Biochar with Inorganic Nitrogen Fertilizer Reduces Direct Greenhouse Gas Emission Flux from Soil

Agricultural waste can have a catastrophic impact on climate change, as it contributes significantly to greenhouse gas (GHG) emissions if not managed sustainably. Swine-digestate-manure-derived biochar may be one sustainable way to manage waste and tackle GHG emissions in temperate climatic conditions. The purpose of this study was to ascertain how such biochar could be used to reduce soil GHG emissions. Spring barley (Hordeum vulgare L.) and pea crops in 2020 and 2021, respectively, were treated with 25 t ha−1 of swine-digestate-manure-derived biochar (B1) and 120 kg ha−1 (N1) and 160 kg ha−1 (N2) of synthetic nitrogen fertilizer (ammonium nitrate). Biochar with or without nitrogen fertilizer substantially lowered GHG emissions compared to the control treatment (without any treatment) or treatments without biochar application. Carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) emissions were directly measured using static chamber technology. Cumulative emissions and global warming potential (GWP) followed the same trend and were significantly lowered in biochar-treated soils. The influences of soil and environmental parameters on GHG emissions were, therefore, investigated. A positive correlation was found between both moisture and temperature and GHG emissions. Thus, biochar made from swine digestate manure may be an effective organic amendment to reduce GHG emissions and address climate change challenges.


Introduction
In recent decades, the increase in human population has caused serious challenges to the agriculture sector and to the agronomist in ensuring food security, causing minimum soil and environmental pollution [1]. Inorganic nitrogen fertilizer consumption in the agricultural sector of the European Union has increased by around 2% over the past ten years to 10.2 million tons [2]. This considerable share of synthesized fertilizer application is due to inefficient use, which causes financial harm, environmental damage, and health risks [3][4][5]. Inorganic fertilizer amendment and soil tillage practices have increased greenhouse gas (GHG) emissions [6][7][8]. By 2030, it is anticipated that the agricultural sector's nitric oxide (N 2 O) emissions may rise by 35-60%. This increase is linked to higher nitrogen content due to fertilizer use and higher production of animal waste [9,10]. Moreover, the increase in the number of livestock is directly proportional to methane (CH 4 ) emissions that, between 1990 and 2030, are anticipated to increase by 60% [11]. The predicted rise in agrofarming emissions is .4%, with a mean increase of 8.3 Pg CO 2 -eq by 2030, assuming the aforesaid rates of rising emissions (10-15%) for the 2020-2030 period. Anthropogenic emissions of GHGs (CO 2 , CH 4 , and N 2 O), have become a substantial contributor to global climate change [12,13]. GHG emissions are highly dependent on soil temperature and moisture,

Experimental Site
The experimental study was conducted during the growing seasons of 2020 and 2021 at the fields of the Lithuania Research Center for Agriculture and Forestry (55 • 40 N, 23 • 87 E). The chemical compositions at depths of 0-10, 0-20, and 0-60 cm for the Endocalcari-Epihypogleyic Cambisol soil used in the experimental fields is shown in Table 1. Biochar was prepared from swine manure digestate at 550 • C. Both biochar and N fertilizer were applied to the soil one week prior to sowing of the crop and were manually applied to each plot (1.5 m 2 ). The experiment was carried out using the spring barley (Hordeum vulgare L.) "Luoke cultivar" and the pea (Pisum sativum) "Respect cultivar" in 2020 and 2021, respectively. The period of growth was from April to August 2020 for spring barley and from April to July 2021 for the pea crop. Data were recorded at each growth stage from seedling until maturity. Lithuanian Hydrometeorological Service-Dotnuva data under the Ministry of Environment data were used (http://www.meteo.lt/, accessed on 12 January 2020) (Figure 1). The chemical changes over two years in the fields studied are given in Table 1.

Soil Physicochemical Properties
Laboratory-based, standardized techniques were used to examine the physicochemical characteristics of both the soil and the biochar. A 1:5 (vol:vol −1 ) soil combination in 1 M KCl solution was used for the electrical conductivity and pH analysis of the soil and biochar [32,33], as well as an extract in distilled water [34]. Soil and biochar organic matter contents were measured using a spectrophotometer at a wave length of 590 nm [35]. A revised ammonium-acetate technique was used to measure cation exchange capacity [36]. Inductively coupled plasma atomic emission spectrometry (Perkin Elmer ICP-OES, Waltham, MA, USA) was used to assess the extractable Mg from DTPA [37]. Using a reference approach, total nitrogen (TN) and accessible phosphorus concentrations were determined [38]. A TGA provided information on biochar ash content, moisture, volatiles, and residual mass.

Soil Physicochemical Properties
Laboratory-based, standardized techniques were used to examine the physicochemical characteristics of both the soil and the biochar. A 1:5 (vol:vol −1 ) soil combination in 1 M KCl solution was used for the electrical conductivity and pH analysis of the soil and biochar [32,33], as well as an extract in distilled water [34]. Soil and biochar organic matter contents were measured using a spectrophotometer at a wave length of 590 nm [35]. A revised ammonium-acetate technique was used to measure cation exchange capacity [36]. Inductively coupled plasma atomic emission spectrometry (Perkin Elmer ICP-OES, Waltham, MA, USA) was used to assess the extractable Mg from DTPA [37]. Using a reference approach, total nitrogen (TN) and accessible phosphorus concentrations were determined [38]. A TGA provided information on biochar ash content, moisture, volatiles, and residual mass.

Gas Sampling and Flux Calculation
Gas chromatography was used to measure the gas flux, and a static chamber gas [13,39] method was modified only slightly for the analysis. A U-shaped groove (50 mm wide and 50 mm deep) was present on the top edge of the chamber base box (frame) to retain a detachable chamber box. Stainless-steel frames were permanently buried 10 cm beneath the surface of the soil. A frame's perimeter covered 0.36 m 2 . The chamber was sealed for 3 min before each flux measurement, and 20 mL of gas sample was drawn using a 20 cm 3 syringe. To increase the consistency of gaseous flux estimates, the gas samples were collected between the hours of 9:00 and 10:00 in the morning. Glass vials with rubber tubing used as a lid were used to collect the gas samples. From the beginning of the cultivating season (one week before the application of biochar) to one month after harvest, the fluxes of CO 2 , N 2 O, and CH 4 were measured at 2-week intervals. Three replicates of each treatment were used; thus, gas samples were collected from each plot. The samples were examined using a gas chromatograph (HP 6890 Series, GC System, Hewlett-Packard, Analytical system Management, Denver, USA) that had nickel catalysts for converting CO 2 to CH 4 and flame ionization and electron capture detectors. The corresponding temperatures were 70, 300, and 350 • C, respectively. The techniques for gas chromatography were explained by [40]. Equations (1) and (2) were used, respectively, to compute the cumulative and GHG flow rates and global warming potential for the growing seasons of 2020 and 2021 (from April to August). Based on the rate of change in GHG concentration within the chamber, which was determined as the slope of the linear regression between the GHG concentration and the gas-sampling time, the flow rate of each GHG was derived.

Calculation of Cumulative Soil GHG Emissions
Between various growth stages, cumulative CO 2 , CH 4 , and N 2 O emissions for each treatment were estimated as indicated by [5,41].
The total cumulative emissions of soil CO 2 , CH 4 , and N 2 O (mgha −1 h −1 ) are represented by the symbol R a , where the initial emissions of soil CO 2 , CH 4 , and N 2 O are represented by R i ; the subsequent emissions of soil CO 2 , CH 4 , and N 2 O are represented by R i + 1 after the subsequent time i; and n is the number of interval days for the emissions of soil CO 2 , CH 4 , and N 2 O.

Global Warming Potential (GWP)
The following equation was used to determine the global warming potential (GWP) of soils treated with biochar and N fertilizer in 2020-2021 (IPCC, 2007).

Soil Temperature and Moisture Measurements
In each growth stage, the soil temperature was measured using a squarely buried thermometer at a depth of 5 cm over the years 2020-21 [5]. Additionally, using an ovendrying method for 24 h at 105 • C, soil samples were taken from 0 to 10 cm deep using a soil auger in order to quantify soil moisture (in mass percent) at each growth stage. The link between soil temperature, moisture content, and CO 2 , N 2 O, and CH 4 emissions was examined using a linear regression.

Statistical Analysis
An analysis of variance (ANOVA) was performed on annual data gathered for each parameter during the 2-year period. The statistical differences were examined using Statistix 8. The Tukey Test was used to assess mean values at a 0.05 probability level. GraphPad Prism 9 was used to plot the data.

Soil CO 2 Emission Flux
Periodic variation was recorded between the seasons of the spring barley crop of 2020 and the pea crop of 2021. The emission of CO 2 during the 2020 spring barley crop was recorded as higher throughout the season, except during jointing stage. During 2021 of the pea crop season, CO 2 emissions were found significantly (≤0.05) lowered under biochartreated soils compares to the spring barley of 2020 ( Figure 2). All biochar-treated soils showed substantially (≤0.05) lowered CO 2 emissions (by 57%, 55%, and 59%, respectively, for B, N 1 B, and N 2 B) compared to non-biochar-treated soils during all stages of the pea crop. The N 2 B treatment significantly (≤0.05) lowered CO 2 emissions during the tillering stage (by 58%) compared to the control treatment. Similarly, the B treatment substantially (≤0.05) lowered CO 2 emissions during the jointing, flowering, and maturity stages by 50%, 51%, and 50%, respectively ( Figure 2). soil auger in order to quantify soil moisture (in mass percent) at each growth stage. The link between soil temperature, moisture content, and CO2, N2O, and CH4 emissions was examined using a linear regression.

Statistical Analysis
An analysis of variance (ANOVA) was performed on annual data gathered for each parameter during the 2-year period. The statistical differences were examined using Statistix 8. The Tukey Test was used to assess mean values at a 0.05 probability level. GraphPad Prism 9 was used to plot the data.

Soil CO2 Emission Flux
Periodic variation was recorded between the seasons of the spring barley crop of 2020 and the pea crop of 2021. The emission of CO2 during the 2020 spring barley crop was recorded as higher throughout the season, except during jointing stage. During 2021 of the pea crop season, CO2 emissions were found significantly (≤0.05) lowered under biochar-treated soils compares to the spring barley of 2020 ( Figure 2). All biochar-treated soils showed substantially (≤0.05) lowered CO2 emissions (by 57%, 55%, and 59%, respectively, for B, N1B, and N2B) compared to non-biochar-treated soils during all stages of the pea crop. The N2B treatment significantly (≤0.05) lowered CO2 emissions during the tillering stage (by 58%) compared to the control treatment. Similarly, the B treatment substantially (≤0.05) lowered CO2 emissions during the jointing, flowering, and maturity stages by 50%, 51%, and 50%, respectively ( Figure 2).

Soil N2O Emission Flux
Biochar treatments had no effect on N2O emissions during the spring barley crop of 2020 throughout all the growth stages. However, there was significant variation recorded during different growth stages of both cropping seasons regarding N2O emissions ( Figure  3). Following the above trend, the biochar-treated soils of B, N1B, and N2B showed substantially (≤0.05) lowered N2O emissions by 48%, 49%, and 48%, respectively, throughout the growth stages of the pea crop compared to non-biochar treatments ( Figure 3). In 2021, during the pea crop season, the N2 treatment (100% nitrogen fertilizer alone) tended to enhance N2O emission, specifically during the seedling and tillering stages.

Soil N 2 O Emission Flux
Biochar treatments had no effect on N 2 O emissions during the spring barley crop of 2020 throughout all the growth stages. However, there was significant variation recorded during different growth stages of both cropping seasons regarding N 2 O emissions ( Figure 3). Following the above trend, the biochar-treated soils of B, N 1 B, and N 2 B showed substantially (≤0.05) lowered N 2 O emissions by 48%, 49%, and 48%, respectively, throughout the growth stages of the pea crop compared to non-biochar treatments (Figure 3). In 2021, during the pea crop season, the N 2 treatment (100% nitrogen fertilizer alone) tended to enhance N 2 O emission, specifically during the seedling and tillering stages.

Soil CH4 Emission Flux
There was no substantial variation recorded for methane gas (CH4) emissions during the spring barley crop of 2020. However, the results indicated significant fluctuation in CH4 emissions during the pea crop growing stages of 2021 ( Figure 4). The N1B treatment significantly (≤0.05) lowered CH4 emissions by 17%, and 19% during the flowering and maturity stages, respectively ( Figure 4). However, biochar did not affect CH4 emissions the way it effected CO2 and N2O emissions. Biochar application did not affect the cumulative emissions of CO2, N2O, and CH4 during the growth stages of the spring barley crop of 2020 (Table. 1). However, there was big variation recorded during the growth stages of the pea crop of 2021. The biochar treatments of B, N1B, and N2B significantly (≤0.05) lowered cumulative CO2 emissions by 16%, 19%, and 17%, respectively, compared to the control treatment. However, the cumulative emission of N2O was significantly (≤0.05) lowered by 6% only in the B treatment compared to the control treatment (Table 2). Similarly, the cumulative emission of CH4 was significantly lowered by

Soil CH 4 Emission Flux
There was no substantial variation recorded for methane gas (CH 4

Soil CH4 Emission Flux
There was no substantial variation recorded for methane gas (CH4) emissions during the spring barley crop of 2020. However, the results indicated significant fluctuation in CH4 emissions during the pea crop growing stages of 2021 ( Figure 4). The N1B treatment significantly (≤0.05) lowered CH4 emissions by 17%, and 19% during the flowering and maturity stages, respectively ( Figure 4). However, biochar did not affect CH4 emissions the way it effected CO2 and N2O emissions. Biochar application did not affect the cumulative emissions of CO2, N2O, and CH4 during the growth stages of the spring barley crop of 2020 (Table. 1). However, there was big variation recorded during the growth stages of the pea crop of 2021. The biochar treatments of B, N1B, and N2B significantly (≤0.05) lowered cumulative CO2 emissions by 16%, 19%, and 17%, respectively, compared to the control treatment. However, the cumulative emission of N2O was significantly (≤0.05) lowered by 6% only in the B treatment compared to the control treatment (Table 2). Similarly, the cumulative emission of CH4 was significantly lowered by 7% in the B and N1B treatments compared to the control treatment (Table 2). Biochar application did not affect the cumulative emissions of CO 2 , N 2 O, and CH 4 during the growth stages of the spring barley crop of 2020 (Table 1). However, there was big variation recorded during the growth stages of the pea crop of 2021. The biochar treatments of B, N 1 B, and N 2 B significantly (≤0.05) lowered cumulative CO 2 emissions by 16%, 19%, and 17%, respectively, compared to the control treatment. However, the cumulative emission of N 2 O was significantly (≤0.05) lowered by 6% only in the B treatment compared to the control treatment (Table 2). Similarly, the cumulative emission of CH 4 was significantly lowered by 7% in the B and N 1 B treatments compared to the control treatment ( Table 2).  The global warming potential (GWP) of CO 2 , N 2 O, and CH 4 emissions followed the same trend as that of cumulative emissions. There was no significant fluctuation recorded for GWP during the growth stages of spring barley in 2020. However, the GWP caused by CO 2 was recorded as substantially (≤0.05) lower in the biochar treatments of B, N 1 B, and N 2 B by 39%, 35%, and 39%, respectively, compared to other treatments during the growth stages of the pea crop of 2021 (Table 3). The GWPs caused by CH 4 and N 2 O were significantly lowered in treatment B by 19% and 34% compared to the control treatment, respectively ( Table 3).
The substantial changes under biochar and N fertilizer rates for soil moisture and temperature are presented in (Figure 5). During 2020-2021, no significant effects of the treatments were recorded for lower soil temperature during the vegetative growth stages of both crops. Furthermore, soil moisture contents were 9.5%, 8.3%, and 7.6% higher in the N 2 B, N 1 B, and B treatments, respectively, during flowering and maturity stages of spring barley. A similar trend was recorded during different growth stages of the pea crop ( Figure 5). treatments were recorded for lower soil temperature during the vegetative growth stages of both crops. Furthermore, soil moisture contents were 9.5%, 8.3%, and 7.6% higher in the N2B, N1B, and B treatments, respectively, during flowering and maturity stages of spring barley. A similar trend was recorded during different growth stages of the pea crop ( Figure 5).

CO2 Emission
The decomposition of organic materials is caused by CO2, N2O, and CH4 emissions. [42,43]. Biochar provides additional environmental advantages since it improves soil fer-

CO 2 Emission
The decomposition of organic materials is caused by CO 2 , N 2 O, and CH 4 emissions [42,43]. Biochar provides additional environmental advantages since it improves soil fertility through decomposition [44,45]. Biochar is a significant source of carbon and helps in increasing SOC buildup [46,47] that, even at low soil temperatures, resulted in higher average CO 2 emissions in biochar-amended soil compared to non-biochar treatments in 2020. This result showed that soil C has a greater potential for soil CO 2 emission variability [48,49] and, hence, increases soil fertility. However, on the other hand, biochar has the potential to mitigate CO 2 emission [50,51]. The seasonal changes in soil CO 2 were dramatically impacted by biochar applications in 2021. Similar fluctuations in soil temperature and moisture were visible for the biochar treatments during the field trial. The agricultural fields consistent results demonstrate that biochar decomposition initially boosted soil CO 2 emission and soil carbon and nitrogen availability [52,53]. However, there was also substantially lowered soil CO 2 after biochar decomposition compared to non-biochar treatments. Unnecessary agronomic practices may influence soil moisture, which can affect soil CO 2 emission [54,55]. For instance, it was reported that different tillage operations lead to GHG emissions [56]. According to the current study, the higher precipitation ( Figure 1) in the first year (2020) compared with that of the second year (2021) could mean that, due to favorable conditions for the decomposition of biochar, the soil CO 2 emission increased [57,58].

N 2 O Emission
The overusage of N fertilizer increases GHG emissions and has negative effects on the ecosystem [59,60]. The current findings showed that, during the growth seasons, the N fertilizer treatments of N 1 , N 2 , and N 3 alone considerably boosted soil N 2 O emissions. (2021). Based on the N used and the emission variables, soil amendment with biochar and synthetic fertilizers can reduce N 2 O emissions [61]. It is challenging to anticipate the emission factors because of the complex chemical compositions of organic fertilizers [5,62]. It is known that N fertilization and mulching treatments together boost N 2 O flux by 71-123% [63][64][65].
Nevertheless, biochar on a field might help to reduce N 2 O emissions [66,67]. Additionally, it was determined from these outcomes that N application with biochar could decrease N 2 O emissions, as observed in the biochar-treated plots compares to non-biochar-treated plots. Moreover, it was also reported that N fertilization could influence degradable N and C, which resulted in improving the intricate microbial interaction between N and C, thus enhancing N 2 O emissions [68,69].
As a comparison to applying N fertilizer alone, using biochar with N fertilizer reduces N 2 O emissions by 25-35% [70,71]. Higher nitrogen fertilizer application rates result in higher GHG emissions, which has an immediate impact on soil N 2 O emissions [5,72,73]. The present study suggested that the N 1 , N 2 , and N 3 treatments are more environmentally unfriendly due to N 2 O emissions, while the B treatment with N fertilizer is ecofriendly.

CH 4 Emission
Compared to CO 2 and N 2 O emissions, only the N 1 B treatment decreased CH 4 emissions during 2021. It was reported that the organic piles' structure was improved with biochar application anaerobically, and biochar could alter the oxidation-reduction potential by enhancing absorptivity, which lowered the mechanism of methanogens and increased that of methanotrophs to mitigate CH 4 emissions [74]. Several of the literature findings have indicated that the interaction between applying biochar to soil and CH 4 flux is not well-known [75,76]. The soil applications of biochar have been shown to enhance [77], lower [77][78][79], or have no substantial influence on CH 4 emission flux [80]. It was reported that biochar addition to soil also promoted methanotrophic CH 4 intake at the oxic-anoxic junction in anaerobic environments. Moreover, the addition of biochar improved the oxida-tion of CH 4 by methanotrophic organisms at the oxic-anoxic root interface, which lowered the concentration of CH 4 that could enter a plant s aerenchyma and escape [79].

Global Warming Potential (GWP)
The overall impact of the main greenhouse gases (i.e., CO 2 , CH 4 , and N 2 O) is driven by GWP [81]. The plots with applied biochar had a much lower net GWP during 2021. However, no substantial difference was reported in 2020, which is in line with the following reports. The non-significant difference in GWP might be due to the non-decomposition of biochar in the first year [82]; however, the decomposition of biochar might be enhanced in the second year, which led to GWP reduction [51]. Overall, studies report that biochar application can significantly mitigate global warming. The biochar C:N ratio may be an important factor that drives GWP under biochar applications [83].

Soil Moisture and Temperature
The incorporation of biochar into soil is treated as sustainable waste. It was reported that the inappropriate management of different wastes (food, agriculture, etc.) creates a global environmental challenge [84]. Thus, biochar addition provides a multitude of advantages in terms of sustainable environment and agriculture aspects [85]. Furthermore, the current study reported that biochar applications significantly (p ≤ 0.05) elevated soil moisture content in the years of 2020-2021. Greater soil moisture content as a result of surface area and porous structure has been observed [22,86]. However, changes in soil temperature could be attributed to weather conditions.

Correlation between Soil Moisture, Temperature, and CO 2 and N 2 O Emissions
According to the current study, CO 2 , N 2 O, and CH 4 emissions were considerably positively associated with soil temperature and moisture. According to a report, the primary variables affecting soil gas emission fluxes are its thermal characteristics [87,88]. Soil CO 2 and CH 4 emissions increase due to fact that higher soil temperature and moisture cause higher biochar decomposition and higher methane oxidation rates [89][90][91]. The reason for higher N 2 O emissions with higher temperatures could be attributed to N fertilization, which releases mineralized N upon decomposition [92]. It was observed that soil temperature and soil moisture had a positive, two-parameter linear association with CO 2 , N 2 O, and CH 4 emissions ( Figure 6). The findings of this study, therefore, show that agricultural management techniques under humid climate conditions affected the rate of soil GHG emission.
For the spring barley and pea crops in 2020-2021, the linear relationship between soil CO 2 , N 2 O, and CH 4 emissions and soil moisture and soil temperature was studied. Figure 6 demonstrates a positive correlation between soil temperature and moisture and soil GHG CO 2 , N 2 O, and CH 4 emissions. The linear CO 2 , N 2 O, and CH 4 R 2 values during 2020 and 2021 were 0.2171 and 0.1353, 0.5550 and 0.3355, and 0.7611 and 0.1981, respectively. According to Figure 6, soil CO 2 , N 2 O, and CH 4 emissions significantly increased when soil temperature and wetness rose. higher biochar decomposition and higher methane oxidation rates [89][90][91]. The reason f higher N2O emissions with higher temperatures could be attributed to N fertilization, whi releases mineralized N upon decomposition [92]. It was observed that soil temperature an soil moisture had a positive, two-parameter linear association with CO2, N2O, and CH4 em sions ( Figure 6). The findings of this study, therefore, show that agricultural manageme techniques under humid climate conditions affected the rate of soil GHG emission.

Conclusions
Biochar application substantially lowered direct CO 2 , N 2 O, and CH 4 emissions from soil in the second year compared to first year for non-biochar-treated plots. Thus, the lower CO 2 , N 2 O, and CH 4 emissions from the agricultural fields confirmed that swine manure digestate biochar could be a suitable remedy for agriculture fields with higher GHG emissions, especially in temperate climatic conditions. Likewise, the cumulative emissions and global warming potential were substantially influenced by biochar during the second year of the experiment. A positive correlation was recorded between GHG emissions and soil moisture and temperature. No negative environmental issues were recorded during the two years of field research. More research is required to explore the long-term implication of swine-digestate-manure-derived biochar.
Author Contributions: All authors contributed to this research paper. M.A. conceived the idea for the article; performed the practical and the literature searches, data preparation, analyses, and figures; and wrote the first draft. D.F., V.F. and V.T. critically revised the work and contributed to writing and editing. E.B.-G. and S.U. helped in biochar preparation and contributed to article conception, critical revision, and editing drafts. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author, Muhammad Ayaz, upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.