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Article

Rice Yield and Greenhouse Gas Emissions Due to Biochar and Straw Application under Optimal Reduced N Fertilizers in a Double Season Rice Cropping System

by
Dandan Li
,
Hao He
,
Guoli Zhou
,
Qianhao He
and
Shuyun Yang
*
School of Resource and Environment, Anhui Agriculture University, Hefei 230036, China
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(4), 1023; https://doi.org/10.3390/agronomy13041023
Submission received: 12 January 2023 / Revised: 14 March 2023 / Accepted: 29 March 2023 / Published: 30 March 2023
(This article belongs to the Special Issue How to Achieve Carbon Neutrality in Agroecosystem?)
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Abstract

:
This study aimed to investigate the impacts of straw and biochar on greenhouse gas (GHG) emissions and grain yield in a double rice cropping system under optimal N fertilizer reduction. Conventional fertilization (CF) was used as the control group, and treatments included optimal fertilization and 15% less nitrogen (OF), together with straw (S) or biochar (B) applied under different fertilization conditions, namely CF + S, CF + B, OF + S, and OF + B. The effects of treatments on soil CH4 and N2O emissions were studied, and changes in soil physicochemical properties were analyzed. The results showed that relative to CF, CF + S and OF + S increased the cumulative CH4 emissions by 11.80% and 2.35%, respectively, while CF + B and OF + B resulted in significant reductions in cumulative CH4 emissions by 27.80% and 28.46%, respectively. Biochar was effective in reducing N2O emissions, and OF further increased the potential, with CF + B and OF + B achieving the best N2O reductions of 30.56% and 32.21%, respectively. Although OF reduced yields by 0.16%, this difference was within reasonable limits; the remaining treatments increased grain yields by 2.55% to 3.47%. CF + B and OF + B reduced the global warming potential (GWP) by 27.93% and 28.63%, respectively, and ultimately reduced the greenhouse gas emission intensity (GHGI) by 30.42% and 30.97%. Both straw and biochar increased the soil organic matter, NH4+-N, and NO3−-N contents, and biochar increased the soil pH, which may be the potential mechanism regulating soil GHG emissions. Overall, OF + B is beneficial for reducing GHG emissions and may be a better agronomic cropping pattern in double season rice growing areas.

1. Introduction

Global warming has become a worldwide issue with serious implications for human life. Methane (CH4) and nitrous oxide (N2O) are important greenhouse gases (GHGs), which contribute 17% and 6%, respectively, to the increase in global radiative forcing [1]. Agricultural production is one of the dominant sources of anthropogenic greenhouse gases, accounting for 51% and 58% of anthropogenic CH4 and N2O, respectively [2,3]. Rice is one of the world’s leading cereal crops and is a staple food for more than half of the world’s population [4]. Notably, large amounts of GHGs are generated during rice cultivation, accounting for 20% and 10% of the total CH4 and N2O emissions, respectively, from agricultural production [4,5]. The double rice cropping system is one of the main rice cropping patterns in East and Southeast Asia, including early and late rice seasons each year, which can make full use of land resources and contribute greatly to food security [6,7]. In general, the GHG emissions per unit area of double-cropping rice fields are also significantly higher than those of other rice cultivation patterns [8]. Under the general trend of global climate change, how to reduce paddy soil GHG emissions under the premise of ensuring grain yield is still a scientific issue that needs to be explored.
China produces approximately one quarter of the world’s straw per year, and the excess straw is discarded or even burned, which causes a huge waste of resources and air pollution [9,10]. As crop straw is rich in organic matter, straw return is a widely used agronomic practice that can improve soil fertility and crop yield and quality [11]. However, it cannot be ignored that straw provides sufficient carbon substrate for methanogens, which will significantly promote CH4 emissions in paddy fields [12]. Yao et al. [13] reported that straw of 4.7 t ha−1 increased cumulative CH4 emissions from paddy soils by 74%. Similarly, Lee et al. [14] reported that rice straw increased soil organic carbon stocks and increased rice yields by 7–26%, but CH4 emissions increased by 27–263% compared to straw removal. However, for the double-cropping rice system, the results of the effect of straw application on N2O emissions from paddy soil are inconsistent, and the impact of a reduction or increase needs to be further evaluated [13,15,16]. In recent years, research on biochar has provided a new way to recycle agricultural straw resources. Due to its large specific surface area, strong adsorption force, and high structural stability, biochar is widely used in soil improvement and GHG emission reduction research [17,18,19]. The application of biochar can improve the porosity of soil planted to rice, destroy the extreme anaerobic environment required by methanogenic bacteria, and reduce paddy soil CH4 emissions by 9–21% [20]. Zhang et al. [21] found that biochar increased soil pH, weakened nitrification, and significantly inhibited N2O emissions from wheat fields by 38–48%. At present, the impacts of straw or biochar application on GHG emissions from double rice cropping systems and the differences between early and late rice remain unclear.
The application of nitrogen fertilizer ensures the yield and quality of rice, but according to statistics, the amount of nitrogen fertilizer applied in China is 3.12 times the global average, and excessive nitrogen fertilizer may increase GHG emissions [22,23]. The soil N and C substrates provided by fertilization ensure sufficient reaction substrates for methanogens while promoting the activity of microorganisms involved in nitrification and denitrification processes and stimulating soil CH4 and N2O emissions [24,25]. Interestingly, different nitrogen application amounts combined with straw or biochar may effectively promote nitrogen utilization and crop yield and quality [26,27]. Tian et al. [28] reported that the application of biochar has a positive effect on the growth and yield of rapeseed and can replace 20% of nitrogen fertilizer. Reduced nitrogen fertilizer (140.3 kg ha−1) combined with biochar application increased rainfed wheat yield and reduced soil cumulative N2O emissions by 7.57–12.93% [29]. However, the study results of Sun et al. [30] in salinizing paddy fields showed that the combination of reduced nitrogen and biochar application increased rice yield by 13.7–38.1% without a significant impact on CH4 emissions. Hence, under the premise of reducing the amount of nitrogen fertilizer, the application of straw and biochar has a wide range of promotion potential. However, the impact of this measure on CH4 and N2O emissions has not yet been demonstrated in a double rice cropping system; more importantly, it is necessary to investigate the intrinsic mechanistic links.
In light of these basic theoretical and practical deficiencies, a typical double-cropping paddy field in the middle and lower reaches of the Yangtze River was taken as the research object, the CH4 and N2O emission fluxes were continuously monitored during the rice growing season, and the grain yield was measured. Paddy soil at different stages was sampled, and the soil physicochemical properties were measured to explain their internal mechanistic relationship with greenhouse gas emissions. This information will contribute to the correction of the nitrogen application rate and straw or biochar application in double-cropping rice systems and is an emerging and attractive method to reduce paddy soil GHG emissions while increasing crop yields.

2. Materials and Methods

2.1. Site Description and Used Material

The experiment was conducted in 2022 at the Chao Lake Agricultural Environment Experimental Station in Chaohu County, Anhui Province, China (117°41’6” E and 31°39’50” N with an elevation of 17 m). The region has a typical subtropical monsoon climate, with an annual mean precipitation of 1046 mm and an average air temperature of 16.0 °C. The Chao Lake area is located in the center of the middle and lower reaches of the Yangtze River and is one of the main rice cultivation areas. Before the experiment, the soil type of this monitoring site in the Chao Lake lowland area was submerged paddy soil with a pH value (H2O) of 6.31, an organic carbon content of 24.20 g kg−1, a total nitrogen content of 1.29 g kg−1, and a physical clay content of 488 g kg−1.
The raw material of biochar was rice straw, which was carbonized under high-temperature anaerobic conditions at 500 °C. The ratio of straw to biochar obtained was 30:1. The pH value of the biochar was 9.04, and the elemental components contained comprised 505.5 g kg−1 of carbon, 434.2 g kg−1 of oxygen, 17.9 g kg−1 of hydrogen, 18.9 g kg−1 of nitrogen, 1.7 g kg−1 of sulfur, 8.9 g kg−1 of potassium, and 776.42 mg kg−1 of effective silicon.

2.2. Experimental Design and Field Management

Six treatments were carried out in this study as follows: conventional fertilization (CF), with a total of 225 kg ha−1 nitrogen fertilizer applied; conventional fertilization + straw return (CF  +  S); conventional fertilization + biochar application (CF  +  B); optimal fertilization and 15% less nitrogen (OF), with a total of 191.25 kg ha−1 nitrogen fertilizer applied; optimal fertilization and 15% less nitrogen + straw return (OF  +  S); and optimal fertilization and 15% less nitrogen + biochar application (OF  +  B).
The inbred indica rice varieties Zaoxian 310 and Wandao 90 were used as early (planted from April to July) and late (planted from August to October) rice, respectively. Three replicates were designed for each treatment, for a total of 18 plots with 40 m2 for each plot (5 × 8 m), and the rice was planted at a density of 20 cm per row and 15 cm apart. The N fertilizer, P fertilizer, and K fertilizer used were urea, superphosphate, and potassium chloride, respectively. For the CF treatment, the nitrogen application method was 5:2:3 (basal fertilizer 50% + tiller fertilizer 20% + panicle fertilizer 30%). For the OF treatment, the nitrogen application method was 4:4:2 (basal fertilizer 40% + tiller fertilizer 40% + panicle fertilizer 20%). Calcium superphosphate and potassium chloride were applied as basal fertilizers at 75 kg ha−1 each. Phosphate fertilizer was applied as a one-time basal fertilizer, and potash was used at a rate of 70% as a basal fertilizer and 30% as a panicle fertilizer. Specifically, straw return refers to mechanically crushed rice straw mixed into the soil at a depth of 3–5 cm, and approximately 7.5 t ha−1 was applied each season; the amount of biochar applied per rice season was 250 kg ha−1.
After transplanting, the water level of the paddy fields was maintained by natural precipitation and artificial irrigation. Approximately 7 days of mid-season drainage was used to suppress rice tillering, and conventional irrigation was applied until 1 week before rice maturity. Regular weed removal and pest control in paddy fields were needed to ensure the normal growth of rice. The specific management regime for the paddy fields during the study is shown in Table 1.

2.3. Measured Responses

2.3.1. Emissions of CH4 and N2O

The static chamber gas chromatography technique was used for GHG emission sampling in the paddy fields. The sampling device consisted of a box and a base. A fixed gas sample collection point was selected at the center of each plot at a distance of 4 m from the perimeter, and the base was placed to hold the sampling box. The sampling boxes were made of 5 mm thick transparent glass (50 × 50 × 60 cm and 50 × 50 × 120 cm). The first size of the box was used when the height of the crop was less than 60 cm. When the height of the crop plants exceeded 60 cm, a two-layer box was used. In short, the box was chosen according to the growth height of the crop. The bread outside the box was covered with sponge and foil to prevent excessive temperatures inside the box due to sunlight. The chamber was equipped with 2 small fans to mix the gas evenly and a thermometer to monitor the temperature inside the chamber. There were small holes on the side of the box to connect the rubber tube for easy gas extraction.
Sampling was started the day after rice transplanting, generally once every four days, and relevant environmental factors and weather conditions were recorded. Samples were collected once daily for one week after important agricultural events, such as fertilizer application and drainage. Emission fluxes of CH4 and N2O from rice were measured between 09:00 and 11:00 a.m., and the base was filled with water to ensure that the chamber was sealed prior to sampling. When sampling, 60 mL of well-mixed gas was extracted from the box and immediately transferred to a special glass tube for measurement. The collected gas samples were measured for CH4 and N2O concentrations by gas chromatography (Brooker 450-GC, Kempen, Germany) within 24 h.
The CH4 and N2O emission fluxes were calculated using the following formula:
F = ρ × V/A × dc/dt × 273/T
where F is the CH4 emission flux (mg m−2 h−1) or N2O emission flux (μg m−2 h−1); ρ is the density of CH4 or N2O in the standard state, with values of 1.25 kg m−3 (N2O) and 0.714 kg m−3 (CH4), respectively; V is the effective volume of the box, referring to the volume of the space from the paddy water surface to the top of the box (m3); A is the area of soil covered by the box (m2); dc/dt is the change in the CH4 or N2O concentration in the sampling box per unit time (μL L−1 h−1 (CH4) and nL L−1 h−1 (N2O)); and T is the temperature inside the box during the sampling period (K).

2.3.2. Calculation of GWP and GHGI

After the crop matured, the grain yield of rice was measured, and the average grain yield (Y, t ha−1) of three groups of repeated plots was randomly selected. The global warming potential (GWP, kg CO2-eq ha−1) and greenhouse gas intensity (GHGI) were used to compare the comprehensive impact of CH4 and N2O emissions at the same scale.
According to the Intergovernmental Panel on Climate Change (IPCC) 2014, CH4 and N2O oxide radiative forcing potentials are converted to CO2 equivalent factors of 28 and 265, respectively. The calculation formula is as follows:
GWP = cumulative CH4 emissions × 28 + cumulative N2O emissions × 265
GHGI = GWP Y−1

2.3.3. Soil Sampling and Analysis

A soil sample was taken from the 0–20 cm soil layer once per month using a stainless steel soil collector, and three samples from each treatment were combined into one composite sample. After sampling, the soil was dried and frozen, and the relevant soil physicochemical properties were determined. The soil organic matter (SOM) content was measured by an elemental total organic carbon analyzer (TOC-LCPH), and the soil pH was measured by the potentiometric method (water-soil ratio of 2.5:1). In this study, the effects of the different treatments on the soil nitrogen content are discussed, and the soil NH4+-N and NO3−-N contents were determined by a flow injection instrument (BDFIA-8000, China).

2.4. Statistical Analysis

The experimental data were statistically analyzed using Excel 2021 and SPSS version 21.0 (International Business Machines Corporation, Armonk, NY, USA), and all figures were created using Origin 2021. One-way ANOVA and multiple comparisons based on the least significant difference (LSD) at the 0.05 probability level were used to determine the significant effects of different treatments on the CH4 and N2O emission fluxes, GWP, GHGI, and soil properties.

3. Results

3.1. CH4 Emissions

As shown in Figure 1, in the complete double rice cropping system, the paddy soil CH4 emissions were mainly concentrated in the early stage and tillering stage, and the emissions were relatively small and stable in the late stage of rice growth. The peak CH4 emission flux of paddy soil under each treatment appeared on roughly the same date, and all occurred after nitrogen fertilization. During the drainage and drying of rice and the field sunning stage, extremely low CH4 emission fluxes close to zero were observed. The emission flux of CH4 from paddy soil under early rice and late rice ranged from 0.14 to 48.26 and 0.17 to 54.37 mg m−2 h−1, respectively.
Compared with CF, for early rice, the cumulative CH4 emissions of the CF + S and OF + S treatments increased by 15.48% and 7.26%, respectively (p < 0.05), and OF + B had the greatest effect on CH4 emission reduction, reaching 30.11% (Figure 2). For late rice, only CF + S significantly increased the cumulative CH4 emissions by 8.56% (p < 0.05), and CF + B reduced the cumulative CH4 emissions by 29.65% with the greatest emission reduction. For the whole double rice cropping system, the cumulative CH4 emissions were ranked as follows: CF + S > OF + S > CF > OF > CF + B > OF + B. Overall, straw return increased the CH4 emissions from the paddy soils, while both the optimal reduction in N fertilizer application and biochar application reduced the CH4 emissions.

3.2. N2O Emissions

Multiple peaks of N2O emissions were evident throughout the rice reproductive period, mainly concentrated in the early and middle rice reproductive periods (Figure 3). The peak N2O emission fluxes occurred after each fertilizer application, and similarly, high N2O emission fluxes were observed at the early stage of paddy field drainage and during the reirrigation period.
All treatments significantly reduced the cumulative N2O emissions from early (7.44–30.46%) and late rice (18.57–37.07%), thus reducing the cumulative N2O emissions from the entire double rice cropping system by 12.67–32.21% (Figure 4). CF + S reduced the cumulative N2O emissions of the double rice cropping system by 12.67%, with 7.44% and 18.57% reductions in early and late rice, respectively. The addition of biochar suppressed N2O emissions from rice soils, with 30.56% and 32.21% reductions with CF + B and OF + B, respectively. Notably, although OF had a significant effect on reducing N2O, under the premise of OF, the application of straw or biochar further reduced the N2O emissions. For early rice and late rice, CF + B and OF + B had the best emission reduction effect, i.e., 30.46% and 37.07%, respectively.

3.3. GWP, Yield, and GHGI

CH4 emissions were the main contributors to the GWP generated by GHGs in the double-cropping rice system, and paddy soil CH4 accounted for 95.73–96.97% and 96.51–97.03% of the total GWP of early rice and late rice, respectively (Table 2). Overall, CF + B and OF + B were the most effective in reducing the GWP produced by GHG emissions in the double rice cropping system, by 27.93% and 28.63%, respectively.
Compared to CF, OF reduced grain yield by 0.16% in the double rice cropping system because it reduced the grain yield of late rice. Otherwise, each treatment increased the grain yield in both early and late rice, with CF + B and OF + S resulting in better grain yield increases in early rice (4.29%) and late rice (3.48%), respectively. The GHGI showed the comprehensive effects of different treatments on crop yield and GHG emissions. Although CF + S increased the grain yield of rice, it increased the GHGI of the double rice cropping system by 7.92% due to its significant increase in CH4 emissions. CF + B and OF + S reduced the GHGI the most, and there was no significant difference between them (p > 0.05).

3.4. Soil Physicochemical Properties

The changes in the main soil physicochemical properties during the cultivation of the whole double rice cropping system are shown in Figure 5. The mean paddy soil pH values varied between 6.38 and 6.50 (early rice) and 6.41 and 6.53 (late rice) in each treatment, and the addition of biochar increased the paddy soil pH; the mean values of CF + B and OF + B were 6.50 and 6.50 (early rice season) and 6.49 and 6.53 (late rice season), respectively. Relative to CF, OF had the potential to reduce soil organic matter, which was reduced by 3.69% and 1.97% in early rice and late rice, respectively. Notably, the addition of straw and biochar under both the CF and OF treatments increased the soil organic matter content in the early and middle stages of rice growth.
The trends of the soil NH4+-N and NO3−-N contents in the paddy fields were similar, with fertilization resulting in a peak in the soil nitrogen content. The range of variation in the soil NH4+-N content for each treatment was 6.49–36.87 (early rice) and 11.23–43.50 (late rice) mg kg−1, and the range of variation in the soil NO3−-N content was 0.43–2.37 (early rice) and 0.52–2.29 (late rice) mg kg−1. The soil NH4+-N and NO3−-N contents increased slightly after the initial transplanting of rice, then increased rapidly after the application of tillering fertilizer and ear fertilizer, and finally fluctuated but generally showed a downward trend.

4. Discussion

There were obvious seasonal changes in the CH4 emissions from the double-season paddy fields, and the peak CH4 emissions were mainly concentrated in the tillering and heading stages. During the rice tillering stage, vigorous rice growth led to well-developed aerenchymas, so a large amount of CH4 was emitted to the atmosphere [31,32]. Meanwhile, the root system grew vigorously and produced a large amount of root exudates, which provided a rich substrate for CH4 production [33]. During the mid-field sunning period, the water was drained, resulting in the destruction of the anaerobic environment in the paddy field; the activity of methanogens was reduced; the CH4 emission flux was close to zero; and the CH4 emissions remained relatively low even after reirrigation [34].
Under conventional fertilization conditions, straw and biochar application to the paddy fields resulted in a respective increase and decrease in soil CH4 emissions. The addition of organic matter, including straw and biochar, increased the soil organic matter content, and related studies have demonstrated a significant positive correlation between the soil organic matter content and soil CH4 emissions [19,35,36]. Straw decomposes in the anaerobic environment of flooded paddy fields, providing additional reactive carbon metabolic substrates for methanogenic microorganisms [36]. This also explains the higher cumulative CH4 emissions observed in late rice than in early rice, probably due to the residual straw roots, which is consistent with the results of Shen et al. [37] and Zhang et al. [38]. Meanwhile, the application of nitrogen fertilizer significantly increased the soil NH4+-N and NO3−-N contents, further increasing the nitrogen substrate required by soil methanogenic bacteria [39]. Conversely, the porous structure of biochar increased soil porosity, allowing easy access to oxygen while increasing the soil pH, which disrupted the strictly anaerobic as well as acidic environment needed by methanogenic bacteria to survive [40]. Optimal nitrogen reduction fertilization promoted the reduction in CH4 emissions from biochar-applied soils and weakened the increase in soil CH4 by straw retention, thus showing that the change in the soil carbon to nitrogen ratio was the dominant factor regulating soil CH4 emissions.
N2O emissions are closely related to fertilizer application, soil properties, and field management practices and largely determine the process of nitrification and denitrification. In the double rice cropping system, N2O emissions peaked after N fertilizer application or at the field sunning stage but remained low in other periods. Nitrogen fertilizer provided a large amount of substrate for nitrifying and denitrifying microorganisms, and soil aeration was good immediately after the rice was transplanted, resulting in high N2O emissions [41,42]. Soil organic matter provided the reaction substrate for denitrifying microorganisms to reduce nitrate nitrogen to produce N2O; therefore, straw or biochar with nitrogen fertilizer stimulated denitrification by increasing the soil organic matter and nitrate nitrogen contents [42]. Meanwhile, the elevated soil organic matter content promoted soil microbial respiration, accelerated soil O2 consumption, and accelerated the formation of the soil anaerobic environment, which in turn stimulated denitrification [43]. Optimizing the reduction in N fertilizer application effectively weakened this process, which is similar to the results of related studies, such as the control of N fertilizer as the dominant factor regulating soil N2O emissions [42,44].
Moreover, this study showed that paddy soil N2O emissions were lower with biochar application than with straw return under both CF and OF. This could be attributed to the following: Firstly, the carbon to nitrogen ratio (C/N) of biochar was higher than that of straw, which showed a relative excess of carbon sources and a relative lack of nitrogen sources, resulting in the uptake of additional nitrogen sources by soil microorganisms to satisfy their own growth and reducing the substrate for nitrification and denitrification [21,45]. Secondly, the adsorption of substrates such as inorganic nitrogen by biochar led to a reduction in nitrogen availability, and biochar has been shown to directly adsorb N2O and thus reduce N2O emissions [45]. Thirdly, biochar functions as an “electron shuttle”, which promotes the transfer of electrons to denitrifying microorganisms, and coupled with its liming effect and potential, it will reduce N2O to N2 [45,46,47].
The GWP of the double rice system was mainly determined by the cumulative CH4 emissions, and the contribution of N2O to GWP was only approximately 3–4%. This may be due to the traditional flooding pattern of paddy fields used in this study, which left the soil in an anaerobic state for a long time, resulting in the inhibition of nitrification reactions and low effectiveness of the denitrification substrate soil NO3−-N [48,49]. Promoting the formation of the end product N2 and reducing the intermediate product N2O, the resulting contribution of N2O to GWP was underestimated to some extent. Nitrogen is an essential element for crop growth and a key nutrient affecting crop yield [8,44]. Although OF reduced the grain yield of late rice, the difference was controlled within a reasonable range. In view of the whole double-season rice cropping system, the additional application of straw or biochar compensates for this loss and ensures stable or even increased grain yields. The application of straw or biochar under both CF and OF conditions increased grain yield in the double rice cropping system, probably by stimulating soil microbial activity and promoting soil nutrient cycling, which is consistent with previous observations in maize and wheat crops [50,51,52]. The biochar application increased the rice yield by 3.29–3.47% with some limitations, which may be related to the number of years. With the gradual mineralization of nitrogen carried by biochar and organic nitrogen in the soil, and the gradual release and fertilization of nitrogen adsorbed by biochar, the application of biochar significantly increases the soil carbon and nitrogen content, which will play an important role in improving rice yield [46,50]. The GHGI showed a combination of changes in GHG emissions as well as yield with different treatments. In this study, the GHGI decreased with decreasing N application, and more importantly, it was also found that biochar application significantly reduced the GHGI compared to straw return under the same nitrogen application rate. This is similar to the report of Liu et al. [53], who concluded that biochar application significantly reduced GWP by 21% and GHGI by 29% while increasing crop yield by 11%. The lower GHGI and some improvements in rice yields imply that optimal N fertilizer reduction in combination with biochar application can simultaneously achieve stable rice yields and lower GHG emissions in paddy fields, which is an agronomic measure worth promoting.

5. Conclusions

Straw significantly increased the soil organic matter content, accelerated the anaerobic environment of the soil, and increased CH4 emissions. Conversely, biochar increased the soil pH and has a porous structure, destroying the anaerobic and acidic environment required for methanogens to survive and thereby reducing CH4 emissions. Relative to CF, CF + S and OF + S increased the cumulative CH4 emissions by 11.80% and 2.35%, respectively, while CF + B and OF + B resulted in significant reductions in cumulative CH4 emissions by 27.80% and 28.46%, respectively. Notably, OF weakened the soil nitrogen substrate and reduced the cumulative N2O emissions, and the application of straw or biochar could further reduce the soil N2O emissions, with CF + B and OF + B achieving the best N2O reductions of 30.56% and 32.21%, respectively. Both straw and biochar increased the paddy soil organic matter content and soil nitrogen content and compensated for the nitrogen fixation effect of reduced nitrogen fertilizer, which was conducive to crop growth and increased the rice grain yield. Although there is a risk that OF may reduce yields, additional applications of straw or biochar can compensate for this loss. CH4 was the main contributor to the GWP of the double rice cropping system, while the addition of biochar lowered the GWP of the entire cropping system, which in turn lowered the GHGI, and OF was able to further optimize this effect. Overall, OF + B exhibited yield assurance as well as lower GHG emissions and can be promoted as an excellent cropping pattern in double-season rice growing areas.

Author Contributions

Methodology, D.L., H.H. and S.Y.; Formal analysis, D.L. and H.H.; Investigation, D.L., H.H., G.Z. and Q.H.; Writing—original draft, D.L.; Writing—review & editing, D.L., H.H. and S.Y. All authors have read and agreed to the published version of the manuscript.

Funding

National Key Research and Development Program of China: 2017YFD0301301.

Data Availability Statement

Data related to the research are reported in the manuscript. Any additional data may be acquired from the corresponding author upon request.

Acknowledgments

This research was supported by the National Key Research and Development Program of China (2017YFD0301301).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Seasonal variations in the CH4 emission flux in the double rice cropping system under different treatments. Error bars represent standard deviations (n = 3).
Figure 1. Seasonal variations in the CH4 emission flux in the double rice cropping system under different treatments. Error bars represent standard deviations (n = 3).
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Figure 2. Cumulative CH4 emissions of early rice/late rice (I) and the total (II) in the double rice cropping system under different treatments. Error bars represent standard deviations (n = 3). Uppercase and lowercase letters indicate significant differences at the 0.05 level between treatments in early and late rice seasons.
Figure 2. Cumulative CH4 emissions of early rice/late rice (I) and the total (II) in the double rice cropping system under different treatments. Error bars represent standard deviations (n = 3). Uppercase and lowercase letters indicate significant differences at the 0.05 level between treatments in early and late rice seasons.
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Figure 3. Seasonal variation in the N2O emission flux in the double rice cropping system under different treatments. Error bars represent standard deviations (n = 3).
Figure 3. Seasonal variation in the N2O emission flux in the double rice cropping system under different treatments. Error bars represent standard deviations (n = 3).
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Figure 4. Cumulative N2O emissions of early rice/late rice (I) and the total (II) in the double rice cropping system under different treatments. Error bars represent standard deviations (n = 3). Uppercase and lowercase letters indicate significant differences at the 0.05 level between treatments in early and late rice seasons.
Figure 4. Cumulative N2O emissions of early rice/late rice (I) and the total (II) in the double rice cropping system under different treatments. Error bars represent standard deviations (n = 3). Uppercase and lowercase letters indicate significant differences at the 0.05 level between treatments in early and late rice seasons.
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Figure 5. Effects of different treatments on the soil pH (I), soil organic matter content (II), soil NH4+-N content (III), and soil NO3−-N content (IV) in the double rice cropping system. Error bars represent standard deviations (n = 3).
Figure 5. Effects of different treatments on the soil pH (I), soil organic matter content (II), soil NH4+-N content (III), and soil NO3−-N content (IV) in the double rice cropping system. Error bars represent standard deviations (n = 3).
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Table
Table 1. Field management during the rice growing season in 2021–2022.
Table 1. Field management during the rice growing season in 2021–2022.
Transplanting Seedlings, Applying Basal FertilizerApplying Tillering FertilizerField SunningIrrigation and RehydrationApplying Panicle FertilizerHarvesting of Rice
Early rice8 June 20208 July 202015 July 202023 July 202016 August 202028 September 2022
Late rice13 June 20216 July 202118 July 202125 July 202118 August 20213 October 2021
Table
Table 2. Grain yield (kg ha−1), yield-scaled GWP (kg CO2-Eq kg−1), and GHGI in the double rice cropping system under different treatments.
Table 2. Grain yield (kg ha−1), yield-scaled GWP (kg CO2-Eq kg−1), and GHGI in the double rice cropping system under different treatments.
TreatmentsCO2–e (CH4)CO2–e (N2O)Total GWPYieldGHGI
Early rice seasonCF7480.2 ± 293.5 c322.7 ± 33.4 a7802.9 ± 302.1 c6897.2 ± 176.5 c1.13 ± 0.10 b
CF + S8638.4 ± 395.1 a298.8 ± 20.3 b8937.2 ± 413.0 a 7102.5 ± 293.4 ab1.26 ± 0.12 a
CF + B5558.4 ± 249.4 e224.5 ± 18.5 f5782.9 ± 260.7 e7193.1 ± 224.8 a0.80 ± 0.09 d
OF6987.2 ± 363.0 d260.5 ± 19.6 c7247.6 ± 359.6 d6925.9 ± 193.5 c1.05 ± 010 c
OF + S8023.3 ± 321.8 b250.7 ± 26.4 d8274.0 ± 331.9 b7072.4 ± 201.5 b1.17 ± 0.11 ab
OF + B5227.9 ± 212.2 f232.9 ± 21.3 e5460.8 ± 226.1 f7129.5 ± 221.8 a0.77 ± 0.08 d
Late rice seasonCF8499.7 ± 321.5 b356.7 ± 31.2 a8856.4 ± 346.8 b7234.4 ± 198.8 c1.22 ± 0.13 ab
CF + S9227.2 ± 414.8 a290.4 ± 24.1 b9517.6 ± 433.1 a7389.1 ± 173.4 b1.29 ± 0.16 a
CF + B5979.1 ± 339.5 f244.1 ± 22.3 d6223.2 ± 361.7 f7429.4 ± 176.4 ab0.84 ± 0.07 d
OF7910.3 ± 386.4 d242.1 ± 17.5 d8152.3 ± 411.0 d7183.3 ± 173.9 d1.13 ± 0.09 c
OF + S8330.4 ± 403.4 c275.5 ± 26.3 c8605.9 ± 421.8 c7486.4 ± 187.5 a1.15 ± 0.10 bc
OF + B6204.0 ± 173.1 e224.5 ± 16.8 e6428.4 ± 203.8 e7466.5 ± 201.9 a0.86 ± 0.11 d
Total seasonsCF15,978.0 ± 538.4 c679.4 ± 32.3 a16,659.3 ± 631.5 b14,131.6 ± 317.0 c1.18 ± 0.17 b
CF + S17,865.6 ± 764.1 a589.2 ± 29.9 b18,454.8 ± 742.5 a14,491.6 ± 420.8 b1.27 ± 0.16 a
CF + B11,537.5 ± 499.2 e468.5 ± 19.7 e 12,006.1 ± 531.7 d14,622.5 ± 297.5 a0.82 ± 0.11 d
OF14,897.4 ± 631.0 d502.6 ± 32.4 d15,400.0 ± 589.1 c14,109.2 ± 316.3 c1.09 ± 0.10 c
OF + S16,355.4 ± 511.8 b526.2 ± 27.5 c16,881.6 ± 530.8 b14,558.7 ± 329.6 ab1.16 ± 0.13 bc
OF + B11,431.8 ± 331.7 e457.4 ± 21.9 f11,889.2 ± 349.3 d14,595.9 ± 376.9 a0.81 ± 0.09 d
Note: According to the IPCC 2014, relative to CO2, the global warming potential (GWP) is calculated as the radiative forcing potential values of CH4 and N2O, which are 28 and 265, respectively. Data are presented as mean (n = 3) ± standard deviation. Letters indicate significant differences between treatments at the 0.05 level.
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Li, D.; He, H.; Zhou, G.; He, Q.; Yang, S. Rice Yield and Greenhouse Gas Emissions Due to Biochar and Straw Application under Optimal Reduced N Fertilizers in a Double Season Rice Cropping System. Agronomy 2023, 13, 1023. https://doi.org/10.3390/agronomy13041023

AMA Style

Li D, He H, Zhou G, He Q, Yang S. Rice Yield and Greenhouse Gas Emissions Due to Biochar and Straw Application under Optimal Reduced N Fertilizers in a Double Season Rice Cropping System. Agronomy. 2023; 13(4):1023. https://doi.org/10.3390/agronomy13041023

Chicago/Turabian Style

Li, Dandan, Hao He, Guoli Zhou, Qianhao He, and Shuyun Yang. 2023. "Rice Yield and Greenhouse Gas Emissions Due to Biochar and Straw Application under Optimal Reduced N Fertilizers in a Double Season Rice Cropping System" Agronomy 13, no. 4: 1023. https://doi.org/10.3390/agronomy13041023

APA Style

Li, D., He, H., Zhou, G., He, Q., & Yang, S. (2023). Rice Yield and Greenhouse Gas Emissions Due to Biochar and Straw Application under Optimal Reduced N Fertilizers in a Double Season Rice Cropping System. Agronomy, 13(4), 1023. https://doi.org/10.3390/agronomy13041023

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