Sunﬂower Husk Biochar as a Key Agrotechnical Factor Enhancing Sustainable Soybean Production

: Climate change has a decisive impact on the physical parameters of soil. To counteract this phenomenon, the ongoing search for more effective agri-technical solutions aims at the improvement of the physical properties of soil over a short time. The study aimed to assess the effect of biochar produced from sunﬂower husks on soil respiration (SR), soil water ﬂux (SWF), and soil temperature (ST), depending on its dose and different soil cover (with and without vegetation). Moreover, the seed yield was assessed depending on the biochar fertilization. Field experiments were conducted on Calcaric/Dolomitic Leptosols (Ochric soil). SR, ST, and SWT were evaluated seven times in three-week intervals during two seasons, over 2018 and 2019. It was found that the time of biochar application had a signiﬁcant effect on the evaluated parameters. In the second year, the authors observed signiﬁcantly ( p < 0.005) higher soil respiration (4.38 µ mol s − 1 m − 2 ), soil temperature (21.2 ◦ C), and the level of water net transfer in the soil (0.38 m mol s − 1 m − 2 ), compared to the ﬁrst year. The most effective biochar dose regarding SR and soybean yield was 60 t ha − 1 . These are promising results, but a more comprehensive cost-beneﬁt analysis is needed to recommend large-scale biochar use at this dose.


Introduction
Soil respiration is an important indicator of soil fertility [1,2]. It includes diversified proportions of both autotrophic (root respiration) and heterotrophic components (microbial and soil fauna respiration), depending on soil type and growing season. The source of CO 2 emitted to the atmosphere from the soil surface is mainly root respiration, as well as decomposition of some root residues, soil organic matter, and plant litter [3,4]. The heterogeneity of the vegetation cover and physical properties of the soil contribute to the spatial variability of soil respiration [5,6]. Soil respiration also depends on the adopted farming system [7,8]. Many researchers argue that the farming system directly affects CO 2 emissions in soil and the content of C, and thus, the impact on global warming [9][10][11]. Switching from traditional to conservation tillage, including no-tillage (NT) cultivation, can reduce CO 2 emissions [12]. Soil management and changes in organic matter content are among the factors controlling CO 2 emissions [13]. Hence, it seems that determining the adaptability of the soil to the changing climatic conditions-reduced precipitation and temperature increase-would allow for safe and optimized soil management to ensure a higher

Experiment Design
Two field experiments were conducted in the years 2018-2019. The experiments were established in a randomized block design with four replicates.

Experiment-1
The single-factor experiment tested the effects of four biochar doses, i.e., 0, 20, 40, and 80 t ha −1 applied on bare soil in March 2018. The biochar was incorporated and mixed into the topsoil layer (30 cm depth) to obtain a uniform mass.
In the first week of March, dragging was carried out to prevent evaporation. Then, after 3 weeks, cultivation was carried out with an active rototiller aggregate up to a depth of 20 cm to loosen the topsoil before applying the biochar to the experimental plots. This was done by hand and then the biochar was mixed with a manual rotary cultivator up to a depth of 20 cm. Each treatment had four replications. Each plot's size was 3 m 2 .

Experiment-2
In 2019, a two-factor experiment was conducted to compare the effects of four doses of biochar application (i.e., 0, 20, 40, and 80 t ha −1 ) on two different soil covers: with and without the plants (soybean).
Each treatment had four replications. The plot size was 3 m 2 each. Soybean was sown in the second week of April at a standard planting rate (80 seeds m −2 ), followed by standard NPK mineral fertilization (30 kg N, 70 kg P 2 O 5 , 100 kg K 2 O). Prior to sowing, the soybean seeds were inoculated with Bradryzobium japonicum bacteria. No pesticides were applied during plant vegetation; weeds were controlled mechanically. In the phase of full maturity, the soybean yield and the height of the first pod deposition were assessed based on the yield structures, as an important parameter of the plants' adaptation to the habitat conditions.

Soil Analysis
The chemical properties of soil were determined by standard methods and conducted in the second year of the study. The pH was measured potentiometrically in 1 M KCl after 24 h in the liquid/soil ratio of 10. Total organic carbon (TOC) was determined by TOC-VCSH (Shimadzu) with Solid Sample Module SSM-5000.
Measurements of soil respiration were conducted with the SRS-SD 1000 m (by ADC BioScientific Ltd., Hoddesdon, UK). Due to the specificity of the SRS-SD device (by ADC BioScientific Ltd., Hoddesdon, UK), CO2 readouts in the soil were registered and recorded after 15 min from the moment the measurement was started. To reduce the measurement errors, readouts were made at the same time of day with similar atmospheric conditions. Measurements were not carried out during or shortly after precipitation. Prior to the measurements, the speed of gas flow was determined at 200 µ mol s −1 , which guaranteed that the balance inside a measurement chamber was achieved after 15 min of active operation of the meter (SRS-SD 1000). The soil respiration, soil temperature, and water flux were measured 7 times during each season in three-week intervals during the two seasons.
Soil respiration (net molar flow of CO2 in/out of the soil; μmol mol -1 ) is: where u is the molar air flow in mol s -1 ; Δc is the difference in CO2 concentration throughout the soil chamber, μmol mol -1 ; Δc = Cref-Can, where Cref is the CO2 flowing into the soil chamber, μmol mol -1 ; and Can is CO2 flowing out from the soil chamber, μmol mol -1 . The net H2O Exchange Rate (Soil Flux) Wflux (m mol s -1 m -2 ) is: where us is the molar flow of air per square meter of soil, m mol m -2 s -1 ; Δe is the differential water vapor concentration, m Bar; and p is the atmospheric pressure, mBar.

Soil Analysis
The chemical properties of soil were determined by standard methods and conducted in the second year of the study. The pH was measured potentiometrically in 1 M KCl after 24 h in the liquid/soil ratio of 10. Total organic carbon (TOC) was determined by TOC-VCSH (Shimadzu) with Solid Sample Module SSM-5000.
Measurements of soil respiration were conducted with the SRS-SD 1000 m (by ADC BioScientific Ltd., Hoddesdon, UK). Due to the specificity of the SRS-SD device (by ADC BioScientific Ltd., Hoddesdon, UK), CO 2 readouts in the soil were registered and recorded after 15 min from the moment the measurement was started. To reduce the measurement errors, readouts were made at the same time of day with similar atmospheric conditions. Measurements were not carried out during or shortly after precipitation. Prior to the measurements, the speed of gas flow was determined at 200 µmol s −1 , which guaranteed that the balance inside a measurement chamber was achieved after 15 min of active operation of the meter (SRS-SD 1000). The soil respiration, soil temperature, and water flux were measured 7 times during each season in three-week intervals during the two seasons.
Soil respiration (net molar flow of CO 2 in/out of the soil; µmol mol −1 ) is: where u is the molar air flow in mol s −1 ; ∆c is the difference in CO 2 concentration throughout the soil chamber, µmol mol −1 ; ∆c = C ref-Can, where C ref is the CO 2 flowing into the soil chamber, µmol mol −1 ; and C an is CO 2 flowing out from the soil chamber, µmol mol −1 .
The net H 2 O Exchange Rate (Soil Flux) W flux (m mol s −1 m −2 ) is: where us is the molar flow of air per square meter of soil, m mol m −2 s −1 ; ∆e is the differential water vapor concentration, m Bar; and p is the atmospheric pressure, mBar.

Statistical Analyses
Results were statistically analyzed. The assumption of normality was checked and based on it, the statistical analysis was conducted. The one-and two-way analysis of variance (ANOVA) tests were performed at α = 0.05, followed by an HSD Tukey's test. The Pearson coefficient of correlation between traits was calculated.

Meteorological Conditions
The course of the weather was similar in the studied growing seasons; however, the distribution of rainfall changed in time ( Figure 2). In 2018, heavy rainfall (over 59 mm) occurred in July, while in 2019, it occurred in April, May, and September ( Figure 2A). In the analyzed period, there were periods without rainfall (June), but also numerous periods of drought ( Figure 2B). More rainfall occurred in 2019; most days with rainfall occurred in May, and the least in June.

Statistical Analyses
Results were statistically analyzed. The assumption of normality was checked and based on it, the statistical analysis was conducted. The one-and two-way analysis of variance (ANOVA) tests were performed at α = 0.05, followed by an HSD Tukey's test. The Pearson coefficient of correlation between traits was calculated.

Meteorological Conditions
The course of the weather was similar in the studied growing seasons; however, the distribution of rainfall changed in time ( Figure 2). In 2018, heavy rainfall (over 59 mm) occurred in July, while in 2019, it occurred in April, May, and September ( Figure 2A). In the analyzed period, there were periods without rainfall (June), but also numerous periods of drought ( Figure 2B). More rainfall occurred in 2019; most days with rainfall occurred in May, and the least in June.

Impact of Biochar Application on Selected Soil Parameters in the Year of Application and after One Year
The significantly positive conditional correlation obtained between soil water flux (SWF), soil respiration (SR), and soil temperature (ST) was related to the date of application of biochar (Table 1). Smaller correlations of other factors of parameters were visible in the first year of the study, which was impacted by the physical properties of the soil, e.g., looseness due to recent biochar application. In the first year of the study, the most significant relationship was found between SR and SWF. As the flow of water between the soil and the atmosphere increased, an increase in soil respiration was observed. In the second year after biochar application, the relationship between SR and SWF increased to ultimately prove the strongest mean correlation (r = 0.76) over the years. Along with the increase in respiration, the water flow in the soil increased significantly. On the other 0 50 100 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2

Impact of Biochar Application on Selected Soil Parameters in the Year of Application and after One Year
The significantly positive conditional correlation obtained between soil water flux (SWF), soil respiration (SR), and soil temperature (ST) was related to the date of application of biochar (Table 1). Smaller correlations of other factors of parameters were visible in the first year of the study, which was impacted by the physical properties of the soil, e.g., looseness due to recent biochar application. In the first year of the study, the most significant relationship was found between SR and SWF. As the flow of water between the soil and the atmosphere increased, an increase in soil respiration was observed. In the second year after biochar application, the relationship between SR and SWF increased to ultimately prove the strongest mean correlation (r = 0.76) over the years. Along with the increase in respiration, the water flow in the soil increased significantly. On the other hand, a weaker correlation was found between soil respiration and temperature (r = 0.55) and between temperature and water flow in the soil (r = 0.44).

Impact of Biochar Application on Selected Soil Parameters in the First and in the Second Year on Bare Soil
The biochar-amended soil was characterized by higher pH and TOC compared to control soil. The pH increased proportionally to the biochar rate ( Table 1). The TOC Agriculture 2021, 11, 305 6 of 14 increase was proportional to the increase of biochar rate mainly in treatments of bare soil. No significant differences of TOC were revealed (Table 2).
Without the use of a protective plant, the analyzed soil parameters significantly varied between seasons (Table 3). The lower efficiency of the respiration process identified in the first year of the study was due to the physical properties of the soil, probably related to the lack of compactness resulting from the timing of biochar application. The water content in the soil was the result of the amount of rainfall and the number of days with rainfall. Higher precipitation was recorded in 2019, as confirmed by the significantly higher values of the obtained soil water flux index. The amount of biochar used significantly impacted the soil respiration process. The best effects were observed in the test objects with 60 t ha −1 biochar applied compared to control. Moreover, the use of biochar significantly improves the water flow in the soil compared to the control object.   Upon analyzing the soil respiration process throughout the growing season, significant object-related differentiation was found, depending on the dose of biochar used (Figure 3). The respiration process fluctuated depending on temperature and humidity. The significantly higher soil temperature in the summer months significantly increased soil respiration. The highest activity of soil respiration, irrespective of the dose of biochar used, was found in August. Biochar had a significant impact on the soil respiration process, which resulted in high readings in objects with a dose of 60 t ha −1 (18 µmol s −1 m −2 ).  Upon analyzing the soil respiration process throughout the growing season, significant object-related differentiation was found, depending on the dose of biochar used (Figure 3). The respiration process fluctuated depending on temperature and humidity. The significantly higher soil temperature in the summer months significantly increased soil respiration. The highest activity of soil respiration, irrespective of the dose of biochar used, was found in August. Biochar had a significant impact on the soil respiration process, which resulted in high readings in objects with a dose of 60 t ha −1 (18 µ mol s −1 m −2 ).

Impact of Biochar Application on Selected Soil Parameters in the Second Year Depending on the Soil Protection Variant
The use of a protective plant in the second year of the study had no significant effect on the soil respiration process and water flow in the soil (Table 4). However, a significant impact of the applied biochar dose on the soil respiration process and soil temperature was observed. Application of an average dose of biochar (60 t ha −1 ) resulted in a significant increase in soil respiration compared to the control. This test object also obtained a slightly higher soil temperature and an increased water flow rate in the soil. The in-depth statistical analysis showed a significant convergence of the analyzed factors on the soil respiration process (Table 3, Figure 4). The use of biochar significantly decreased the respiratory activity of the soil, especially in the 40 t ha −1 dose. However, applying a higher dose did not increase soil respiration.
The course of soil respiration in the analyzed period (May-October) depended on the adopted soil cover variant ( Figure 5). The lack of plant cover slightly increased the respiratory activity of the soil in May-July, but it significantly increased it in the summer months, i.e., August-September. Application of an average dose of biochar (60 t ha −1 ) resulted in a significant increase in soil respiration compared to the control.
The minimal soil cover and characteristic of plants in the juvenile phase (June) resulted in a slight increase in soil respiration after the use of biochar (Figure 5b). A significant observation in soil respiration was found in August and September (during the period of intensive growth of plant biomass and roots) in objects with a high dose of biochar. The biochar used had a significant impact on the soybean yield ( Figure 6). Soybean yields were significantly higher in the object where the average dose of biochar was applied (60 t ha −1 ) compared to the control. However, no significant variation in the plant morphotype was found. The height of the first fruiting node on plants was similar regardless of the biochar dose used (Figure 7).      The course of soil respiration in the analyzed period (May-October) depended on the adopted soil cover variant ( Figure 5). The lack of plant cover slightly increased the respiratory activity of the soil in May-July, but it significantly increased it in the summer months, i.e., August-September. Application of an average dose of biochar (60 t ha −1 ) resulted in a significant increase in soil respiration compared to the control. The minimal soil cover and characteristic of plants in the juvenile phase (June) resulted in a slight increase in soil respiration after the use of biochar (Figure 5b). A significant observation in soil respiration was found in August and September (during the period of intensive growth of plant biomass and roots) in objects with a high dose of biochar. The biochar used had a significant impact on the soybean yield ( Figure 6). Soybean yields were significantly higher in the object where the average dose of biochar was applied (60 t ha −1 ) compared to the control. However, no significant variation in the plant morphotype was found. The height of the first fruiting node on plants was similar regardless of the biochar dose used (Figure 7).

Discussion
Our results showed that biochar application increased soil respiration compared with the control treatment, which is in contradiction with several studies based on short-

Discussion
Our results showed that biochar application increased soil respiration compared with the control treatment, which is in contradiction with several studies based on short-term incubation [1,37,38]. According to Lu et al. [25], the effects of biochar on soil respiration are varied because of differences in biochar type, soil type, soil moisture and temperature conditions, and crop planting. There was a significant negative correlation between soil respiration and soil moisture [25]. Their results indicated that rainfall during the maize-growing season suppressed soil respiration and limited the effects of biochar. The effect of soil temperature on soil respiration was greater than that of soil moisture, and soil respiration due to biochar incorporation was more sensitive to the soil temperature than that of control treatments. The research confirmed the above results since seasonal variations in soil respiratory activity, conditioned by the course of the weather, were shown. The lower efficiency of the respiration process was found in the first year of the study, which was impacted by the physical properties of the soil, e.g., lack of compactness due to recent biochar application. Moreover, the soil respiration activity was found to be highly dependent on the water flow rate and temperature. The significantly higher soil temperature in the summer months significantly increased soil respiration. The highest activity of soil respiration, irrespective of the dose of biochar used, was found in August. The presented results have been partially supported by the research of Rutigliano et al. [32], who observed that the speed of respiration was growing within the first 3 months and was statistically higher than the control, but after 14 months, there was no difference between the samples.
Lu et al. [25] analyzed soil respiratory activity in the following four years of consecutive application of straw biochar. The authors highlighted that application of straw biochar neither increased nor inhibited soil respiration throughout the entire maize-growing season compared to the control. In our own research, the authors showed that the use of biochar has a positive effect on the soil respiration process, but it depends on the soil protection variant. In the case of biochar application without soil protection, the positive effect of soil respiration was noticed regardless of biochar dose differentiation, compared to control. The differences in soil respiration between biochar treatments were significant in the ex-periment with soil protection. The use of biochar (up to 60 t ha −1 ) in the experiment with soybean as a soil protector significantly increased the respiratory activity of soil compared to the control.
Zhang et al. [39] proved that the soil respiration of fields treated with returned wheat straw was 547 kg C ha −1 year −1 higher than in fields without residue in the same region. In the experiment, the authors proved relevant differences in respiration of soil conditioned by biochar compared to control conditions. However, the biochar application in different doses did not change soil respiration significantly. Shah et al. [38] tested the effect of different doses of biochar (5, 10, 20 t ha −1 ) on soil respiration. The authors showed that with the increase in the dose of biochar, the soil respiratory activity increased. Similar conclusions were presented by Kubaczyński et al. [37], who stated that in short-term incubations, soil respiration was positively correlated with increasing biochar dose, while during long-term (several years) observation, the impact of biochar dose on the amount of emitted CO 2 was not so significant. It is worthwhile to conduct short-and long-term field studies in this area. In our own research, the authors showed that the soil respiratory activity increased proportionally to biocarbon fertilization. The best results were obtained in an object with 60 t ha −1 biochar, beyond which the soil respiratory activity slightly decreased.
Seremesic et al. [40] tested the effect of biochar at various doses (12.5, 25.75, 125 t ha −1 ) and different soil types (Alluvium (A), Chernozem (C), and Humogley) on the biometric parameters of soybeans. The authors showed that soybean shoot biomass was significantly affected by soil type and biochar level. Soil types had less effect on morphological trait manifestation in soybeans. Sun et al. [41] suggested that biochar incorporation to brown soil can benefit soybean production by N retention in the soil and enhanced microbial turnover that resulted in P and K feedback. Results obtained by Seremesic et al. [40] correspond with a study of Yin et al. [42] on acid black soil, in which soybean yield increased by 35.97% compared to the control. Significant effects of biochar application on the soybean shoot were observed on Humogley soil compared to soybean height that was observed on Chernozem. Regarding shoot biomass, Humogley significantly influenced its formation compared to Alluvial soil. The obtained result could be explained with an improved water retention capacity of Humogley.
The obtained results of the soil tests for Calcaric/Dolomitic Leptosols prove that high soybean yields can be obtained with appropriate biocarbon fertilization. The authors showed that the soybean yield was significantly differentiated as impacted by the applied doses of biochar. Significantly higher soybean yields were obtained in the object with a dose of 60 t ha −1 biochar compared to control. However, the biochar application resulted in no significant difference in the formation of the first fruiting node on plants. Only slightly lower-placed pods were observed in test objects with a high dose of biochar. Upon analyzing the impact of biochar application on the soil respiration process throughout the growing season of soybean, the authors showed a significant difference between the objects. A significant observation in soil respiration was found in August and September (during the period of intensive growth of plant biomass and roots) in objects with a high dose of biochar.
Yooyen et al. [43] compared the effects of different doses of Blachia siamensis Gagnep. biochar (10, 20, 30 t ha −1 ) on soybean yield. Growth and yields of soybean, including stem height, number of nodes, dry matter of stems, dry matter of leaves, dry matter of pods, and dry matter of seeds in the biochar treatments, show statistically significant differences at p < 0.05 compared to control (BC 0). The most significant result obtained in this study was the statistically significant increase of pods and seeds (p < 0.05). Moreover, according to the results, treatments with 20 t ha −1 and 30 t ha −1 of biochar yielded seeds 28.0 percent and 36.8 percent heavier, respectively, compared to the untreated control. In our own research, the authors showed that the biochar application increased the seed yield of the soybean, but the impact on the height of the first pod was not relevant. The highest yield (3.8 t ha −1 ) was obtained in an object with 60 t ha −1 biochar, and with a higher dose, the yield slightly decreased.

Conclusions
The respiration process fluctuated depending on temperature and humidity. The significantly higher soil temperature in the summer months significantly increased soil respiration. The highest activity of soil respiration, irrespective of the dose of biochar used, was found in August. Biochar had a significant impact on the soil respiration process, which resulted in high readings in objects with a dose of 60 t ha −1 (18 µmol s −1 m −2 ). The use of a protective plant in the second year of biochar application had no significant effect on the soil respiration process and water flow in the soil. However, a significant impact of the applied biochar dose was observed on the correlation between soybean cultivation on the soil respiration process and soil temperature. Among the compared treatments, a significantly higher soil respiration activity was found in the object after the application of 60 t ha −1 biochar, which increased soybean yield by an average of 2 t ha −1 compared to the control. The dose of 60 t ha −1 of biochar from the sunflower husk can be recommended for soybean cultivation since it increases the physical properties of sandy soil.