Introduction

Recent decades have seen a rise in global warming, a long-term phenomenon resulting in increasing average temperatures of the air and the ocean on the surface of the earth. Future generations are going to see an exacerbation of the temperature. The main reasons for global warming are increasing emissions of CO2, CH4, and N2O due to various human activities. Especially, CH4 and N2O are important greenhouse gases; CH4 and N2O contribute to 25 times and 298 times the greenhouse effect by CO2, respectively, and 50% and 60% of greenhouse gas are generated by agricultural activity [1, 2]. Hence, these causes require to be controlled.

Rice (Oryza sativa L.) is one of the important cereal resources worldwide. It is differently cultivated under the soil, climatic, and hydrological conditions, and is generally managed under flooded conditions in paddy fields [3]. Sufficient land is required for cultivation since more than 50% of the global population consumes rice. However, paddy fields are subject to unique problems [4, 5]. During the rice cultivation period, rice paddies are constantly flooded, and most of the greenhouse gas emitted from rice paddies is CH4 [6]. CH4 generation is affected by soil organic matter, pH, temperature, and soil physical properties [7]. It is reported that the causes of N2O generation include anaerobic degradation of organic material, kind of crops, and soil characteristics; moreover, fertilizers also contribute greatly towards the emissions [8]. South Korea is one of the countries consuming the maximum inorganic fertilizer per unit area of farmland in the world, thereby generating great amounts of N2O. Hence, alternate methods that decrease the CH4 and N2O emissions during rice cultivation in rice paddies, are the need of the hour [9,10,11].

Along with global warming, by-products discarded after crop harvest results in environmental pollution [12]. Studies investigating the composting or bio-oil for recycling and utilizing of by-products are therefore being conducted. However, every by-product produced per year is not disposable, and most are either incinerated or neglected.

South Korea produces large quantities of barley straw after barley harvesting. Decomposition of the barley straw in the soil is an important organic resource to improve the fertility of the soil, and efficiently improve the physical condition [13]. Moreover, decomposition of the barley straw is reported to improve crop growth through delivering various nourishments. However, when the barley straws are resolved farm working such as plowing and rice planting work would be inconvenient and if the straws do not decompose enough, rice growth of early-stage would bring inhibition due to organic acids and nitrogen starvation situation in the paddy field [14, 15]. This phenomenon occurs when the incorporation of the barley straw into soil coincides with the time of rice transplanting. Additionally, since burning barley straws results in air contaminants, proper management of barley straw is required for stable rice production in paddy fields [16, 17].

Biochar contributes an ideal method that simultaneously recycles agricultural waste, increases crop yield, and reduces GHG emissions from paddy fields. Hence, if barley straws are converted into biochar and subsequently applied to rice paddies, it could help improve the soil physical property and rice growth with reduction of CH4 and N2O, and would also be an ideal management method [18, 19]. Moreover, biochar application into the soil can help organic farming by excluding inorganic fertilizers and pesticides, a preferred method for increasing eco-friendly agriculture and reducing the environmental load and concerns of safe farm products.

This study, therefore, focuses on biomass utilization, biomass recycling, crop yield, soil fertility, and greenhouse gas emissions. We also aim to evaluate the effect of combined biochar and barley straw on rice yield, and soil properties, and the CH4 and N2O emissions in paddy fields.

Materials and methods

Raw materials

The properties of raw soil collected from the topsoil are presented in Table 1. The raw soil comprises sandy loam soil and has a bulk density of 1.31 Mg m−3, porosity 56.3%, pH 5.87, and CEC 6.64 cmolc kg−1.

Table 1 Soil properties of the experimental site used in this study
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Barley straw was used to produce biochar as a raw feedstock. The biochar was produced at Sunchon National University in a covered stainless container under oxygen-limited conditions and pyrolyzed in a furnace (DK-1015(E), STI tech, Gumi, Republic of Korea). The container was purged with N2 gas to provide limited oxygen before insertion into the furnace. The furnace controller was programmed to drive the internal biomass chamber to a temperature of 400 °C at a rate of 3 °C min−1, after which the peak temperature was sustained for 1 h. The barley straw biochar (BC) was ground and filtered through a 2 mm sieve for the field experiment. The pH and nutrient contents of BC used in the experiment were pH 7.72, 0.42% TN, 0.30% P2O5, 1.41% K2O, and 21.7 cmolc kg−1 CEC; the BC mostly comprised C (70% >). The OM, TN, P2O5, and K2O contents of BS were 93.2%, 0.27%, 0.11%, and 2.34% respectively (Table 2).

Table 2 Chemical characteristics of biochar and barley straw
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Experimental design

The field experiment to evaluate growth characteristics of rice, soil properties, and changes of greenhouse gas emission on biochar applications under paddy (34° 56ʹ 33ʺ N, 127° 33ʹ 56ʺ E) conditions was conducted at Sepung-ri, Gwangyang-eup, Gwangyang-si, Jeollanam-do, South Korea. Figure 1 shows the temperature and precipitation during the experimental period in the above area.

Fig. 1
figure 1

Mean daily precipitation and air temperature during rice cultivation

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The field experiment was conducted during the rice cultivation season of 2015. Each treatment was separated by a Cn, IF, BC, BS, and BC + BS treatment, respectively (Table 3). BC was applied at 2000 kg ha−1, whereas IF application was conducted using the Soil Management and Fertilizer Recommendation provided by the Rural Development Administration (RDA), Korea. Inorganic fertilizers were applied N–P2O5–K2O = 50–45–40 kg ha−1 as basal application. First topdressing (20 kg N ha−1) was spread about 3 weeks after rice transplanting and second topdressing (20 kg N ha−1 and 17 kg K2O ha−1) on 8 weeks after rice transplanting. BC and BS were evenly spread by a tractor in the field, and incorporated into the soil at a depth of 15 cm, 14 days before rice transplanting. The rice seedlings were transplanted with 20 cm spacing on 31 May, and rice harvesting was conducted on 30 September.

Table 3 Treatment conditions used in this experiment
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Sampling and analysis on plant and soil

To enable sampling of rice plants, one area of each treatment was randomly selected, excluding the edge effects. Rice yield and growth parameters, including culm length, panicle length, number of panicles per hill, number of grains per panicle, 1000 grain, and straws obtained from all treatments were measured at the rice harvesting stage (30 September). The harvested rice plants were then divided into four aliquots: stem, leaf, rice bran, and brown rice. These were dried in a dry oven at 70 °C for 2 days, after which the characteristics of nutrient uptake (TN, TP, and K) were determined, as described in NIAST [20]. Total nutrient contents were calculated as the sum of N, P2O5, and K2O contents. Briefly, the collected rice plant samples were digested with H2SO4 + HClO4, following which the TN and TP analyses were carried out using the Kjeldahl method and UV spectrophotometry, respectively. The K value was analyzed using an inductively coupled plasma-atomic emission spectrometer (ICP-AEC, Optima 3300EV, Perkin-Elmer, Waltham, MA, USA).

After rice harvesting, soil samples were collected from the surface layer (15 cm depth) in each treatment group, air-dried, and passed through a 2 mm mesh. The chemical properties of the sieved soils were analyzed by different standard methods. The soil pH and EC were measured using a soil–water ratio of 1:5, after shaking the mixture for 30 min. Organic matter (OM) and total nitrogen (TN) analysis were performed using the Tyurin method and Kjeldahl method, respectively. Available phosphorus (Avail. P2O5) was calculated by the Lancaster method, and the exchangeable cations in soil were extracted by 1 N-NH4OAc. At the end of the experiment, the bulk density and porosity of soil were measured at different random places in each field, using the core method. In this study, soil analysis was achieved as described in NIAST [20].

Monitoring of CH4 and N2O

The CH4 and N2O fluxes were monitored through a static chamber. Gas sampling in the paddy field was performed between 1 and 2 p.m. every 7 days. Gases for measuring CH4 and N2O were collected at 0, 10, 20, and 30 min after chamber closure, using a syringe. The measurements of CH4 and N2O were simultaneously analyzed on a gas chromatograph (GC-2014, Shimadzu) with a flame ionization detector (FID) and an electron capture detector (ECD), respectively, as described in the GC manual. The temperature of the equipment for gas analysis was controlled at 55 °C for a column, 100 °C for injector, and 230 °C for detector in FID, and at 50 °C for column and 310 °C for detector in ECD. Carrier gases were used with a gas mixture of argon and methane for CH4, and nitrogen gas for N2O. Fluxes of CH4 and N2O were calculated using the following equation [21]:

$${\text{F }} = \, \rho \, \times \, \left( {{\text{V}}/{\text{A}}} \right) \, \times \, \left( {\Delta {\text{c}}/\Delta {\text{t}}} \right) \, \times \, \left( {{273}/{\text{T}}} \right)$$

where F is CH4 and N2O flux, ρ is CH4 and N2O density, V is the volume of the chamber (m3), A is the area of the chamber (m2), Δc × Δt is an average increase of gas concentration, and T is 273 + mean temperature in the chamber (°C).

The total CH4 and N2O fluxes for the entire rice cultivation were computed as described by Singh et al. [22]:

$${\text{Total CH}}_{{4}}\, {\text{and N}}_{{2}} {\text{O flux = }}\sum\limits_{i}^{n} ({\mathrm{Ri }\times \mathrm{ Di})}$$

where Ri is the rate of CH4 and N2O emission in the sampling interval, Di is the number of days in the sampling interval, and n is the number of sampling intervals.

The potential for global warming and greenhouse gas intensity were calculated as follows:

$${\text{Total Global warming potential }}\left( {{\text{g CO}}_{{2}} {\text{m}}^{{ - {2}}} } \right) \, = {25} \times {\text{ TF }}{-}{\text{ CH}}_{{4}} + {298} \times {\text{ TF }}{-}{\text{ N}}_{{2}} {\text{O}}$$
$${\text{Greenhouse gas intensity }} = {\text{ GWP}}/{\text{Y}}$$

The effect of BC on global warming potential (GWP) is described in the IPCC [23] as the total flux (TF) of CH4 and N2O over the rice cultivation, and greenhouse gas intensity (GHGI) using GWP and rice yield (Y) is described by Zhang et al. [11].

Statistical analysis

Statistical analyses of all data were performed using SPSS Version 22. The mean values were measured as an average of three replicates. Each mean value was subjected to analysis of variance (ANOVA) and a comparison of the treatments was performed with Duncan’s multiple range test (DMRT) at 5% probability.

Results

Growth characteristics and nutrient content of rice plant

The results related to rice growth are presented in Tables 4 and 5. In general, rice growth was best in the BC + BS treatment group. The culm length of rice plants was determined to be 54.3, 67.0, 66.7, 61.7, and 66.5 cm in Cn, IF, BC, BS, and BC + BS treatments, respectively, which showed no difference between treatments excluding the Cn group. The panicle length of the rice plant showed similar measurements as the culm length. The number of panicles per hill and number of grains per panicle for all treatments ranged from 10.8 ~ 12.2 and 89.6 ~ 99.8 ea respectively, and were significantly not different. The straw and yield of rice plants were generally higher in the order of BC + BS, IF, BC, BS, and Cn treatments.

Table 4 Growth characteristics of rice by biochar application
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Table 5 Nutrient contents of harvested rice plant
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The TN, TP, and K contents of the rice plant in the different sections are presented in Table 5. The TN content measured was in the order of leaf > brown rice > stem ≧ rice bran. The TP content was highest in brown rice, and the K content in all treatments ranged from 0.96 ~ 1.12% for stem, 0.62 ~ 0.72% for leaf, 0.69 ~ 0.83% for rice bran, and 0.13 ~ 0.17% for brown rice.

Soil physicochemical properties

The physicochemical properties of soil after rice harvesting are presented in Fig. 2. BC application generally and significantly improved the conditions of raw soil, as compared to soil BC treatment. The bulk density and porosity of soil after rice harvesting were affected by BC and BS application. The bulk density of soil markedly decreased by 0.08 ~ 0.09 Mg m−3 in the BC treatments groups, as compared to Cn and IF treatments. The porosity of the soil in BC, BS, and BC + BS treatments was higher (range, 2.38 ~ 3.31%) than determined in other treatments. Ranges of soil chemical properties after rice harvesting are as follows: pH 5.89 to 6.20, EC 0.18 to 0.25 dS m−1, OM content 15.8 to 18.7 g kg−1, TN content 1.44 to 1.72 g kg−1, Avail. P2O5 content 57.2 to 61.7 mg kg−1, CEC 6.71 to 7.43 cmolc kg−1.

Fig. 2
figure 2

Changes in soil chemical characteristics after rice harvesting for each tested treatment group (*Means by the same within a column are not significantly different at 0.05 probability level according to Duncan’s Multiple Range Test)

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Changes in CH4 and N2O emission rates

The change in CH4 emission rate varied significantly by BC application and sampling dates (Fig. 3). The CH4 emission rates with and without BC treatments respectively ranged from 6.16 ~ 6.20 and 11.4 ~ 12.2 mg m−2 h−1 at 7 days, 9.34 ~ 12.5 and 15.5 ~ 16.5 mg m−2 h−1 at 14 days, and 27.6 ~ 28.0 and 41.4 ~ 44.7 mg m−2 h−1 at 21 days after transplanting. Thereafter, CH4 emission rates in Cn, IF, BC, BS, and BC + BS treatments were maintained in the range of 6.34 ~ 19.7, 5.32 ~ 23.9, 1.32 ~ 14.8, 4.54 ~ 20.5, and 1.45 ~ 13.4 mg m−2 h−1, respectively, until 126 days after transplanting. In particular, a minimum CH4 emission rate was observed at the rice harvesting stage for all treatments, due to temporary aerobic conditions. Thus, the pattern of CH4 emission rate for all treatments increased rapidly until 21 days after transplanting, and thereafter gradually decreases until the rice harvesting stage.

Fig. 3
figure 3

Changes in CH4 and N2O emission rate during rice cultivation

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Changes in the rate of N2O emission also varied significantly by amounts of inorganic fertilizer added and sampling dates (Fig. 3). The N2O emission rate ranged between 6.03 ~ 174 ug m−2 h−1 (average 54.6 ug m−2 h−1) for Cn, 94.7 ~ 653 ug m−2 h−1 (average 304 ug m−2 h−1) for IF, 13.8 ~ 353 ug m−2 h−1 (average 81.5 ug m−2 h−1) for BC, 39.1 ~ 322 ug m−2 h−1 (average 123 ug m−2 h−1) for BS, and 36.5 ~ 247 ug m−2 h−1 (average 106 ug m−2 h−1) for BC + BS treatment during rice cultivation. Generally, the peak of N2O production was significantly affected by inorganic fertilizer application. Thus, the pattern of N2O emission rate varied as per the widely different treatments applied.

Discussion

Biochar or organic material application to crop fields is recommended for improving plant growth and multifaceted functions of the soil [24,25,26,27,28]. Our study, our findings indicate that combined BC + BS application promotes rice yield, improves soil fertility, and decreases fluxes of CH4 and N2O.

Biochar is a highly heterogeneous material [29], and its application to soil is known to alter the physical [30], chemical [31], and biological [32] properties of soil. Moreover, biochar application contributes to the increased availability of nutrients for crop growth [33], and is a direct source of nutrients for plant uptake [34]. These studies have indicated a positive effect of biochar application for increasing crop yield in agricultural fields. For example, Gwenzi et al. [35] reported that biochar application improves maize growth, and Liu et al. [36] reported that rapeseed and sweet potato yields are increased after biochar treatments (40 t ha−1) (36.0% and 53.8%, respectively) as compared to their controls. Furthermore, several researchers have determined multiple applications of biochar and nutrients (use as fertilizer, and organic matter to improve crop growth) due to the high nutrient retention capacity of biochar [28, 37,38,39]. In the present study, the combined BC and BS exert the maximum effect on the rice yield and yield components in the paddy field. In particular, rice yields are significantly increased by the BC + BS treatment (increments of 8.5% for IF, 3.3% for BC, 2.0% for BS, and 15.1% for BC + BS), relative to the Cn treatment (Table 3). This is in agreement with previous studies: Liu et al. [40] reported 8.5 ~ 10.7% increased rice yield in rice straw biochar application; Zhang et al. [11] showed that rice yield in biochar treatments of 10 t, 20 t, and 40 t ha−1 increased in the range 9.2–27.6% during two cycles; Zhang et al. [41] also reported that biochar treatments (10 t and 40 t ha−1), regardless of N fertilization, improve the rice yield by 9–14%, compared to their control treatment. Moreover, Zhang et al. [42] and Thammasom et al. [43] reported that straw incorporation in rice paddy is effective in increasing rice yield due to improve soil fertility through decomposition of organic matter. The results from the current study observed higher inorganic element contents of rice plants in BC + BS treatment through the improvement of soil pH and CEC after biochar application, and supply of nutrients by adding straw (Fig. 4).

Fig. 4
figure 4

Distribution of inorganic element contents by parts of a rice plant with different treatments. (*,**,***,****Means by the same within a column are not significantly different at 0.05 probability level according to Duncan’s Multiple Range Test)

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The important physical properties of soil include compaction, aggregation, pore, air, and water content [15, 44, 45]. Bulk density and porosity are important indicators of the physical status of the soil. In this study, the results obtained in field experiments indicate that BC and BS application significantly affect the soil's physical properties. Biochar or organic materials such as straw, green manure, and compost are known to effectually improve the soil as well as crop productivity [46,47,48]. This could be attributed to the low density of BC and BS relative to the soil, thereby resulting in a direct dilution effect [49]. The soil diluted by BC or BS may change the internal structure and surface area of soil due to increased soil porosity, thus improving the flow of water and air, and also the aggregate stability [50, 51]. Tan et al. [45] reported that bulk density of soil treated with 200 g kg−1 biochar decreased by 20.45% as compared to control treatment. Blanco-Canqui [52] reported the results of another researcher that biochar application reduces the bulk density by 12% on an average, regardless of soil condition and biochar rate. Moreover, Thammasom et al. [43] reported that biochar and straw application significantly decrease bulk density in paddy soil. Similar to the previous reports, our study also showed that the application of BC and BS lowers the bulk density of soil and soil porosity.

Soil chemical properties in paddy fields were generally improved by BC and BS application, with significantly greater changes observed after BC treatments as compared to BS treatment. BC application showed significant increases in pH and CEC as compared to the Cn and IF treatments. In particular, improvement of soil chemical factors was more effective with BC + BS treatment than other treatments. This phenomenon may be attributable to the higher pH and CEC of BC than soil, and nutrients supplied by BS. These findings are in agreement with the reports of Tan et al. [45], which reveal that soil pH by biochar application increases by 0.61–0.85 units as compared to control soil under different soil types. Liu et al. [40] also reported over 2 seasons that the pH of applied BC soil in a paddy field increases by 0.07–0.18 units compared with their control soil. Butterly et al. [53] observed an increase in soil pH due to the incorporation of higher alkalinity material. Thammasom et al. [43] reported that CEC of soil using biochar and straw application significantly increases than control soil because of mineralization of inorganic elements through decomposition of biochar and straw. It has also been reported that the pH and CEC in soil are closely related [54,55,56,57]. Considering the above, we determined that rice yield in BC + BS treatment is higher than other treatments due to improved nutrient availability by increasing soil fertility.

In paddy fields, GHG emission is affected by various environmental factors such as microbial activity, aeration, temperature, fertilizer, and resource of nutrient supply [7, 58,59,60]. Our study assessed the positive effect of biochar application with barley straw on CH4, N2O, GWP, and GHGI (Table 5). The total CH4 fluxes from the rice cultivation field were 42.0, 41.0, 25.2, 38.8, and 25.7 g m−2 after treatment with Cn, IF, BC, BS, and BC + BS, respectively. These results indicate that treatments with BC application lowers the CH4 emission more than treatment without BC. We further determined that the reduction efficiency of CH4 fluxes by BC application was 33.8–40.0% more than soils treated with Cn, IF, and BS. The total N2O fluxes were higher in the order IF (0.92 g m−2) > BS (0.37 g m−2) ≧ BC + BS (0.32 g m−2) > BC (0.25 g m−2) > Cn (0.16 g m−2). We found significantly decreased fluxes of CH4 and N2O under different applications of BC. Several studies have previously reported the advantage of biochar application on emissions of CH4 and N2O during crop cultivation [10, 56, 57, 61,62,63,64,65,66].

As shown in Table 6, GWP was calculated using total CH4 and N2O fluxes obtained from results during rice cultivation. In declining order, GWP in tested soils were IF (13.0 ton CO2 ha−1) > Cn > BS > BC + BS > BC treatments. BC application decreases both, mean CH4 and N2O, emissions in the rice field. The BC and BC + BS treatments decrease the GWP by 32.9–36.1%, 43.2–45.9%, and 31.7–34.9% when compared to Cn, IF, and BS, respectively. Benefits of biochar application include restriction of methanogen activity [9], increasing CH4 oxidation, soil pH and aeration [60, 67], increased denitrification rate [64], enhancement of the last step from N2O to N2, the activity of N2O sorption surface [68], and inhibition of N2O production [69]. These mechanisms are commonly involved in improving soil physical properties after biochar application. Our results also revealed that GHGI (GWP/Y) is dependent on the application of BC and BS; we determined the highest rice yield and decreased CH4 and N2O emission in BC + BS treatment during the rice cultivation period.

Table 6 Total fluxes of CH4 and N2O, GWP, and GHGI during rice cultivation
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In our experiment, the combined treatment of BC and BS is more effective than the individual application of IF, BC, and BS treatments on rice yield and GHGs emission. Taken together, our results suggest that BC application with BS is a potentially useful cropping system for environmental-friendly agriculture and global warming mitigation in the rice paddy.