Ecological and Economic Benefits of Greenhouse Gas Emission Reduction Strategies in Rice Production: A Case Study of the Southern Rice Propagation Base in Hainan Province

Abstract

Developing tailored emission reduction strategies and estimating their potential is crucial for achieving low-carbon rice production in a specific region, as well as for advancing China’s dual carbon goals in the agricultural sector. By utilizing water-saving and drought-resistant rice (WDR) with enhanced water and nitrogen utilization efficiency, the mitigation strategies were constructed for rice production systems, and their potential for emission reduction was estimated in the southern rice propagation base of Hainan Province. This study revealed that the implementation of a reduction strategy, which involves dry direct seeding and dry cultivation, combined with a 53% reduction in nitrogen fertilizer, can effectively synergize the mitigation of methane (CH₄) and nitrous oxide (N₂O) emissions from rice paddies. Compared with traditional flooded rice cultivation, this integrated approach exhibits an impressive potential for reducing net greenhouse gas (GHG) emissions by 97% while simultaneously doubling economic benefits. Moreover, when combined with plastic film mulching, the strategy not only sustains rice yields but also achieves a remarkable emission reduction of 92%, leading to a fourfold increase in economic benefits. Our study provides a comprehensive low-carbon sustainable development strategy for rice production in the southern rice propagation base of Hainan Province and offers valuable insights for researching GHG emissions in other regions or crops. These emission reduction pathways and the assessment method could contribute to the realization of low-carbon agriculture.

1. Introduction

With the increase in human activities, anthropogenic greenhouse gas (GHG) emissions have become the primary cause of global warming and climate change [1]. Agriculture is the dominant emission source for non-carbon dioxide GHG in all regions, particularly methane (CH₄) and nitrous oxide (N₂O). Rice cultivation, as a significant contributor in the agricultural sector, emitted 30 (25–38) Tg CH₄ per year for the decade spanning from 2008 to 2017, which accounts for approximately 8% of global anthropogenic CH₄ emissions [2] and also serves as a crucial source of N₂O [3]. Therefore, looking for feasible ways to reduce GHG emissions from rice paddies holds immense importance for achieving sustainable agricultural development.

Traditional flooded rice cultivation not only consumes a large amount of agricultural water but also is a significant cause of CH₄ emissions [4,5]. On the other hand, fertilization management provides substrates for the nitrification and denitrification processes in soil, leading to N₂O emissions [6,7]. Previous studies have shown that water-saving irrigation technology not only inhibits CH₄ production but also promotes CH₄ oxidation from flooded rice paddies [8,9]. Other factors influencing CH₄ and N₂O emissions include rice variety [10,11], tillage and fertilization management [12,13,14,15], and soil physicochemistry [16]. For instance, altering agricultural management practices, such as implementing water-saving irrigation and optimizing fertilizer management, can mitigate CH₄ and N₂O emissions from rice paddies.

Water-saving and drought-resistant rice (WDR) exhibits high tolerance to drought and efficient utilization of nitrogen fertilizer, making it suitable for cultivation in anaerobic rice paddies with low nitrogen input [17,18]. For example, the adoption of dry-cultivated WDR has been widely promoted in China to ensure stable production and reduce GHG emissions [19,20]. In recent years, plastic film mulching has emerged as a common practice in crop production to minimize water consumption while simultaneously enhancing crop yield and mitigating GHG emissions [21,22]. This approach offers both economic benefits and environmental advantages. However, the mitigation potential of dry-cultivated WDR with plastic film mulching and its effect on rice yields is still unknown.

Hainan Province, located in the tropics, offers excellent light and thermal conditions for rice production and propagation. In order to achieve low-carbon agriculture, it is crucial to explore sustainable management systems and emission reduction strategies while assessing the potential for reducing GHG emissions. To provide a scientific basis for the government to promote sustainable agricultural management, we conducted in situ experiments focusing on drought-tolerant rice with high water and fertilizer utilization efficiency. Specifically, this study aimed to investigate the following: (1) the effect of water-saving irrigation and nitrogen reduction on rice yield; (2) the monitoring and evaluation of GHG emission patterns and intensities from rice paddies; and (3) an economic analysis of cost-effectiveness. By exploring feasible emission reduction strategies for rice paddies in Hainan Province, this research aims to explore a feasible low-carbon development path for rice production in the southern rice propagation base.

2. Materials and Methods

2.1. Site Description

The experiment was conducted in 2023 at the southern rice propagation base of Shanghai Academy of Agricultural Sciences, which is located in Guangpo Town, Lingshui County, Hainan Province (18.549° N, 110.046° E). Please refer to Figure 1 for the detailed location. The total area covers over 67 ha for conventional breeding and includes large areas that require transformation and leveling. It has a tropical island monsoon climate with abundant sunlight and heat. The average annual temperature is 25.2 °C, and the average annual rainfall ranges from 1500 to 2500 mm. The soil organic matter content in Lingshui County is relatively low at an average of 15.49 g kg⁻¹.

2.2. Experimental Design

The local, traditional rice planting management was used as the baseline scenario. Two emission reduction strategies were established, with WDR variety as the core experimental material. These strategies were evaluated at three treatment levels: (1) Baseline scenario (BL): local rice variety + continuous flooding + conventional nitrogen application rate. (2) Emission reduction scenario 1 (SR1): WDR variety + dry direct seeding and dry cultivation + 53% nitrogen reduction. (3) Emission reduction scenario 2 (SR2): WDR variety + plastic film mulching + dry direct seeding and dry cultivation + 53% nitrogen reduction. Each treatment was randomly replicated three times. The experimental plots under the BL treatment were continuously planted with rice throughout the year. However, for the SR1 and SR2 treatments, WDR varieties were cultivated in newly-reclaimed land this year. The plots were kept independent of each other to prevent any interference from water and fertilizer.

In the BL treatment, seedlings were manually transplanted on 14 January 2023 and harvested on 23 May 2023. The experimental variety was Teyou 009 (a local rice variety). Throughout the rice growing season, the water level in the paddy field was maintained at around 10 cm and drained approximately one month before harvest. The nitrogen fertilizer application rate was 260 kg N ha⁻¹, with a ratio of 6:4 for base fertilizer to tillering fertilizer.

In the SR1 and SR2 treatments, Hanyou 73 (WDR variety) was dry direct-seeded on 10 January 2023 and dry direct-seeded by machine mulching, respectively. Both of them were then harvested on 23 May 2023. Micro-sprinkler irrigation was used for supplemental irrigation five times, while no surface water existed throughout the growth period. The nitrogen fertilizer application rate was 120 kg N ha⁻¹, and the ratio of base fertilizer to tillering fertilizer was also 6:4. Other field management practices followed local planting methods, and all treatments adhered to agricultural guidance for pesticide use. Additionally, herbicides were sprayed on WDR variety during the sowing period and stem–leaf period for weed control.

2.3. Greenhouse Gas Monitoring and Emission Reduction Estimation

The static transparent chamber technique was used to monitor GHG emissions from paddy fields [9]. The chamber used in this study included a lid and frames. Before rice planting, Plexiglas base frames with a water tank and side holes were pre-installed. The dimensions of the bases were 50 cm in length, 40 cm in width, and 20 cm in height. Gas sampling was carried out within the lids, which had dimensions of 46 cm × 40 cm × 50 cm. A fan was installed at the top of the lid to facilitate gas mixing inside the chamber. As the rice plants grew, extension chambers of varying heights (20 cm, 40 cm, or 60 cm) were selected to adjust the height of the sampling chamber.

An auto-sampler was used for collecting gas samples. It consisted of a circuit board, an electromagnetic valve, an air pump, and a battery. During gas sampling, the chamber was placed in the water tank and sealed with water to contain gases. After connecting the auto-sampler and chamber, four gas samples were collected at six-minute intervals and stored in aluminum bags (LB-101, Delin Dalian, China). Samples were taken once a week from 8:30 am to 11:00 am on rainless days.

The gas samples were analyzed for CH₄ and N₂O concentrations using a gas chromatograph (7820A, Agilent Technologies, Inc., Santa Clara, CA, USA) with a hydrogen flame ionization detector and an electron capture detector. Linear regression was used to calculate CH₄ or N₂O fluxes, while cumulative CH₄ or N₂O emissions during rice growing season were calculated by linear interpolation.

Net GHG emissions (nGHG, CO₂e ha⁻¹) comprehensively evaluated both CH₄ and N₂O emissions from paddy fields by converting them into CO₂-equivalent (CO₂e) emissions using global warming potentials (GWPs). The mitigation potential of SR scenarios (M_GHG, CO₂e ha⁻¹) was calculated as follows:

$$\mathrm{n}\mathrm{G}\mathrm{H}\mathrm{G}={\mathrm{E}}_{{\mathrm{C}\mathrm{H}}_4}\times 27+{\mathrm{E}}_{{\mathrm{N}}_2\mathrm{O}}\times 273$$

(1)

$${\mathrm{M}}_{\mathrm{G}\mathrm{H}\mathrm{G}}={\mathrm{n}\mathrm{G}\mathrm{H}\mathrm{G}}_{\mathrm{B}\mathrm{L}}-{\mathrm{n}\mathrm{G}\mathrm{H}\mathrm{G}}_{\mathrm{S}\mathrm{R}}$$

(2)

where GWP values for CH₄ and N₂O are represented as 27 and 273 on a hundred-year timescale. Cumulative CH₄ and N₂O emissions during the rice growing season are represented as ${\mathrm{E}}_{{\mathrm{C}\mathrm{H}}_4}$ (kg CH₄ ha⁻¹) and ${\mathrm{E}}_{{\mathrm{N}}_2\mathrm{O}}$ (kg N₂O ha⁻¹). And nGHG_BL (CO₂e ha⁻¹) and nGHG_SR (CO₂e ha⁻¹) represent nGHG emissions under baseline and emission reduction scenarios.

2.4. Rice Production

Rice yield measurements were taken for each plot according to the planting area, fresh weight and water content of rice, and standard moisture content. The GHG emission intensity per unit of rice yield (GHGI_yield) was calculated based on the ratio between nGHG and rice yields for each treatment, which is shown in Equation (3). The unit of GHGI_yield is kg CO₂e kg⁻¹.

The economic benefits of three strategies were calculated using input–output analysis. Net income was calculated by subtracting gross input costs, including agricultural inputs (seeds, fertilizers, pesticides, and plastic films), machinery costs, and management costs (labor costs), from gross values. The benefits (gross value) were earned from selling rice grain and were assumed to be obtained from carbon credits after entering the carbon trading market. All costs were calculated based on experimental year prices.

GHG emission intensity per unit of revenue (GHGI_CNY), which could be represented by the ratio between nGHG and net income (Equation (4)), was used to represent economic ecological benefit. The units of net income, gross value, and GHGI_CNY are CNY ha⁻¹, CNY ha⁻¹, and kg CO₂e CNY⁻¹, respectively.

$${\mathrm{G}\mathrm{H}\mathrm{G}\mathrm{I}}_{\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}=\raisebox{1ex}{$\mathrm{n}\mathrm{G}\mathrm{H}\mathrm{G}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}$}\right.$$

(3)

$${\mathrm{G}\mathrm{H}\mathrm{G}\mathrm{I}}_{\mathrm{C}\mathrm{N}\mathrm{Y}}=\raisebox{1ex}{$\mathrm{n}\mathrm{G}\mathrm{H}\mathrm{G}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{N}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{e}$}\right.$$

(4)

3. Results

3.1. Greenhouse Gas Emissions

The CH₄ flux under the BL treatment showed a uni-modal trend and reached its maximum values during the tillering stage. In contrast, the CH₄ flux under the SR1 and SR2 treatments was more consistent and approached zero (Figure 2A). The average CH₄ flux during the rice growing season for BL, SR1, and SR2 was 8.72, 0.04, and 0.29 mg m⁻² h⁻¹, respectively. There was a significant difference between BL and the reduction scenarios (p < 0.05), but no significant difference was found between SR1 and SR2. The N₂O flux mainly occurred after water and fertilizer management, with average values of 0.0423, 0.0285, and 0.0299 mg m⁻² h⁻¹ for BL, SR1, and SR2, respectively (Figure 2B). There was no significant difference in N₂O flux between the three treatments.

The SR1 and SR2 treatments significantly reduced GHG emissions from rice paddies during rice production. Compared with BL, the seasonal CH₄ emissions under the SR1 treatment were negligible, acting as a weak absorption sink for CH₄, and the seasonal N₂O emissions could be reduced by 37%. The seasonal CH₄ and N₂O emission reductions under the SR2 treatment were 96% and 32%, respectively. Overall, both of these emission reduction strategies could significantly reduce the comprehensive GHG emissions from rice paddies. The nGHG emission reductions for SR1 and SR2 were 6836 and 7185 kg CO₂e ha⁻¹, respectively, representing a large mitigation potential of 97% and 92%.

3.2. Rice Yields and Ecological and Economic Benefits

The rice yields for BL, SR1, and SR2 were 6858, 5426, and 6741 kg ha⁻¹, respectively, as shown in

Table 1. Rice yields and economic and ecological benefits of rice production in southern rice propagation base under different scenarios.

	Rice Yield (kg ha−1)	GHGIyield (kg CO2e kg−1)	Gross Value (CNY ha−1)	Input Costs (CNY ha−1)	Net Income (CNY ha−1)	GHGICNY (kg CO2e CNY−1)
BL	6858	1.079	17,282	16,050	1232	6.006
SR1	5426	0.040	14,103	10,210	3893	0.055
SR2	6741	0.084	17,397	11,100	6297	0.090

. The yield of SR1 was significantly lower than that of BL, with a reduction rate of 21%, while there was no significant difference between SR2 and BL, with a reduction rate of only 1.7%. GHGI_yield was affected by both the rice yield and nGHG emissions. The GHGI_yield values for the three treatments, from low to high, were SR1 < SR2 < BL, with values of 0.04, 0.08, and 1.08 kg CO₂e kg⁻¹ yield, respectively. This indicates that in the scenario of SR1, the carbon emissions per unit of rice production for WDR were only 4% of those for conventional rice production, while the carbon emissions per unit of production for SR2 were 8% of those for conventional production.

Based on the minimum guidance price of rice in 2023 (2.52 CNY kg⁻¹ rice grain for early indica rice), the rice production values for BL, SR1, and SR2 treatments were 17,282, 13,672, and 16,987 CNY ha⁻¹, respectively. After deducting total input costs and accounting for carbon emission reduction benefits, the economic benefits of rice production under SR1 and SR2 were two and four times higher than those under BL. The trends in GHGI_CNY were also observed as SR1 < SR2 < BL, with values of 0.055, 0.090, and 6.006 t CO₂e CNY⁻¹. Both SR1 and SR2 are beneficial to increasing the economic benefits of rice production and reducing carbon emissions per unit of yield and value. Among them, SR2 has the highest economic benefit, while SR1 has the lowest carbon emissions per unit of yield and value.

4. Discussion

4.1. Ecological Benefits

Adopting water-saving irrigation techniques and optimizing fertilizer management are the most effective strategies for reducing GHG emissions in rice paddies. The results of this study revealed that the SR1 and SR2 treatments generated remarkable economic benefit and ecological benefit (Figure 3). By transitioning from flooding management to dry cultivation and reducing the amount of nitrogen fertilizer application, SR1 and SR2 collectively mitigated seasonal CH₄ emissions and N₂O emissions compared with BL. These reduction strategies had a substantial impact on GHG emissions, with 97% and 92% mitigation potential in SR1 and SR2, respectively, which was mainly due to CH₄ mitigation. Moreover, when promoting these strategies, it is important to carefully consider the balance between GHG emissions and rice yields. Considering the ideal rice yields of the reduction strategies in newly-reclaimed land, the GHGI_yield was also dramatically lower for the SR1 and SR2 treatments than for the BL treatment, with a 96% and 92% reduction, respectively.

Normally, water-saving irrigation techniques, such as alternate wetting–drying water-saving irrigation (AWD), typically reduce CH₄ emissions but increase N₂O emissions [15]. However, compared with AWD, irrigation based on soil water potential (SWP) (maintaining soil water potential at −10 kPa within the top 15 cm of the soil profile) reduced CH₄ and N₂O emissions by up to 34% and 66%, respectively, although it was accompanied by significantly lower yields [23]. To ensure the rice yield under water-saving conditions, it is necessary to cultivate rice varieties with enhanced drought tolerance [24]. Previous studies have confirmed that SWP-controlled water-saving irrigation (dry cultivation), coupled with a drought-resistant rice (WDR) variety, ensures high rice yields while decreasing CH₄ emissions but increasing N₂O emissions [9]. Fertilizer management practices affect GHG emissions from rice systems, too. For example, N₂O emissions increased significantly with an increasing N rate, and CH₄ emissions decreased by 15% at high N rates [6]. Li, et al. [25] found that, compared with conventional management, the combination of water-saving irrigation (at shallow water depths) and modified N fertilizers (nitrapyrin–urea composition plus hydroquinone) mitigated 50% of GHG emissions by reducing 54% of CH₄ emissions and 5% of N₂O emissions, while improving rice yield. This finding is consistent with the results of this study. Shifting from flooded water management to non-flooded cultivation gives rise to soil redox potential, inhibiting CH₄ production and promoting CH₄ oxidation [16]. Additionally, reducing the application rate of chemical fertilizer is more favorable for decreasing N₂O emissions [26]. In summary, modified water and fertilizer, in combination with the WDR variety (SR1 and SR2), is a win–win strategy to mitigate GHG emissions while maintaining rice production.

Furthermore, plastic film mulching could significantly increase crop yield and water use efficiency by modifying the crop growth environment [27]. A meta-analysis based on 313 studies showed that plastic film mulching reduced CH₄ emissions by 69%, increased N₂O emissions by 85%, reduced nGHG emissions by 51%, and improved rice yield by 1.6% compared with continuous flooding irrigation [22]. Plastic film mulching in upland rice significantly decreased CH₄ emissions compared with that in traditional paddy fields due to lower soil moisture during the rice growth stage [28]. However, in upland soil, plastic film mulching led to an increase in seasonal CH₄ and N₂O emissions (5–10% increase for CH₄ and 130–260% increase for N₂O) over non-mulching treatments but also significantly improved meteorological characteristics, resulting in higher grain yields [29]. Consistent with these previous research findings, using film mulching as part of a reduction strategy (SR2) maintained rice yields compared with traditional management (BL) and increased rice yields by 20% compared with the reduction strategy without film mulching (SR1). In contrast, in the reduction strategies, SR2 increased GHGI_yield by 111% more than that in SR1 due to significantly increased CH₄ and N₂O emissions. Regardless of whether film mulching was used or not, the GHGI_yield was significantly lower in the reduction strategies than in traditional management. Therefore, film mulching can be a useful agricultural practice for increasing crop productivity while decreasing GHGI in rice production.

4.2. Economic Benefits

In addition to the aforementioned environmental benefits, the adoption of reduction strategies can also bring economic benefits. The economic benefits of SR1 and SR2 were 3893 and 6297 CNY ha⁻¹, respectively, which were two times and five times higher than those of BL. This suggests that the adoption of WDR planting strategies can not only reduce GHG emissions but also provide economic benefits to local farmers.

Within the context of rice production, the sustainability and profitability of rice systems are dominated not only by grain yields but also by economic and social indicators [30]. In particular, decreasing nitrogen application rates, labor inputs, and irrigation energy inputs have increased both net income and social benefits. Rahim, et al. [31] found that net income was estimated to increase by 30% through the combination of optimizing the usage of agricultural inputs (seeds and fertilizers). Water-saving irrigation technologies have also shown a certain downward trend in irrigation energy consumption and labor inputs [32]. While plastic film mulching was beneficial for increasing rice yield, it led to high economic inputs [33]. On the other hand, engaging in carbon trading markets could enable the reduction of GHG emissions through exchanges [34]. Low-carbon economic benefits would be an effective way to extend the boundaries of social and economic efficiency within rice production. In conclusion, this study demonstrates that the adoption of reduction strategies (SR1 and SR2) can significantly reduce GHG emissions from rice paddies while bringing economic benefits to farmers. However, further research is needed to optimize the planting strategies and enhance their economic and environmental benefits.

4.3. Limitations and Prospects

The emission reduction strategies for rice production, based on water and fertilizer management and variety optimization, can effectively achieve emission mitigation. However, in order to enter the carbon market for carbon trading, it is also necessary to have market recognition and supporting methodologies. To ensure a transparent, fair, and trustworthy carbon trading market, it is important to develop operational, scientific, and feasible measurement methods and standards for carbon trading. Additionally, the monitoring of key parameters in emission reduction projects is essential to guarantee the effectiveness of emission reduction effects. In addition, further studies are needed to investigate changes in soil organic carbon when flooded rice paddies are converted to non-flooded conditions (dry cultivation). It is important to examine how the decomposition rate of organic matter or the soil carbon pool change under dry cultivation compared with the flooded management, as well as to understand any alterations in the mechanism of soil organic carbon retention. These investigations are crucial for a comprehensive evaluation of the environmental benefits associated with these emission reduction strategies.

On the other hand, there exists a potential risk of yield loss after rice production is transformed from traditional flooded management to dry cultivation. Further optimization of agricultural management practices, such as sufficient irrigation and fertilization, is still needed to ensure food security. Additionally, the emission reduction strategy (SR1) is prone to problems, such as weed overgrowth in actual production. Before promoting this emission reduction strategy, it is necessary to fully consider weed control issues and further develop and integrate operable and easy-to-promote technologies for weed prevention and control (such as SR2). At the same time, in order to comprehensively evaluate the economic and ecological benefits of the emission reduction strategies, this study introduced the concept of unit net income carbon emission intensity while calculating the carbon emission intensity per unit net output. Due to variations in economic and ecological benefits across different regions, adjustments and optimizations should be made according to actual conditions when conducting horizontal or vertical comparisons. The feasibility and effectiveness of these adjustments need to be verified through practical implementation. Additionally, considering the extensive rice cultivation area in Hainan Province, it is necessary to formulate more emission reduction strategies and analyze their potential for reducing emissions in order to effectively address global climate change while ensuring food security.

5. Conclusions

Drought-tolerant rice varieties have great potential to help mitigate and adapt to global climate change. This study used the WDR variety (Hanyou 73) as the core of the reduction strategies. A comprehensive evaluation confirmed a 21% reduction in rice yields for the drought-tolerant rice variety’s emission reduction strategy (SR1) compared with BL. However, the nGHG emission reduction potential was as high as 97%, with economic benefits increasing twofold. Furthermore, when coupled with plastic film mulching, the rice yield showed no significant difference when compared with BL, and the emission reduction potential was 92%, resulting in four times higher economic benefits. Overall, this optimization led to improved yields, economic gains, and ecological benefits.

Therefore, by optimizing rice field management and planting methods, GHG emissions in rice paddies can be effectively reduced while ensuring rice yields. When applying or promoting emission reduction technologies in practice, various factors should be comprehensively considered, and comprehensive evaluation should be taken, such as reasonable fertilization, optimized water management, and selecting adaptive rice varieties to achieve the goal of reducing greenhouse gas emissions in rice paddies.