3.2. Organic Matter and Carbon Sequestration from Experimental Treatments
Soil organic matter varied significantly (
p-value ≤ 0.05) when measured just after soil amendment applications and at the end of the growing season (
Table 4). This reflects inter-relation and thus interdependence of the important soil health indicator, i.e., SOM on soil amendments. The importance of biodigestate and biochar thus becomes important to explore for improving soil health, carbon sequestration potential of the organic amendments, environmental safety, and tuber yield from agricultural fields in Prince Edward Island.
From the
p-values, it is evident that the significant effect of soil amendments on SOM was nominal at the time of amendment applications and became obvious towards the end of the growing season (
Figure 4). At the beginning of the growing season, BDIF had significantly lower SOM than the other experimental treatments (
Figure 4a). However, towards the end of the growing season, BD, BC, and BCIF treatments had significantly higher enhancement in SOM contents than the other treatments (
Figure 4b).
These results implied that BD, BC, and BCIF treatments showed a significantly higher response in sequestering the highest organic carbon (implied from SOM values) in the soil. As a result, the nutrient management involving organic amendments of biochar, biodigestate, and a mixture of biochar with inorganic fertilizer can significantly enhance carbon sequestration in agriculture soils. Similar findings have been reported in the literature. For example, the study conducted by Cardelli et al. [
26] revealed that the residual materials from waste such as biodigestate and biochar enrich the soil by sequestering carbon into the soil. Alburquerque et al. [
27] also reported that the addition of biodigestate helps to make organic matter easily available to the plant, and it also enhances the soil organic matter.
Adding treatments like biochar and biodigestate helps to alleviate soil compaction. Biochar has the potential to soften the soil and increase fertility status [
28]. Biochar has a great impact on the bulk density of the soil. It has a direct relationship with the bulk density, and adding biochar to the soil helps to decrease the soil bulk density. Bulk density has an indirect relation with soil porosity and a direct relation with soil compaction; the higher the bulk density is, the lower the soil porosity will be, and it will cause compaction. As biochar helps to reduce bulk density, it also reduces the compaction level in the soil [
29]. Soil compaction is a major problem in agricultural soils due to heavy equipment used for tilling agricultural fields, and it is recognized that compaction not only disturbs the soil fertility and its structure but also increases the emissions of GHGs from the soil [
30].
From an environmental perspective, one of the greatest benefits of biochar is its ability to store carbon in the soil. Charcoal is one of the most stable carbon compounds, which means that it takes a long time for it to degrade [
13]. This contrasts with regular compost, which is quickly consumed by soil microorganisms and converted into carbon dioxide and methane [
31]. This makes biochar a long-term method of sequestering carbon into the soil—the carbon that would otherwise be quickly lost as greenhouse gases into the atmosphere. Leading researchers on the sustainability of biochar have declared that “annual net emissions of carbon dioxide (CO
2), methane, and nitrous oxide could be reduced by a maximum of 1.8 Pg CO
2-C equivalent (CO
2-C
e) per year (12% of current anthropogenic CO
2-C
e emissions; 1 Pg = 1 Gt), and total net emissions over a century by 130 Pg CO
2-C
e, without endangering food security, habitat, or soil conservation” [
32].
3.3. Trends in the Emission of Greenhouse Gases
The experimental treatments and data collections events had significant effects (
p-value ≤ 0.05) on the emission of CO
2 (
Table 5 and
Table 6). The plots amended with inorganic fertilizer had the highest emission of CO
2 followed by the plots of biodigestate and biochar when mixed with inorganic fertilizer (
Figure 5a). However, the pure biodigestate-amended plots had the least emissions of CO
2 followed by the purely biochar-amended plots. These results are in concurrence with the findings reported in the literature. For example, Cardelli et al. [
26] has reported that biochar and biodigestate and their combinations can reduce the CO
2 emissions from agricultural soils.
With an increase in temperature during the growing season, there was higher CO
2 emission during the summer months of June, July, and August than during the colder months of the growing season, reflecting a significant effect of the data collection events and thus temperature on CO
2 emission (
Table 5 and
Figure 5b). The least CO
2 emissions were recorded on 29 October 2021 (
Figure 5).
Emissions of CH
4 experienced a significant effect (
p-value ≤ 0.05) of experimental treatments (
Table 5) and the events of data collection (
Figure 6). The biochar amendment treatment had the lowest emission of CH
4 followed by the treatment of biochar’s combination with inorganic fertilizer (
Figure 6a). Biodigestate and inorganic fertilizer treatments had the highest emissions of CH
4. Thus, the biochar amendment was the best treatment among all other treatments to emit the least amount of CH
4 from the soil. In one of the reports, it is suggested that biochar is the most valuable source to reduce greenhouse gas emissions. It is pertinent to mention that CH
4 is 25 times more potent than CO
2 at trapping heat in the atmosphere; therefore, biochar helps to reduce the greenhouse gas effect on the atmosphere which is useful in mitigating climate change impact through the sustainable approach of agricultural practices [
33].
Like on CO
2 emissions, the events of data collection/GHG monitoring and thus temperature had a significant effect causing treatment plots to emit the least amount of CH
4 emission during colder months, i.e., September and October (
Figure 6b). Biochar has been recommended in the literature as a soil amendment to improve soil fertility and CH
4 emissions from rice cultivation [
34].
There was a significant effect (
p-value ≤ 0.05) of experimental treatments and a nonsignificant effect (
p-value > 0.05) of data collection events on the emission of N
2O (
Table 5 and
Table 6). The biochar amendment treatment had the lowest emission of N
2O followed by the control treatment (
Figure 7a). Inorganic fertilizer and its mixture with biodigestate had the highest emissions of N
2O from their respective treatment plots. Thus, the biochar amendment was the best treatment among all other treatments to emit the least amount of N
2O from the soils amended with it.
It is generally accepted that biochar application to soil may benefit both crop and soil productivity [
35,
36,
37]. However, the literature has also reported increased GHGs emissions, especially CH
4 emissions [
38,
39]. Further, Zhang et al. [
35] reported that biochar application promoted higher emissions of CH
4 from rice cultivation by an average of 45.9%. Xie et al. [
40] demonstrated that biochar amendment reduced rice grain yield by an average of 24.8%. The effects of biochar are, in fact, dependent on soil type [
40,
41], feedstock type, biochar production conditions [
39,
40,
41,
42], and application rate [
38,
43]. Feng et al. [
41] showed that the same biochar feedstock, pyrolyzed at different temperatures, resulted in varying CH
4 mitigation effects in different soils.
Like on CO
2 and CH
4 emission trends, the events of data collection/GHG monitoring and thus temperature had a significant effect causing treatment plots to emit the highest amount of N
2O during the hotter month than during colder months (
Figure 7b). Although CH
4 and N
2O emissions are way smaller than CO
2, they are more potent than CO
2 and have multiple times more capacity to deteriorate the environment than CO
2 [
41].
Figure 8 presents monthly means (averaged for two events of data collection in June, July, and September and three events of data collection during August (
Table 2) of GHGs emissions from experimental plots during the first four months of the 2021 potato growing season. It is pertinent to mention here that regardless of the data collections events, biochar and biodigestate and their combinations with inorganic fertilizer had lower emissions as compared with emissions from the control or the only inorganic fertilizer treatments. The same was true for CO
2 (
Figure 8a), CH
4 (
Figure 8b), and N
2O (
Figure 8c).
Several previous studies have illustrated the value of biochar in sequestering carbon [
44,
45]. Lefebvre et al. [
45] modeled the potential of storing carbon through the production of biochar from sugarcane residues, concluding that this could reduce carbon dioxide emissions in the area under study by up to 31%. It would also greatly increase the carbon content of agricultural soils, improving crop health. Mona et al. [
46] identified the benefits of utilizing microalgae as a feedstock for biochar production, explaining that microalgae can further reduce emissions by absorbing carbon from thermal power plants. In their 2015 study, Agegnehu et al. [
16] observed that a mixture of compost and biochar decreased soil emissions by 16–33% as compared to mineral fertilizer in a peanut field. Another study showed that turning one tonne of crop residue into biochar could help sequester 920 kg of carbon dioxide [
47].
To better understand the trend of GHG emissions and to relate the emissions with temperature and experimental treatments, CO
2 emissions during June, July, August, and September 2021 have been plotted against the six experimental treatments in
Figure 9. The month of August had higher CO
2 emissions than the other months maybe because of higher atmospheric/soil temperature. It is understood that organic amendments had kept soil temperature low during the comparatively hotter month, such as August. For all ranges of temperature (during the four months of monitoring), CO
2 emission from inorganic fertilizer treatment was higher than all other treatments. In other words, organic fertilizers (i.e., biodigestate and biochar) and their combinations with inorganic fertilizers as well as control had significantly mitigated CO
2 emissions.
Biochar is beneficial if the soil is very acidic, but if the soil is at a more optimum pH—or if the crops being grown, such as blueberries, prefer acidic soils—too much biochar can upset this balance as reported by Cox [
13] who recommends using ~2.5 cm of biochar for regular soil or up to ~5 cm of biochar for poor/compacted soil to avoid this issue. Moreover, a soil’s pH level influences biochar’s effectiveness in storing carbon; more acidic soils induce more rapid decomposition of the biochar, leading to an increase in CO
2 emissions. However, biochar’s capacity to raise soil pH helps offset this issue [
48].
3.4. Effect of Amendments on Soil Microorganisms
Although the physical data was not collected, it is important to discuss the effect of carbon levels produced from various experimental treatments on soil organisms and ultimately on plant growth. Biochar is reported to alter soil microbial populations and their activities [
49,
50,
51]. Britniky et al. [
49] conducted a three-year experiment to investigate the interactive effects of biochar soil amendment mixed with NPK and cattle manure, on microbial biomass carbon, soil dehydrogenase activity, and soil microbial community abundance in luvisols of arable land in the Czech Republic. They found that the co-application of biochar with manure changes soil properties in favor of increased microbial biomass and their activity. Biochar provides better aeration, improved water content in soils, plant nutrition, and a boost to plant cultivation [
52,
53,
54,
55]. Increasing plant nutrients sourced from biochar can help improve plant cultivation [
56]. Abbas et al. [
15] evaluated the effect of the application of biochar in combination with the recommended synthetic fertilizer on soil properties, maize (
Zea mays L.) plant growth characteristics, and maize grain yield and quality parameters, and they concluded that the potential of biochar application in combination with nitrification inhibitor may be used as the best nutrient management practice for enhanced soil fertility and crop yield.
Many environmental factors including solar radiation, temperature, precipitation, and atmospheric greenhouse gas emissions have huge impacts on crop cultivation and soil organisms; therefore, an optimal plant cultivation environment would require an intricate balance of all these components [
57]. However, due to climate change and greenhouse gas mitigation, this is not always the case, as we observe elevated levels of CO
2 in the atmosphere [
57,
58]. A study conducted by He et. al. [
58] used phylogenetic microarrays (PhyloChip) to assess the effects of elevated CO
2 on the nature of soil for plant cultivation and soil microbial communities. With regards to the nature of the soil, some of the changes they observed with elevated CO
2 included an increase in plant biomass, a decrease in the aboveground N concentration, increased soil pH, and an increase in soil moisture. They also examined the richness of soil microbial communities, determined by several operational taxonomic units (OTUs), to investigate the impact of elevated CO
2 on soil microorganisms. Their findings revealed lower numbers of OTUs at elevated CO
2, suggesting that the richness of soil microbial communities was decreased, proposing a shift in microbial community composition at higher levels of CO
2. Additionally, higher levels of CO
2 also show increased activity of enzymes present in the soil [
59].