reflections, considering biochar can increase the carbon storage capacity of the soil and stabilize the soil's organic carbon pools (Wang et al. 2014). As the effect of global warming is increasing day by day, biochar can also decrease the CO₂ emissions from agricultural soils (Deng et al. 2017). Thus, it can contribute to the increase in soil and crop yield by providing organic carbon increases and sequestration in the soil, and agricultural and environmental sustainability can be achieved by decreasing the emission values. However, considering that CO₂ emissions from soils can vary with soil moisture and soil temperature (Mancinelli et al. 2015; Yerli & Sahin 2021) and that the physical, chemical, and biological properties of soils can directly affect CO₂ emissions (Haddaway et al. 2017), it is possible to manage CO₂ emissions from soils effectively only with a good understanding of the CO₂ emissions relationships (Yerli et al. 2022a). Increasing moisture and temperature in the soil can trigger microorganism activities and increase CO₂ emissions by oxidizing organic carbon. After organic carbon encounters O₂, it turns into CO₂ and spreads from the soil to the atmosphere (Yerli et al. 2019). Zhao et al. (2020) reported a positive correlation between CO₂ emissions and soil temperature, and Buragiene et al. (2019) reported a positive correlation between CO₂ emissions and soil moisture. However, this is not always the case. Because situations such as water ingress into the soil increase the soil moisture and decrease the soil temperature by creating a cooling effect on the soil (Mancinelli et al. 2015), microorganism activities and thus oxidation can be effective between certain soil moisture and temperature values, and the increase in soil temperature due to a chemical reaction that occurs as a result of the oxidation of organic carbon additives mixed into the soil (Yerli & Sahin 2021) makes it difficult to understand and manage the CO₂ emissions from the soil. Yerli et al. (2022b) determined that CO₂ emissions from the soil showed positive correlations with H₂O emissions and soil moisture at 5-, 10-, and 20-cm soil depths, but the correlation with soil temperature was negative at the same soil depths.

Many studies have shown that biochar decreases CO₂ emissions from the soil (Liu et al. 2011; Wang et al. 2014; Zhang et al. 2017; Wu et al. 2018; Azeem et al. 2019; Akinyemi & Adesina 2020; Lehmann et al. 2021). Lehmann (2007) reported that biochar absorbs organic carbon and stabilizes soil carbon stocks due to its high surface area and porosity, thus decreasing CO₂ emissions. However, CO₂ emissions may also increase depending on the raw material content of biochar and preparation processes, such as stability, pyrolysis, and temperature, as well as the characteristics of the soil in which the biochar is mixed and its interaction with the soil (Zimmerman et al. 2011; Singh et al. 2012). In a study examining the effects of pine wood and grass-derived biochar applications in loamy soil on CO₂ emissions, it was determined that pine wood biochar did not affect CO₂ emissions, while grass-derived biochar increased CO₂ emissions (Hilscher et al. 2009). Karhu et al. (2011) reported that birch biochar mixed with soil has a negative effect on decreasing CO₂ emissions. Mohamed et al. (2015) evaluated the effects of woody waste mixed with sandy soil and its biochar on CO₂ emissions, and they found that both applications increased CO₂ emissions, but this increase was 3 times higher than biochar in the woody waste application and 6 times more than the control.

The insight shows that the mechanism of CO₂ emissions from the soil has a complex structure and that the effect of biochar on decreasing emissions varies. The studies have gained momentum to evaluate the effects of biochar applications on decreasing CO₂ emissions. However, there are no studies in the literature that compare the effects of different biochar sources on CO₂ emissions during the wetting–drying process of the soil and evaluate the soil moisture, H₂O emissions, and soil temperature that affect CO₂ emissions. Therefore, the objectives of this study were (i) to determine whether biochar applications would decrease the CO₂ emissions from the soil in the wetting–drying process, (ii) to evaluate the changes in CO₂ emissions depending on the raw material contents of the biochar, and (iii) to discuss the relationships of CO₂ emissions with the soil moisture, H₂O emissions, and soil temperature measured during the wetting–drying process. To achieve these objectives, this study investigated CO₂ emissions during the wetting–drying process of soil including biochars with different raw materials, and evaluated the relationships of CO₂ emissions with soil moisture and temperature and H₂O emissions.

The experiment was carried out in the laboratory of Van Yuzuncu Yıl University, Faculty of Agriculture, Department of Biosystem Engineering. During the experiment, the mean air temperature was 24 ± 2 °C and the air humidity was 38 ± 5%. The experiment was conducted with three replications using a completely randomized factorial design with hazelnut, straw, apple, and sawdust biochar materials and as a control in the soil in which biochar was not applied. Each wetting–drying cycle was completed in 1 week and repeated three times in total for 3 weeks.

Prior to the experiment, the particle size distribution (texture), pH, EC, total N, organic matter and organic carbon contents of the experimental soil were analyzed (Table 1). The particle size distribution was determined by the Bouyoucos hydrometer method (Gee & Bauder 1986), and the soil texture was sandy loam (sand: 66.3%, silt: 15.6%, clay: 18.1%) according to the USDA classification. According to the pH and EC determined by pH-meter and EC-meter in 1:2.5 saturation extract, the experimental soil was very slightly alkaline and nonsaline. The total N determined by the Kjeldahl method (Bremner & Mulvaney 1982) was insufficient, and the organic matter content determined by the Walkley-Black method (Nelson & Sommers 1982) and the organic carbon calculated from the organic matter content (Avramidis et al. 2015) were low.

All biochar materials were prepared using the same procedures. First, the materials were dried, sieved, and homogenized, and finally, subjected to pyrolysis at 400 °C. Prior to the experiment, analyses and calculations were made to determine the pH, EC, total N, organic matter, and organic carbon contents of the biochar materials in the same way as the analysis and calculations of the soil (Table 1).

Air-dried soil, sieved through a 2-mm sieve, was mixed with 1% biochar on a weight basis and tapped into pots with volumes of 1.5 l (diameter: 13 cm, height: 11 cm) by preserving the soil bulk density (1.31 Mg m⁻³). All pots were incubated for two weeks at moisture level at field capacity. In the planning of irrigation events, the dry weights of control application pots with the soil were determined. To determine the water retained at field capacity (pot capacity), the pots without biochar (control) were first saturated with water and then surface covered to prevent evaporation. When the drainage completely stopped, the pots were weighed, and this moisture after conversion by bulk density, wet and dry weights was expressed as field capacity as the volume (0.320 m³ m⁻³). The irrigations were applied with the same water amounts in all applications to complete missing moisture to the field capacity considering the weights in control pots when each wetting–drying process was completed.

The CO₂ emission from the soil was measured daily during three wetting–drying periods with the EGM-5 infrared gas analyzer device (CFX-2, PPSystems, Stotfold, UK) by taking three readings from each pot. Soil moisture, H₂O emissions, and soil temperature were also measured simultaneously with CO₂ emissions. Soil moisture was determined by weighing the pots, while a CO₂ measuring device was used for H₂O emission and soil temperature measurements. The H₂O emission was automatically recorded by a CO₂ measuring device during CO₂ measurement. The STP-1 temperature probe connected to the CO₂ emission device was used to measure soil temperature at a 5-cm soil depth (Yerli & Sahin 2021; Yerli et al. 2022a, 2022b).

Data analyses were performed using the General Linear Model in the SPSS program (Ver. 23). Duncan multiple comparison test at the 5% level of probability was used to compare significant means. Pearson correlation analysis was used to determine the relationships of CO₂ emissions with soil moisture, H₂O emissions, and soil temperature.

The decrease in H₂O emissions in parallel with the increase in moisture retention in biochar applications can be explained by the porous structure of biochar. Biochar retains moisture not only inside the pores but also between the particles and micropores, so it takes a longer time for the biochar applied soil to lose water and dry out (Cakmakci & Sahin 2022). Ahmad Bhat et al. (2022) stated that the improved hydraulic properties of the soil with biochar increased moisture retention in the soil. Ahmed et al. (2019) stated that biochar application improved the soil water holding capacity, thus preserving soil moisture and decreasing H₂O emissions, and reported that this contribution was due to the spongy structure of the biochar. Chen et al. (2010) also indicated that biochar application increases water storage in pores by improving soil aggregation and soil pore size distribution. The high soil moisture of the straw biochar among biochar applications can be related to its higher organic matter content than other biochar materials (Table 1). Lal (2020) reported that soil organic matter is very important in soil water-holding ability and H₂O emissions from soil. Charles Gould (2015) stated that by increasing the soil organic matter from 1 to 2%, the soil water storage can be increased by approximately 3 l for each 0.0283 m³ of soil. Organic matter additives decrease H₂O emissions from the soil by increasing the pore number and size and distribution of the soil and also the specific surface area of the soil and provide moisture to the soil (Devi et al. 2019; Yerli & Sahin 2021).

Longer preservation of soil moisture can significantly affect the heat capacity and thermal conductivity of the soil, resulting in a decrease in soil temperature. Licht & Kaisi (2005) reported that increased soil moisture causes lower soil temperatures. Soil temperature tends to decrease depending on the cooling of the surface soil and the increase in moisture values in the soil by supplying water to the soil, thus changing its thermal conductivity (Yerli et al. 2022a). Similarly, Mancinelli et al. (2015) also stated that water ingress into the soil can create a cooling effect on the soil by increasing soil moisture.

Biochar is a very effective material to decrease the emission of greenhouse gases from the soil, especially CO₂ (He et al. 2017). Organic carbons in the soil applied biochar become labile, thereby stabilizing the organic carbon in the soil (Wang et al. 2014), and the emission from soils decreases (Deng et al. 2017). Biochar can stabilize the carbon stocks of the soil for hundreds of years with its recalcitrant organic carbon (Schmidt et al. 2002). Due to the porosity of the biochar integrated with the soil, it absorbs organic carbon in the environment and decreases CO₂ emissions from the soil (Lehmann 2007). Case et al. (2014) stated that biochar traps the CO₂ emissions from the soil in its structure. Ge et al. (2020) indicated that biochar applied to soil limits CO₂ emissions from the soil by being effective on the labile carbon fraction, soil aggregates and soil respiration components. In addition, as a result of many studies, it was found that different biochar materials applied to the soil have a positive effect on decreasing CO₂ emissions from the soil (Liu et al. 2011; Wang et al. 2014; Zhang et al. 2017; Wu et al. 2018; Azeem et al. 2019; Akinyemi & Adesina 2020; Lehmann et al. 2021).

Mukherjee & Lal (2013) reported that the CO₂ emissions from the soil decreased with the application of different biochar materials to the soil, but the emission rates can differ because biochars change the soil properties in different ways depending on their different characteristics. He et al. (2017) indicated that the effect of biochar on decreasing CO₂ emissions from the soil may vary depending on the properties of the soil to which the biochar is mixed and its interaction with the soil. Therefore, the lower CO₂ emissions in straw biochar among biochar applications can be explained by its higher EC value (Table 1). Yerli & Sahin (2021) reported that CO₂ emissions vary depending on the EC value of organic residues mixed into the soil. Increasing EC negatively affects soil microorganism activity and weakens oxidation processes (Zhai et al. 2011). Sakin & Yanardag (2019) stated that CO₂ emissions decreased with increasing EC values in the soil. In addition, due to the higher organic matter content of the straw biochar (Table 1), changing soil environmental conditions based on changed soil properties with a decrease in mineralization may have decreased CO₂ emissions. Chaudhari et al. (2013) reported that bacteria, fungi, and actinomycetes in the soil decreased by 40% with the addition of organic matter to the soil, decreasing the bulk density of the soil and thus, oxidation processes in the soil were adversely affected.

As a result of this study, it was determined that (i) all biochar applications decreased CO₂ emissions, but the biochar material that decreased the emissions the most was straw biochar; (ii) with the biochar applications, the moisture retention in the soil increased, and accordingly, the H₂O emissions and soil temperature decreased, and the straw biochar was more effective; and (iii) CO₂ emissions have strong positive linear relationships with soil moisture, H₂O emissions, and soil temperature. It has been concluded that the content and properties of the biochar applied to the soil are very important to decrease the CO₂ emissions from the soil and that the CO₂ emissions can be managed with soil moisture, H₂O emissions, and soil temperature. Therefore, it can be suggested to apply biochar to the soil by evaluating these strategies and to monitor the soil moisture and temperature to decrease emissions for more effective agricultural and environmental management, and it can be recommended to develop studies in this context.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.