By Dr. Praneet Ngamsnae
Human activities emit approximately 50 billion tons of greenhouse gases annually. The proportion of emissions from each sector includes energy and heat production (73.2%), agriculture, forestry, and land use (18.4%), industry (5.2%), and waste (3.2%). It is important to note that the proportion of greenhouse gases from each sector may vary from country to country (Ritchie 2020).
In the agricultural sector, rice cultivation is one of the largest contributors to greenhouse gas emissions. Paddy rice accounts for 8% of global anthropogenic methane emissions annually and is also a significant source of nitrous oxide emissions due to the use of fertilizers. These two gases are hazardous, with a Global Warming Potential (GWP) 25 and 298 times greater than carbon dioxide, respectively (UNFCC).
Methane Generation in Rice Fields
When rice fields are inundated, and both fertilizer and organic matter are concurrently deposited, conditions of oxygen deficiency arise. This anoxic environment activates methanogenic bacteria. Consequently, methane produced in this process is released into the atmosphere through the rice stalks (Rahman and Yamamoto 2020).
Nitrous Oxide Generation in Rice Fields
Nitrous oxide is generated through the anaerobic decomposition of nitrogen fertilizer. In rice fields, which are sites of significant organic matter accumulation, the submerged conditions influence the microbial denitrification. During denitrification, soil microbes convert nitrate (NO3–) to nitrous oxide (N2O) and nitrogen gas (N2), which is then released into the atmosphere (Gupta 2021).
The quantity of greenhouse gases emitted from rice fields is contingent upon the intensity of nitrogen fertilization, drainage practices, and microbial activity. Collectively, these account for up to 70% of the gases released from the rice monoculture system.
Therefore, improper management of fertilization in rice fields can lead to significant losses of trace elements and nutrients. In particular, excess nitrogen is lost through the process of denitrification, resulting in its release into the atmosphere rather than being utilized by the rice plants.
For this reason, it is necessary to develop improved techniques and new methods of rice cultivation to reduce or mitigate the conditions and factors that lead to fertilizer loss and production of the greenhouse gases, which contribute to climate problems.
Impact of Greenhouse Gases on Nutritional Value of Rice
Rising atmospheric carbon dioxide levels can diminish the nutritional value of rice, a crucial global food source. Elevated carbon dioxide disrupts photosynthesis, resulting in significant losses of essential vitamins and minerals in rice. An international study, published in Science Advances (Zhu 2018), found that increased carbon dioxide levels reduce the nutrient content in rice, including vitamins B1, B2, B5, B9, protein, iron, and zinc.
Experiments on 18 rice varieties in China and Japan exposed to carbon dioxide levels of 568-590 ppm, higher than the current global average of 410 ppm, showed that the plants absorbed fewer soil nutrients, leading to increased carbon accumulation in the grains. This reduction in nutrients will have significant implications for the billions of people worldwide who depend on rice as a staple food. Of particularly concern is the reduction of folate (vitamin B9) in rice by up to 30%, as folate is vital for healthy infant development.
Aquatic Animals in Rice Fields Help Reduce Emissions
Integrating the practice of raising aquatic animals, such as fish or shrimp, into rice fields, also known as rice-fish farming (Ngamsnae 2024), can significantly contribute to the reduction of greenhouse gas emissions.
Reducing Methane Emissions in Rice Fields
Rice monoculture in flooded fields accounts for about 8% of global methane emissions. Methane emissions are primarily driven by the anaerobic decomposition of hard-to-degrade organic materials, such as plant residues, organic substances, and organic fertilizers, under low-oxygen, flooded conditions.
Aquatic animals, especially bottom feeders like crabs and certain fish, increase dissolved oxygen in the water and soil through their movements and digging activities. This shift from anaerobic to aerobic decomposition significantly reduces methane production and release (Xing 2019). Studies have shown that combining crayfish farming with rice cultivation can lower methane emissions by 18.1-19.6% compared to rice monoculture.
This reduction in methane production in integrated rice and aquatic animal farming systems is due to the enhanced activity of methane-oxidizing bacteria (MOB) in the water, which utilize methane as an energy source. Additionally, the rice plant’s root system supplies extra organic matter, provides food for these bacteria, increases their activity, and thus reduces methane production further (Liu 2024).
Therefore, integrating rice cultivation with aquatic animal farming is an effective strategy for reducing methane emissions. This method promotes sustainable and environmentally friendly food production practices. Improved water quality and reduced eutrophication lead to higher oxygen levels, foster aerobic conditions, and consequently reduce methane production.
Reducing Nitrous Oxide Emissions in Rice Fields
Rice fields are a significant source of nitrous oxide emissions because the anaerobic decomposition of organic matter in submerged conditions affects the nitrification and denitrification processes of microorganisms (Wu 2018). The amount of nitrous oxide released is influenced by the concentration of nitrogen fertilizer application, water drainage practices, and microbial activity, collectively accounting for up to 70% of the gas emissions from monoculture rice systems (Braker 2011; Syakila 2011).
Transitioning from rice monoculture to integrated rice-aquaculture systems, such as incorporating fish, mud crabs or crayfish, can reduce nitrous oxide emissions by 19.7-28.2% (Wang 2019). Additionally, this integrated system can decrease nitrogen loss through ammonia volatilization and nitrate leaching, further mitigating nitrogen emissions in rice fields.
Comparison of Greenhouse Gas Emissions
Rice Monoculture: The predominant practice of flooding paddy fields fosters an anaerobic environment conducive to the activity of methanogenic microorganisms that decompose organic matter, resulting in the production of methane, nitrous oxide, and carbon dioxide. The anaerobic decomposition of organic material in inundated fields is a principal contributor to these emissions.
Integrated Rice-Aquatic Animal Systems: The integration of rice cultivation with aquatic animal farming has been demonstrated to mitigate greenhouse gas emissions. A comprehensive global meta-analysis has reported that co-cultivation of rice with aquatic animals reduces emissions of nitrous oxide and methane by 17% and 11%, respectively (Huang 2024). Notably, the extent of emission reductions is contingent upon the specific type of aquatic animals incorporated. Among various integrated systems, the rice-crayfish system exhibits the highest efficacy, achieving reductions in nitrous oxide and methane emissions by 32% and 45%, respectively.
In summary, although both rice monoculture and integrated rice-aquatic animal systems contribute to greenhouse gas emissions, the latter demonstrates a lower emission profile. The magnitude of emission reduction varies with the type of aquatic animal integrated into the rice fields. These findings underscore the potential of integrated rice-aquatic animal systems as a viable strategy for reducing greenhouse gas emissions associated with rice cultivation.
Transitioning to Co-Culture Systems
Some general guidelines for farmers interested in transitioning to co-culture systems are outlined below.
Step 1 Education and Training: Farmers should initially educate themselves about co-culture systems, including their benefits and operational mechanisms. This can be achieved by attending workshops, seminars or training programs, engaging relevant literature, and consulting field experts.
Step 2 Strategic Planning: A thorough planning process is essential for a successful transition. Farmers must decide on the type of aquatic animals to cultivate alongside rice, considering local climate conditions, soil properties, and market demand.
Step 3 Infrastructure Development: Depending on the chosen co-culture system, farmers may need to modify existing infrastructure or invest in new facilities. This could involve constructing ponds for aquatic animals, installing appropriate irrigation systems, and acquiring necessary equipment.
Step 4 Implementation: With the infrastructure in place, farmers can commence the implementation phase. This involves planting rice and introducing the selected aquatic animals into the system.
Step 5 Monitoring and Management: Continuous monitoring and effective management are critical to ensure the system’s success. Farmers should regularly assess the health of both rice and aquatic animals, test soil and water quality, and manage pests and diseases.
Step 6 Market Access: Farmers must also consider market access for their products. This includes identifying potential buyers, marketing their products, and joining cooperatives or other marketing organizations to enhance market reach.
Step 7 Continuous Learning and Adaptation: As farmers gain experience with co-culture systems, they should continuously learn and adapt their practices to enhance productivity and sustainability.
It is important to recognize that these steps can vary based on the local conditions, the specific type of co-culture system, and other contextual factors. Therefore, farmers are advised to seek guidance from local agricultural extension services or other experts during the planning and implementation of a transition to co-culture systems.
Recommendations to Stakeholders
Key stakeholders in addressing greenhouse gas emissions from rice fields include governments, farmers, research institutions, non-governmental organizations (NGOs), businesses, and consumers. Governments and international organizations play a pivotal role by establishing policies and providing necessary resources. Farmers are responsible for implementing sustainable agricultural practices. Research institutions contribute by developing these practices and offering training programs, while NGOs focus on raising awareness and supporting farmers. Businesses can adopt sustainable practices and invest in innovative technologies. Consumers can drive demand by choosing sustainably produced products.
Important measures to mitigate emissions include altering farming practices, such as adopting organic farming techniques and incorporating rice straw into the soil prior to flooding. The adoption of new technologies, such as precision agriculture, can enhance resource efficiency. Education and training programs for farmers on sustainable practices are essential. Governments can implement policies and regulations that promote sustainable farming. Continued research and development are necessary to improve existing practices, and to develop new technologies aimed at reducing greenhouse gas emissions.
Conclusion
The mitigation of greenhouse gas emissions from rice fields is critical, as rice cultivation significantly contributes to methane and nitrous oxide emissions, both of which have high global warming potential. Integrating rice cultivation with aquaculture, such as raising fish or shrimp in rice fields, offers a promising solution. This practice improves oxygen levels in the water and soil, thereby reducing methane production through aerobic rather than anaerobic decomposition. It also reduces nitrous oxide emissions by improving nitrogen utilization and decreasing nitrogen loss. Research indicates that integrated rice-aquatic animal systems can reduce methane emissions by 18.1-19.6% and nitrous oxide emissions by 19.7-28.2%, depending on the type of aquatic animal involved. This sustainable approach not only mitigates greenhouse gas emissions, but also enhances soil fertility and crop yields, providing a viable strategy for environmentally friendly and productive rice farming.
AQUADAPT 