The results of the experiment show that the method of organic matter management and the use of slow-release fertilizers impacted plant yields. In the three-year period, the yield of corn ranged from 6.15 t/ha to 11.86 t/ha. Significant differences in yielding were found in individual years. On average, the lowest yield was obtained in 2019 due to insufficient rainfall during the growing season (Table 7)
Table 7
GHG emissions in individual objects, per experiment years, in CO2 eq /t grain
| Total emission | Fertilizers and seeds and plant protection product | Emission from the mineralization of crop residues | Production and use of biochar | Mineralization of soil organic matter | Diesel combustion |
2019 |
A | 1364.51 | 1044.19 | 103.27 | 0.00 | 60.90 | 156.14 |
B | 1417.00 | 834.95 | 403.63 | 0.00 | 49.02 | 129.41 |
C | 852.64 | 542.35 | 98.54 | 61.50 | 32.55 | 117.71 |
D | 895.38 | 551.36 | 104.06 | 64.94 | 41.01 | 134.01 |
E | 896.07 | 561.59 | 107.21 | 70.01 | 33.67 | 123.60 |
F | 739.53 | 411.69 | 108.79 | 70.19 | 31.15 | 117.71 |
2020 |
A | 811.43 | 562.47 | 103.27 | 0.00 | 33.71 | 111.98 |
B | 1053.00 | 515.53 | 403.63 | 0.00 | 30.98 | 102.86 |
C | 777.45 | 474.40 | 98.54 | 61.50 | 28.71 | 114.29 |
D | 658.13 | 350.52 | 104.06 | 64.94 | 26.97 | 111.64 |
E | 695.72 | 389.13 | 107.21 | 69.11 | 23.93 | 106.34 |
F | 630.19 | 318.47 | 108.79 | 69.70 | 24.64 | 108.59 |
2021 |
A | 884.31 | 625.95 | 103.27 | 0.00 | 37.29 | 117.8 |
B | 1114.36 | 569.38 | 403.63 | 0.00 | 34.02 | 107.3 |
C | 830.13 | 520.38 | 98.54 | 61.50 | 31.31 | 118.4 |
D | 708.75 | 393.22 | 104.06 | 64.94 | 29.95 | 116.6 |
E | 788.99 | 469.47 | 107.21 | 69.53 | 28.46 | 114.3 |
F | 709.73 | 384.94 | 108.79 | 70.04 | 29.28 | 116.7 |
The average yield for all objects was 7.48 t/ha. The highest yields were obtained in 2021, at 10.23 t/ha. In the first year of research, the highest crop of plants was obtained in objects where biochar and fertilizer based on urea and biochar (variants C, D, E, F) were used. One of the most important positive aspects of using biochar as a soil additive is improving the efficiency of water use in light of the current water shortage in Poland. The year 2019 was characterized by a serious water deficit, therefore the introduced biochar could increase the efficiency of water use, as confirmed by the research of Seyedsadr et al. (2022). Even in the case of surface introduction of biochar, a positive effect on the availability of water and nutrients for plants was observed (Gao, De Luc, 2021; Huang et al, 2021a, b). The results presented by these authors indicate the possibility of effective use of biochar on permanent lands, which broadens the prospects for the use of biochar in agriculture in the context of increasing soil water sorption and improving its fertility. In turn, Liang et al. (2021) found no unequivocal influence of biochar on soil fertility. These authors proved the positive effect of leaving straw in the soil on plant yields and soil properties. However, they did not find any difference between the variants where straw, straw compost and straw biochar were used. In the presented research, the highest average yield over three years was obtained in the objects where fertilizer based on urea and urea was applied. There was no statistical difference between object E and F, despite the reduction of nitrogen dose by 20% in object F. The obtained results prove the ability of biochar to reduce nitrogen losses and increase the efficiency of nitrogen fertilization. Similar results were obtained by other authors such as Krause et al. (2018), Jin et al. (2019), Hu et al (2021).
The use of biochar on agricultural land improves soil fertility, which consequently increases the use of nutrients, water and microelements contained in the soil. The reduction of GHG emissions from the use of biochar is related to the sorption of carbon dioxide in soil air and the reduced emissions of nitrogen oxides, which are considered the most important agricultural GHGs (Krause et al. 2018). Gross et al. (2022) found that the addition of biochar to the soil increased its bioaccumulation in the soil. These authors unequivocally state that the use of biochar limits GHG emission from agricultural production. In their research, they proved that biochar increases the level of carbon accumulation in soil. Two years after the use of 7 Mg C/ha of biochar, carbon sequestration from the atmosphere increased to approximately 2.5 Mg/ha. In addition, these authors found approximately 21% lower GHG emissions compared to the object fertilized with an equivalent amount of manure. These studies did not take into account the biochar production system boundary and the transport of biomass and biochar from their production sites to agricultural production sites. Anand et al. (2022) calculated that the introduction of 1 Mg of biochar to the soil reduces the loss of fertilizing elements by 4 kg. The reduction of the losses of fertilizing elements results from the high sorption capacity of biochar. Awad et al. (2018) observed that incorporating the mineral fraction into biochar, using components such as oyster shells or polymer compounds, notably enhanced its capacity to mitigate greenhouse gas emissions from agricultural ecosystems. Xu et al. (2016) highlighted that biochar's CO2 sorption capacity can reach 34 mg/g of biochar, dependent on metallic element content. Shin et al. (2021) and Oliveira et al. (2017) independently found that biochar addition significantly increased carbon sequestration and intensified humification processes, resulting in reduced carbon dioxide emissions and enhanced soil properties. Biochar's impact on soil microorganism communities and chemisorption of carbon dioxide were also noted to contribute to decreased emissions (Zhang et al. 2016, Yang et al. 2021). Managing soil to curtail carbon emissions and promote long-term carbon accumulation is crucial in natural resource management. Biochar is recognized as a sustainable soil carbon source and an effective means of carbon sequestration (Jiang et al. 2022). Furthermore, Xiong et al. (2021) demonstrated that torrefaction of straw can enhance soil organic matter management, albeit with the potential for increased organic matter outflow and surface runoff, necessitating thorough soil mixing upon biochar introduction.
The emission of CO2 equivalent ranged from 630.2 to 1339.8 kg of CO2 per ton of marketable corn yield within the adopted system boundary. This parameter's value was mainly dependent on the yield size, which was primarily influenced by the research year. On average, over subsequent years of research, the values were 965.9, 771.0, and 839.4 CO2 eq/t of marketable corn yield. Throughout all the years of research, the highest level of greenhouse gas emissions was observed in the scenario where the straw remained in the field. Corn grain production in this scenario was linked to GHG emissions ranging from 1052 to 1339 kg of CO2 eq/t of corn grain. Removing straw from the field reduced GHG emissions by up to 25%. To conduct a more comprehensive analysis of the influence of implemented technological adjustments on the evolution of greenhouse gas emissions from corn production, the sources were categorized into 5 groups: emissions associated with diesel combustion, emissions linked to soil organic matter mineralization, emissions tied to biochar production and usage, emissions associated with crop residue mineralization, and emissions related to the usage of fertilizers, seeds, and plant protection products (refer to Fig. 3).
Figure 3 HIRE
Figure 3. Distribution of CO2 emissions depending on the experiment variant. Average emission value analysis depending on the method and time (year).
The analysis of the results of the share of individual GHG emission sources in the total emission leads to an unequivocal conclusion that the distribution of CO2 eq emissions is differentiated for technological variants under study. When analyzing the results for 2019, it can be noticed that the greatest differences in the distribution occur in the “fertilizer use” group. The percentage share of this factor ranged from 57.3% for object B to 75.8% for object A (Fig. 1, 2, 3). There are also noticeable differences in the share of GHG emissions related to the mineralization of crop residues. The highest share of this factor, amounting to 30.1%, was found in object B, where straw was left, and the lowest − 8.1% in object A. In the case of emissions related to the production and use of biochar, its share in objects C -F ranged between 6.5% and 8.9%. The difference in the percentage share of the other two factors is not significant. Based on the independence test (χ2 = 441.94, p-value = 0.000) it can be concluded that there is a significant variation in the distribution of the share of individual factors on CO2 eq emissions for the tested methods jointly. Additionally, based on the independence test, it was verified which distributions of the methods differ in pairs. As a result of this analysis, it was shown that both the distributions for method A and for method B differ from the distributions of all other methods. Moreover, the distribution in the C method significantly differs from the distributions obtained in the D and F methods. The results of the analysis carried out for the data from 2019 were repeated in the next two years, i.e. 2020 and 2021.
In the second step of the statistical analysis, to verify whether the average CO2 emissions for the production of 1 ton of corn differs significantly for individual methods, an analysis of variance was used for experiments with repeated measures. To verify whether it is possible to use the one-dimensional approach, the Mauchley sphericity test (1940) was performed. Its results are presented in Table 8.
Table 8
The results of Mauchly’s sphericity test
Result | W | Chi-sq | df | p |
Year | 0.872373 | 2.321146 | 2 | 0.313307 |
As the test value is high and close to 1 (W = 0.872373), it entails the test probability level p = 0.0313307, which means that there are no grounds to reject the spherical hypothesis. Therefore, it is possible to use the one-dimensional approach. Due to the fact that there are two factors in the analysis, i.e. time and method, the mean boundary effects are presented in Figs. 4 and 5.
Figures 4 and 5. Graphical interpretation of the influence of the factors, experiment variant and year of experiment/CO2 emission
Figures 4 and 5 HIRE
Please note: vertical bars represent the 95% confidence interval.
The results in the diagram related to the cultivation method (left) suggest that object A is the most unfavorable in terms of CO2 eq emissions per ton of corn produced. The average emission value for this method, for four plots over three years, is approximately 1180 kg CO2 eq/t of grains of corn. Lowest average CO2 eq emissions was obtained for methods F and D and it was approximately 720–730 kg CO2 eq/t of corn grain (Figs. 4 and 5). The highest CO2 emissions was observed in the first year of cultivation, i.e. 2019, and was 1030 kg of CO2 eq/t of grains of corn on average (for all objects). It was the lowest in the second year of cultivation, i.e. in 2020, at 775 kg of CO2 eq/t grain of corn.
Figure 6 shows the combined effect of method and time on average CO2 eq emissions. Average CO2 eq emissions when using method B is the highest in each year compared to other methods. However, there is no method that is by far the best in terms of CO2 emissions. Namely, in 2019 and 2020 the lowest emission was obtained for method F, and in the last year for method D. Method A gives a high emission result (similar to method B) in the first year, while in subsequent objects the emission significantly decreases.
Figure 6. Graphical interpretation of the influence of the factors, experiment variant and year of experiment/CO2 emission.
Figure 6 HIRE
To verify whether the existing differences in the impact of factors, both individual and jointly, on the average CO2 eq emission, an analysis of variance with repeated measures was performed for the studied data. The main results of this analysis are presented in Tables 9, 10, 11.
Table 9
Results of repeated ANOVA measures
Result | SS | df | MS | F | p |
Free term | 55665332 | 1 | 55665332 | 17780.47 | 0.000000 |
Method | 1777428 | 5 | 355486 | 113.55 | 0.000000 |
Error | 56353 | 18 | 3131 | | |
Year | 853525 | 2 | 426762 | 73.36 | 0.000000 |
YEAR * method | 214208 | 10 | 21421 | 3.68 | 0.001843 |
Error | 209438 | 36 | 5818 | | |
It turns out that both main effects, i.e. method and year, are statistically significant (p < 0.001), and their interaction effect is also significant (p < 0.01). Thus, the differences observed in the charts turned out to be statistically significant. This means that individual crop residue management strategies resulted in significant differences in CO2eq emissions throughout the period considered. CO2 eq emissions differed significantly in individual years of cultivation (regardless of the method used). The type of method used differentiates the average emissions in each of the analyzed periods separately. The above results were obtained for the analysis of all methods and periods jointly. To verify which corn crop residue management strategies differ significantly from the others in terms of CO2 eq emissions, Scheffe's post-hoc test was carried out separately for the factors: “Variant of experiment” and “Year”.1
Table 10
Scheffe's test results for the “Variant of experiment” factor (the values of probabilities below the adopted threshold of 0.05 are marked in red).
Variant of experiment | A | B | C | D | E | F |
A | | 0.000060 | 0.000510 | 0.000000 | 0.000009 | 0.000000 |
B | 0.000060 | | 0.000000 | 0.000000 | 0.000000 | 0.000000 |
C | 0.000510 | 0.000000 | | 0.004462 | 0.393675 | 0.000658 |
D | 0.000000 | 0.000000 | 0.004462 | | 0.255009 | 0.954633 |
E | 0.000009 | 0.000000 | 0.393675 | 0.255009 | | 0.051012 |
F | 0.000000 | 0.000000 | 0.000658 | 0.954633 | 0.051012 | |
The above table of results gives grounds to conclude that both the average emission for method A and for method B differ significantly from the average emission for other methods because all p values are lower than 0.05. The use of method C does not differentiate CO2 emissions only in relation to method E, while method D gives a result similar to E and F, and significantly different from methods A, B, C. Method E differs significantly only from A and B, and method F additionally from C.
Table 11
Scheffe test results for the factor “year”
Year | 2019 | 2020 | 2021 |
2019 | | 0.000000 | 0.000000 |
2020 | 0.000000 | | 0.030282 |
2021 | 0.000000 | 0.030282 | |
The Scheffe test results for the time factor presented in the table show that there are significant differences in the average amount of CO2 emissions in each of the analyzed periods (all probabilities are lower than 0.05). This means that in the second year of cultivation, the emission is significantly lower than in the first and third years, while in the third year it increases significantly compared to the second year, but is significantly lower than in the first year.
GHG emissions related to diesel combustion ranged from 107.3 to 149.4 kg of CO2 eq/t of produce. The share of this source of GHGs in individual research objects ranged between 9.2% and 17.2% (Figs. 1.2 and 3). The emissions related to the use of mineral fertilizers had the highest share of total GHG emissions in all research objects. In each case, this value exceeded 50%. Regardless of the year of research, the highest share of this source was found in object A, and the lowest in object B. When analyzing the absolute values, it was found that the highest value of emissions related to fertilizer use was found in objects A, B and C and the lowest value in objects D, E and F. The average value of GHG emissions related to the use of mineral fertilizers for all years was 719 CO2 eq/t in facility A, while in facility F, where the fertilizer produced on the basis of biochar and urea was applied, the value was 395.9 CO2 eq/t. In this study, CO2 eq emissions related to diesel combustion, the use of seeds and mineral fertilizers was between 2,000 and 3,000 kg/ha and was comparable to the data presented by Holka and Bieńkowski (2020). Qi et al. (2018) found that GHG emissions from corn cultivation range between 400 and almost 800 kg of CO2 eq/t depending on the level of intensification of production. However, these authors excluded the emissions related to the mineralization of crop residues and soil organic matter from the system boundary. Liu et al. (2021) report the values of the carbon footprint for corn cultivation in the Chinese province of Hebei at a higher level than that obtained in this research, i.e., approximately 2,000 CO2 eq/t. These authors emphasize that in assessing the carbon footprint of agricultural systems, soil-bound carbon should be taken into account for a clearer approach to calculating the environmental impact of agriculture. N2O emissions from nitrogen unused by plants were a significant source of GHG emissions. Carbon dioxide emissions from soil are an important component of the total impact of plant production on the intensity of climate warming. The results of the own research show that the level of GHG emissions from soil ranged from about 1000 CO2 eq/ha to almost 4,000 CO2 eq/ha. The highest values of this parameter were found in objects where straw was left in the field and the distribution of crop residues was the most important parameter influencing the GHG emission from this source. Chi et al. (2020) found CO2 emissions from the soil during the growing season of corn and wheat at 530 to over 600 kg when straw was left in the field. Assuming that the decomposition of organic matter also takes place in addition to the growth of plants, the total level of GHG emissions could be higher than estimated in this research. Pareja-Sánchez et al. (2019) found the annual amount of CO2 emissions from soil, in which corn was grown at approximately 1,800–4,500 CO2/ha. Large differences in carbon dioxide emissions resulted from the variable level of nitrogen fertilization. These authors emphasize the role of nitrogen as a factor that models carbon transformation in soil. Kumar et al. (2021) calculated the level of CO2 emissions from the soil in corn cultivation at approximately 1,000–3,000 kg of CO2/ha. These authors found a significant positive relationship between the level of nitrogen fertilization and the emission of carbon dioxide from the soil surface. In turn, Álvaro-Fuentes et al. (2016) found no unequivocal trend in CO2 emissions from the soil in the cultivation of unfertilized corn and fertilized with nitrogen at the dose of 300 kg N/ha. The values of emissions from the soil surface reported by these authors ranged from 4,300 to 5,600. Mdhluli and Harding (2021) prove that harvesting residual crop of corn and converting it for energy purposes could ensure better environmental effects than leaving it at the production site. These authors emphasize that residual crop of corn is poor in nitrogen and other macronutrients, and transforming it into highly efficient energy sources is the right direction of residual crop management. The use of residual crop after the torrefaction of organic matter could be more effective than introducing raw organic matter into the soil. However, many studies indicate that in the long term, such an approach will lead to a deterioration of soil fertility. The production potential of the habitat has the greatest impact on the level of GHG emissions from corn cultivation, which is related to the fertility of the soil and the production potential of the cultivated varieties. These parameters are not directly related to production technology, but in many cases play the greatest role in modeling the value of carbon footprint (Boone et al. 2016). Mineral fertilizers accounted for the largest share of GHG emissions in the presented research. For all facilities, except for the use of biochar fertilizers, the emission level from this source exceeded 3300 CO2 eq/ha. These values are comparable to those presented by Qi et al. (2018). The use of slow-release fertilizers in its own research resulted in a significant reduction in the share of this source in the total GHG emissions from corn production. When applied properly, slow-release fertilizers can significantly increase the efficiency of nitrogen fertilization, which reduces GHG emissions per plant yield (Sikora et al. 2020B). Leaving a residual crop at the plant production site is not the optimal way to manage carbon resources (Wang et al. 2021a). Organic matter left in the form of mulch or mixed with soil undergoes intensive mineralization, which leads to the emission of significant amounts of carbon dioxide. In the authors' own research, the mineralization of straw in the soil accounted for over 30% of the total GHG emissions related to corn production. In the remaining objects, this value was approximately 10% and was related to the mineralization of the roots and the uncollected part of the post-harvest residues. Cereal straw is not rich in nitrogen compounds; therefore, its decomposing biomass is not a significant source of this element. In intensive cultivation and high nitrogen fertilization, the humification processes are very limited, which means that the remaining straw is not a source of humus (Bei et al. 2022). Huang et al. (2021a) found that after eight years of rice cultivation the content of total organic carbon in the soil increased when straw was left in the field, compared to the objects with the straw removed. However, in the same studies, no statistically significant difference was observed in the accumulation of carbon in the soil between the objects where straw was left, and where straw was torrefacted and ash was left. The results presented by these authors prove the questionable value of straw as a permanent source of carbon bound carbon in the soil. The direction and intensity of organic compound transformations in soil is determined by many factors and is related to the complex ecological relations within the soil microbiome. Yuan et al. (2021) found an increase in the content of labile carbon fractions after six years of leaving wheat and rice straw in the field. However, the authors found that the permanent binding of organic carbon increased significantly when potassium was applied directly on the straw. This was accompanied by an increase in plant yield by approximately 20% compared to objects where straw was collected. Xie et al. (2021) emphasize that a critical element influencing the sequestration of carbon in straw left in the field is the right amount of available nitrogen and phosphorus. Too much nitrogen increases the level of intensification of organic matter mineralization, which is not beneficial from the point of view of carbon sequestration.
The available research results indicate that the environmental effectiveness of leaving straw at the cultivation site is determined by many factors related to production technology, soil properties, and climatic conditions. Therefore, it is not possible to provide unified recommendations on the management of harvest residues in light of specific environmental effects. Only the development of a technology to transform biomass into a more durable form and introduce it into the soil could provide the input data for the development of a methodology for the permanent sequestration of organic carbon in the soil. On the one hand, this would reduce the value of the carbon footprint of cultivated plants and, on the other hand, increase the level of carbon sequestration in the soil, improving its fertility.
The scientific literature emphasizes the potential of biochar in sequestering carbon in soil through the use of pyrolized organic matter, forming persistent organic compounds. To comprehensively evaluate the environmental impact of biochar, specific studies on its permanent carbon sequestration potential using distinct raw materials are crucial. Additionally, for enhanced organic carbon sequestration and reduced greenhouse gas emissions, exploring the incorporation of mineral additives to optimize biochar-based products is essential.