Assessment of Agro-Environmental Impacts for Supplemented Methods to Biochar Manure Pellets during Rice (Oryza sativa L.) Cultivation

The agro-environmental impact of supplemented biochar manure pellet fertilizer (SBMPF) application was evaluated by exploring changes of the chemical properties of paddy water and soil, carbon sequestration, and grain yield during rice cultivation. The treatments consisted of (1) the control (no biochar), (2) pig manure compost pellet (PMCP), (3) biochar manure pellets (BMP) with urea solution heated at 60 °C (BMP-U60), (4) BMP with N, P, and K solutions at room temperature (BMP-NPK), and (5) BMP with urea and K solutions at room temperature (BMP-UK). The NO3−–N and PO4−–P concentrations in the control and PMCP in the paddy water were relatively higher compared to SBMPF applied plots. For paddy soil, NH4+–N concentration in the control was lower compared to the other SBMPFs treatments 41 days after rice transplant. Additionally, it is possible that the SBMPFs could decrease the phosphorus levels in agricultural ecosystems. Also, the highest carbon sequestration was 2.67 tonnes C ha−1 in the BMP-UK treatment, while the lowest was 1.14 tonnes C ha−1 in the BMP-U60 treatment. The grain yields from the SBMPFs treatments except for the BMP-UK were significantly higher than the control. Overall, it appeared that the supplemented BMP-NPK application was one of the best SBMPFs considered with respect to agro-environmental impacts during rice cultivation.

NH 3 volatilization, and runoff losses. New strategies such as biochar-manure pelletizing methods are available to minimize N loss from the application of animal-waste compost. New approaches that would improve the efficiency of compost are significant to agricultural production in Korea, because the amount of animal waste must be disposed in an effective manner with a minimal impact on agricultural eco-systems.
In general, the production of biochar pellets with poultry litter mixed with switch grass (BMP) is relatively simple. Pellet is blended poultry litter with powder of switchgrass, and then BMP is produced with slow pyrolysis [24]. Several scientists reported that the synergistic effects of biochar blended with inorganic fertilizer or biochar mixed with nutrient-rich compost were observed to improve crop yields [25][26][27]. There is only limited information on the field application of supplemented biochar manure pellets with inorganic fertilizers (SBMPFs). SBMPF provides supplemental nutrients and can also regulate nutrient loss or release rate by functioning as a slow release fertilizer. Slow-release fertilizers gradually discharge nutrients to the soil during the growing season and provide sufficient nutrients to crops while minimizing leaching losses [28], which can increase farmers' profits and minimize environmental impacts [29]. Ultimately, this application ameliorates the loss of income in agro-business and mitigates the potential contamination of agricultural watersheds. SBMPFs thus represent an efficient way to decrease field application costs and biochar loss during soil application [19].
However, only limited information on blended biochar pellets functioning as slow-release fertilizers is available. Kim et al. [30] indicated that the application of a combination of biochar and slow release fertilizers yielded the lowest methane emissions among the treatments due to the inhibition of methanogenic bacteria via increased soil aeration and improved rice yield compared to the control.
Additional benefit for cropland application of biochar is carbon sequestration [31,32]. Biochar has a much longer residency period (up to 1000 years) compared to raw materials because of its recalcitrance to biotic and abiotic degradation [33]. However, biochar is partly degraded and oxidized into CO 2 when incorporated into soils [34] and up to 50% of feedstock carbon may be lost during pyrolysis [31,35]. Therefore, reduction of carbon during biochar production and increasing its stability in the soil would improve its potential for carbon sequestration. In terms of soil carbon sequestration Energies 2020, 13, 2070 3 of 14 and the mitigation of CO 2 -equiv. (carbon dioxide equivalency) emission, biochar incorporated with cow manure compost can sequester 2.3 tonnes C ha −1 , and ranges from 7.3 to 8.4 tonnes ha −1 for mitigating CO 2 -equiv. emission in the cornfield [36]. Shin et al. [37] indicated that the application of biochar pellets blended with organic compost is a promising way to increase carbon sequestration during crop cultivation. For the application of BMP, carbon sequestration and mitigation of CO 2 -equiv. emission were 1.65 tonnes ha −1 and 6.06 tonnes ha −1 greater than those of the control, respectively, during rice cultivation [38]. Soil carbon sequestration from the application of biochar made of wood branch increased from 1.87 to 13.37 tonnes ha −1 , while the plots with rice straw application demonstrated decreased soil carbon from 2.56 to 0.92 tonnes ha −1 [39].
The objective of this study was to evaluate the agro-environmental impact of supplemented biochar manure pellet fertilizers (SBMPFs) application on the agro-ecosystems and soil carbon sequestration during the rice growing season. It is hypothesized that the SBMPFs can significantly mitigate non-point pollution sources and increase potential carbon sequestration in agro-ecosystems.

Biochar Production
Biochar derived from rice hull was purchased from a local farming cooperative society in Go-chang, JeonBuk, South Korea. The top to bottom pyrolysis method to produce biochar was employed, wherein rice hull is burned from the upper level to bottom, and reduces oxygen flux from the exterior of the pyrolysis system at 29.4 KPa of air suction rate. The maximum temperatures during pyrolysis were from 490 • C at the top and 550 • C at the bottom of the pyrolysis system. The loading volume in each batch was 1.5 m 3 of rice. The biochar was milled with a grinder to pass through a 2-mm sieve before chemical analysis. The same raw materials were used for both the biochar and pig manure compost, and their chemical properties are shown in Table 1 [37,38]. The moisture contents of the biochar and pig manure compost were 5.5% and 27.2%, respectively. The biochar was generally alkaline with a pH of 9.7 and low in total nitrogen (TN), 2.0 g kg −1 .

Production of Supplemented Biochar Manure Pellet
The processing of SBMPFs is described in Figure 1. Prior to pelleting, biochar was processed in a series of sieves (0.5-5 mm) to ensure even particle distribution. In producing biochar pellets, 40% biochar was mixed with 60% pig manure compost as a binder. The SBMPF was completely mixed by using an agitator while spraying different nutrient solutions in the mixtures, and then feeding it into a commercial pellet mill (7.5 KW, 10HP, KumKang Engineering Pellet Mill Co., Daegu, South Korea). Different biochar pellets (Patent number: 10-1889400) treated with (1) urea solution heated at 60 • C (BMP-U60), (2) N, P, and K nutrient solutions at room temperature (BMP-NPK), (3) urea and K solutions at room temperature (BMP-UK), and (4) pig manure compost only (PMCP) pelletized. The size of BMPFs was approximately Ø 0.51 cm × 0.78 cm. The total carbon, TN (total nitrogen), TP (total phosphorus), and TK (total potassium) contents of BMPF embedded with different treatments are described in Table 2. Their total carbon and nitrogen contents varied from 225 g kg −1 to 289 g kg −1 and from 29.1 g kg −1 to 102.0 g kg −1 , respectively. It was observed that the BMP-U60 had the highest nitrogen content of 102.0 g kg −1 and BMP-UK had the lowest nitrogen content of 84.0 g kg −1 .

Field Experiment
The experimental field was cultivated with rice monoculture, and it has clay loamy soil. It is located at 35° 49.510´N of latitude and 127° 2.536´E of longitude in the National Institute of Agricultural Sciences (NIA), Rural Development Administration (RDA), Jeonju, Republic of Korea. The precipitation amount and average temperature were 718 mm and 22.3 °C during the rice cultivation season, respectively. Additionally, the solar radiation quantity and duration of sunshine are measured at 2753.2 MJ and 949.9 hours during the cultivation period, respectively. The rice variety used in this experiment was Shindongjin, with a planting distance of 30 × 60 cm. The experimental design was a block design with five treatments consisting of 1) the control, 2) PMCP, 3) BMP-U60, 4) BMP-NPK, and 5) BMP-UK with three replications and 16 m 2 of the plot size. The amount of fertilizer and manure compost applied in the control and PMCP treatment were 90-45-57 kg ha −1 (N-P-K) and 2600 kg ha −1 , respectively, which was based on National Institute of Agricultural Sciences (NIA) recommended rates for rice cultivation [40]. The SBMPFs were incorporated into the soil based on 90 N kg ha −1 for whole basal application at 5 days prior to rice transplanting. Water logging time was 6 days prior to rice transplanting. The date of rice transplant was May 23, and drainage times were 14 days, 35 days, and 93 days after transplanting with one-week drainage. Rice was harvested 154 days after transplanting period. To evaluate the agricultural impact of different SBMPFs, major plant

Field Experiment
The experimental field was cultivated with rice monoculture, and it has clay loamy soil. It is located at 35 • 49.510 N of latitude and 127 • 2.536 E of longitude in the National Institute of Agricultural Sciences (NIA), Rural Development Administration (RDA), Jeonju, Republic of Korea. The precipitation amount and average temperature were 718 mm and 22.3 • C during the rice cultivation season, respectively. Additionally, the solar radiation quantity and duration of sunshine are measured at 2753.2 MJ and 949.9 h during the cultivation period, respectively. The rice variety used in this experiment was Shindongjin, with a planting distance of 30 × 60 cm. The experimental design was a block design with five treatments consisting of (1) the control, (2) PMCP, (3) BMP-U60, (4) BMP-NPK, and (5) BMP-UK with three replications and 16 m 2 of the plot size. The amount of fertilizer and manure compost applied in the control and PMCP treatment were 90-45-57 kg ha −1 (N-P-K) and 2600 kg ha −1 , respectively, which was based on National Institute of Agricultural Sciences (NIA) recommended rates for rice cultivation [40]. The SBMPFs were incorporated into the soil based on 90 N kg ha −1 for whole basal application at 5 days prior to rice transplanting. Water logging time was 6 days prior to rice transplanting. The date of rice transplant was May 23, and drainage times were 14 days, 35 days, and 93 days after transplanting with one-week drainage. Rice was harvested 154 days after transplanting period. To evaluate the agricultural impact of different SBMPFs, major plant nutrients were analyzed from the Energies 2020, 13, 2070 5 of 14 surface water and soil in the paddy during rice cultivation. For rice growth responses, the plant height and number of tillers were measured about 100 days after rice transplanting, while the grain yield and dry weight of rice straw were weighed after harvest. For the effect of SBMPF applications in the paddy, the physicochemical properties of the soil used are presented in Table 3. Table 3. Soil physicochemical properties of experimental field 1 .

Chemical Analysis of Paddy Soil and Water
After rice transplantation in the paddy, surface soil and water samples were collected every 20 days. The collected water samples were filtered through Whatman 2. The surface water was analyzed for NH 4  [41] from dried soil samples that passed through 2 mm sieves were stored in a refrigerator at 4 • C until PO 4 − , K + and SiO 2 were analyzed using a UV spectrophotometer (C-Mac, Dae-Jeon, Korea). Total carbon (TC) in soils was analyzed with total organic carbon (TOC) analyzer (Elementa vario TOC cube, Hanau, Germany). The combustion temperature was 950 • C and tungsten trioxide (WO 3 ) was used as the catalyst. With 350mg of soil samples, total nitrogen (TN) contents were determined by dry combustion with 250mg of L-Glutamic acid, standard compound, by using vario Max CN (Elementar, Hanau, Germany).

Data Processing and Carbon Balance Calculations
The soil carbon sequestration via BMPFs application was calculated from the difference of the residual amount of soil carbon between the control and different treatments after rice harvest by using the following equation [38]: where SS TC (kg ha −1 ) is the potential sequestration amount of soil carbon, T (kg ha −1 ) is the treatment of SBMPFs, NT (kg ha −1 ) is the control, TC is total carbon content (g kg −1 ), i is the sampling date, Li and Ii are carbon contents of the last and initial samplings which analyzed the soil carbon content (g kg −1 ), and SW is the soil weight (bulk density, 1.3; 10cm of plowing soil depth, kg ha −1 ). The mitigation of CO 2 emission for SBMPFs application was also estimated using equation [38]: where SS TC is the amount of soil carbon sequestration (tonnes ha −1 ) and CF SC is the conversion factor of CO 2 emission from soil carbon (1 kg C = 3.664 kg CO 2 -equiv.). Profit analysis for the mitigation of CO 2 emission was also calculated by using the equation [38]:

Statistical Analysis
Statistical analysis was conducted using SAS version 9.2 Software (SAS, Inc., Cary, NC, USA), with an ANOVA with Duncan multiple range tests for the comparison of treatments with carbon contents at 1st day of rice transplanting and day after harvesting, carbon sequestration, and growth components during rice cultivation. Standard deviation was used for comparisons of paddy water and soil chemical properties.

Paddy Water Quality
The NH 4 + -N and NO 3 − -N concentrations in the surface paddy water are presented in Figure 2.
At the first day of rice transplanting, the NH 4 + -N concentration of surface paddy water in the MBP-NPK was significantly higher than the other treatments, but its control showed nearly the same values than the other treatments. However, the NO 3 − -N concentrations in the control and PMCP were only significantly higher than those in the SBMPF treatments. It was observed that NH 4 + -N concentrations in the treatments were higher on the first day of rice transplants, but similar to the rest of the days. The loss of nitrogen under the application of SBMPF was almost complete within 21 days after rice transplantation. This might be due to the adsorption of NH 4 + -N by the applied biochar in the soil.
Regardless of the treatments at 112 days of rice transplanting, the NO 3 − -N concentrations were higher compared with other sampling days (93 days) due to the start of drainage of the surface water in the rice paddy. The study showed that the application of SBMPs can be a solution to mitigate the loss of nitrogen and phosphorus [44]. The PO 4 − -P, K + , and SiO 2 concentrations in the surface paddy water under application of BMPFs are described in Figure 3. The measured PO 4 --P concentration in the control and PMCP treatment was 2.8-5.3 times higher than the value in BMP-U60, BMP-UK, and BMP-NPK, respectively, until 21 days after rice transplantation. The PO 4 --P concentrations were not significantly different (p > 0.05) from 41 days to 93 days after rice transplanting among the treatments. The greatest differences in K+ concentrations can be seen at 41 days after transplant. The higher values in the control and PMCP were 28.5 mg L −1 , and the lowest in the BMP-U60 was 9.6 mg L −1 , but not significantly different (p > 0.05) with that of BMP-UK. The PO4 − -P, K + , and SiO2 concentrations in the surface paddy water under application of BMPFs are described in Figure 3. The measured PO4 --P concentration in the control and PMCP treatment was 2.8-5.3 times higher than the value in BMP-U60, BMP-UK, and BMP-NPK, respectively, until 21 days after rice transplantation. The PO4 --P concentrations were not significantly different (p > 0.05) from 41 days to 93 days after rice transplanting among the treatments. The greatest differences in K+ concentrations can be seen at 41 days after transplant. The higher values in the control and PMCP were 28.5 mg L −1 , and the lowest in the BMP-U60 was 9.6 mg L −1 , but not significantly different (p > 0.05) with that of BMP-UK.
Silicon (Si) in soil exists in an unavailable form, but the Si in crop residues is a useful structure (H4SiO4) compared with Si fertilizer for crop uptake [45]. This recycled Si is leached into soil after the decomposition of crop residues. It is observed that SiO2 concentration ranged from 10 mg L −1 to 35 mg L −1 during the cultivation period, and the highest SiO2 concentration was 34.4 mg L −1 in the BMP-UK at after 41 and 112 days of rice transplanting. However, SiO2 concentrations in the paddy water under the application of SBMPFs were higher than those of the control and PMCP at 112 days after transplant. The most commonly used silicon fertilizer is wollastonite for soil application because of its high solubility for plant uptake (2.3%-3.6%) [46]. Recently, much attention has been paid to biochar as an alternative soil ameliorant because it could slowly release 43 mg kg -1 for the available plant uptake of silica [47]. The 1% KOH solution treated biochar application to soil significantly increased available form of silicon in the plant [48]. In this study, the SiO2 concentration was significantly increased at the harvesting time under the application of SBMPFs. Thus, the Silicon (Si) in soil exists in an unavailable form, but the Si in crop residues is a useful structure (H 4 SiO 4 ) compared with Si fertilizer for crop uptake [45]. This recycled Si is leached into soil after the decomposition of crop residues. It is observed that SiO 2 concentration ranged from 10 mg L −1 to 35 mg L −1 during the cultivation period, and the highest SiO 2 concentration was 34.4 mg L −1 in the BMP-UK at after 41 and 112 days of rice transplanting. However, SiO 2 concentrations in the paddy water under the application of SBMPFs were higher than those of the control and PMCP at 112 days after transplant. The most commonly used silicon fertilizer is wollastonite for soil application because of its high solubility for plant uptake (2.3-3.6%) [46]. Recently, much attention has been paid to biochar as an alternative soil ameliorant because it could slowly release 43 mg kg −1 for the available plant uptake of silica [47]. The 1% KOH solution treated biochar application to soil significantly increased available form of silicon in the plant [48]. In this study, the SiO 2 concentration was significantly increased at the harvesting time under the application of SBMPFs. Thus, the incorporation of SBMPFs had the potential ability to recycle silica. Overall, the PO 4 -P, K + , and SiO 2 concentrations were significantly higher than the other sampling days (93 days) due to the start of drainage of the surface water in the paddy field. incorporation of SBMPFs had the potential ability to recycle silica. Overall, the PO4 -P, K + , and SiO2 concentrations were significantly higher than the other sampling days (93 days) due to the start of drainage of the surface water in the paddy field.

Nutrients in Paddy Soil
Urea application is usually the main source of ammonium ions because urea can be hydrolyzed into NH4 + and OH − by the ammonification reaction within short periods after application in the paddy soil. The major nutrient concentrations in the soil are described in Figure 4. NH4 + -N concentration in the BMP-NPK was highest among the treatments at 41 days after rice transplanting. Total nitrogen losses were reduced with the incorporation of rice straw in the rice paddy soil due to increasing immobilization [49] and denitrification [50]. P2O5 concentrations except the PMCP were not significantly different during 21 days after rice transplanting among treatments. The K2O concentrations in the soil treated with BMPFs continuously decreased during rice cultivation due to the K + solubility, except for the BMP-U60 treatment. Biochar application increased the availability of K + and P because it was a net source of cations due to increased soil capacity to hold exchangeable cations [51,52]. The application of biochar produced from rice straw increased the available P and K + by 15.3% and 28.6% in the soil, respectively. However, biochar application did not significantly increase total nitrogen compared with the control in the rice paddy [53]. Overall, the release of major nutrients to soil under the application of SBMPFs was significantly lower compared with those from the control and PMCP.

Nutrients in Paddy Soil
Urea application is usually the main source of ammonium ions because urea can be hydrolyzed into NH 4 + and OH − by the ammonification reaction within short periods after application in the paddy soil. The major nutrient concentrations in the soil are described in Figure 4. NH 4 + -N concentration in the BMP-NPK was highest among the treatments at 41 days after rice transplanting. Total nitrogen losses were reduced with the incorporation of rice straw in the rice paddy soil due to increasing immobilization [49] and denitrification [50]. P 2 O 5 concentrations except the PMCP were not significantly different during 21 days after rice transplanting among treatments. The K 2 O concentrations in the soil treated with BMPFs continuously decreased during rice cultivation due to the K + solubility, except for the BMP-U60 treatment. Biochar application increased the availability of K + and P because it was a net source of cations due to increased soil capacity to hold exchangeable cations [51,52]. The application of biochar produced from rice straw increased the available P and K + by 15.3% and 28.6% in the soil, respectively. However, biochar application did not significantly increase total nitrogen compared with the control in the rice paddy [53]. Overall, the release of major nutrients to soil under the application of SBMPFs was significantly lower compared with those from the control and PMCP.

Carbon Sequestration and Profit Analysis
Soil carbon sequestration was only considered after soil analysis from rice paddy incorporated SBMPFs at day 1 of rice transplanting and the day after harvesting. Changes of total carbon contents in paddy soil under different treatments at the initial stage and after harvesting are described in Table  4. The carbon contents on first day of rice transplanting and the day after harvesting were significantly (p < 0.001) different in the treatments. There was minimal difference in total carbon content in the control between the first day of rice transplanting and after harvesting.

Carbon Sequestration and Profit Analysis
Soil carbon sequestration was only considered after soil analysis from rice paddy incorporated SBMPFs at day 1 of rice transplanting and the day after harvesting. Changes of total carbon contents in paddy soil under different treatments at the initial stage and after harvesting are described in Table 4. The carbon contents on first day of rice transplanting and the day after harvesting were significantly (p < 0.001) different in the treatments. There was minimal difference in total carbon content in the control between the first day of rice transplanting and after harvesting.
The application of biochar incorporated to the soil has been suggested as a promising method for carbon sequestration as well as another method for mitigating greenhouse gas, increasing crop yields and enhancing the sorption of pollutants [49,54]. Regarding carbon sequestration, it might be distinguished that short term released CO 2 refers to the retention time of sequestrated carbon in soil from organic matter decomposition, while long term, it is stored as biochar from thermal conversion materials [38]. For the application of different types of SBMPFs, carbon sequestration, mitigation of CO 2 , and profit analysis were calculated by using Equations (1)-(3), respectively ( Table 5). The analysis of carbon sequestration showed 2.67 tonnes C ha −1 in the BMP-UK as the best treatment for carbon sequestration, and 1.14 tonnes C ha −1 in the BMP-U60 as the worst. It appeared that their recovery rates varied from 25.4% to 48.5% of SBMPFs applied to the rice paddy. It was observed that the mitigation of CO 2 increased with the application of BMPFs, and the highest was 5.09 tonnes C ha −1 in the BMP-UK. The profit under SBMPFs application was estimated to range from $6.56 ha −1 to $68.80 ha −1 during rice cultivation for KAU. The target of the Korean government is to reduce greenhouse gas emissions by 1.48 million tonnes CO 2 -equiv. (5.2%) of the 28.49 million tonnes CO 2 -equiv. total greenhouse emissions in the agricultural sector by 2020 [55]. Therefore, it is estimated that the 482,085 ha −1 (29.3%) of 1,644,000 ha −1 total area of rice cultivation with the BMP-NPK application in Korea [56] is required to accomplish this goal.
In order to establish carbon trading in the agriculture sector, policymakers should prepare a draft policy specifically for mitigating greenhouse gas emissions by providing support to farmers of about $58 per hectare of cultivated rice paddy through the application of BMP-NPK. The application of BMPFs did not only increase carbon storage, but also enhanced rice yield and soil fertility [38].

Rice Growth Responses to Supplemented Biochar Manure Pellet
Growth responses to the application of SBMPFs are shown in Table 6. The plant height in BMP-U60 was 15.2% higher than the control, and rice yield in the BMP-U60 was increased by 15.7% compared with the control, even when the application amount of pig manure compost applied was reduced to about 1000 kg ha −1 . This result might be due to the enhanced nutrient use efficiency under application of BMPFs functioning as a slow release fertilizer. Min et al. [4] reported that supplemented BMPFs application enhanced rice yield. Shin et al. [38] also reported similar results in their study. With the whole basal application of SBMPFs in the rice field prior to rice transplanting, it could prevent additional fertilizer application. Puga et al. [57] conducted similar research to evaluate the effects of biochar-based N fertilizers on nitrogen use efficiency (NUE) and maize yield. Their results showed that an average maize yield was increased 26% in the application of biochar-based N fertilizers (51% biochar with 10% N) compared with urea only treatment, and the NUE was 12% improved. Pokharel and Chang [58] also reported that manure pellet with wood chip biochar significantly increased plant grain yield by 36.3 and 16.1%, compared to the control, while woodchip with biochar applications significantly decreased plant grain yield.

Conclusions
Different supplemented biochar manure pellet fertilizers were tested to assess their agroenvironmental impacts on paddy water and soil systems during rice cultivation. With regard to the water quality of paddy, the NO 3 − -N and PO 4 − -P in control and PMCP were relatively higher than those of the SBMPFs applied plots. Non-point pollutants in runoff water to small stream near the rice cultivation area were reduced with application of SBMPFs. Considering the soil chemical properties, NH 4 + -N concentration in control was lower compared with the SBMPFs treatment at 41 days after rice transplant. However, the available P 2 O 5 concentrations were almost stage-state among all the treatments from 21 days after rice plant until the harvest period, except for the first day of rice transplant in the PMCP. It is possible that the SBMPFs can be applied with whole basal application without additional application of chemical fertilizers. Also, the highest carbon sequestration was 2.67 tonnes C ha −1 in BMP-UK treatment, and the lowest was 1.14 tonnes C ha −1 in the BMP-U60 treatment. The grain yields from the SBMPF applied plots, except for BMP-UK, were significantly higher than the yield from the control even though amounts of pig manure compost applied were decreased from 1881.8 kg ha −1 to 2070.8 kg. Therefore, the application of SBMPFs can contribute to reducing the agro-environmental impacts of runoff as well as enhance carbon sequestration and rice yield in agro-ecosystems.