(The Conversation)
Rice feeds more than half the world. From terraced paddies in Southeast Asia to irrigated fields in China and India, it underpins daily meals for billions of people.
But the same flooded soils that help rice thrive also create ideal conditions for microbes that release climate-warming gases.
In a new study, our team of environment and agri-culture scientists found that greenhouse gas emissions from rice paddies have near-ly doubled globally since the 1960s, averaging about 1.1 billion tons of carbon dioxide-equivalent emis-sions per year in the 2010s. Thatâs roughly equal to the annual emissions of 239 mil-lion cars.
This makes rice-growing the largest emissions source in agriculture outside of live-stock, and rice demand is expected to keep rising.
Farmers have ways to reduce their rice cropsâ emis-sions without lowering their yields. If every grower used the best currently available âclimate-smartâ options, we found that global rice emis-sions could be reduced by about 10% by midcentury. However, greater reductions are needed to slow climate change, which would require developing additional, more effective strategies.
Why rice emissions have increased
Rice emissions have risen for two reasons: the expan-sion of rice cultivation area and the intensification of management practices.
Just over half of the global increase is from the expan-sion of rice-growing areas. In Africa, for example, the rice-growing area has roughly doubled since the 1960s, helping drive a twofold rise in methane emissions in the region.
At the same time, rice farmers are using more fer-tilizers and organic amend-ments, such as straw and manure, planting more pro-ductive rice varieties and growing the plants closer together. The result is more rice but also more greenhouse gas emissions.
We found that one practice in particular- leaving rice stalks in the field after har-vest and then plowing them into the soil to improve soil fertility- was responsible for about 18% of riceâs increase in overall net emissions since the 1960s. The reason: It in-creases the organic matter in the soil, which microbes then decompose, creating more methane emissions.
Rising global tem-peratures fur-ther acceler-ate microbial activity in the soils, mean-ing even more emissions.
Fertilizer is another major contributor to emissions. Use of syn-thetic nitro-gen increased by about 76% after 2000, boosting nitrous oxide- another powerful greenhouse gas. It contrib-uted about 9% of the increase in total global net emissions from human activities.
Irrigation practices also affect emissions. In the past, irrigated rice paddies were kept flooded throughout the growing season, resulting in constant greenhouse gas emissions produced by mi-crobes that thrive in the wet environment.
Over the past two de-cades, however, more farm-ers have used intermittent flooding – draining their fields periodically. This change has lowered methane emis-sions compared with keeping the paddies continuously flooded. However, we found a slight increase in nitro-gen oxide emissions as soils cycled between wet and dry, which induces microbes to transform nitrogen in organic matter into nitrogen oxide gases, particularly nitrous oxide.
Climate impact of rice production
Putting a full climate price tag on rice production is harder than measuring one greenhouse gas at a time.
Rice paddies emit meth-ane and nitrous oxide from wet or flooded soils. They also remove carbon dioxide from the atmosphere as rice grows, and they lose carbon from their soils between crop seasons.
A credible global estimate requires consistently ac-counting for different gases and soil carbon changes, as well as the uncertainty in-volved in tracking data across space and time.
To do that, we combined three approaches: An ecosys-tem computer model allowed us to simulate crop growth, water conditions and soil processes to estimate changes in methane, nitrous oxide and soil carbon together.
An artificial intelligence-powered machine learning model improved estimates where measurements were sparse to cover all rice re-gions in the world.
And a meta-analysis of more than 1,200 field ex-periment sites provided direct evidence of how practices such as irrigation, fertilizer use and management of crop residue affect emissions.
Together, they allowed us to quantify emissions from 1961 to 2020, determine what drove those emissions, and test the potential of mitiga-tion techniques under future climate conditions.
What works and doesnât for climate mitigation
There are ways to reduce emissions from rice pro-duction without sacrificing yield.
Our study found that re-ducing fertilizer use and residue applications, manag-ing irrigation to allow dry periods in between flooded ones and reducing tillage could, together, reduce global greenhouse gas emissions from rice by about 10% by midcentury.
We were surprised to find that replacing chemical fer-tilizers with more organic choices is not always better from a greenhouse gas per-spective, although it may raise nitrous oxide emissions. This strategy is particularly effec-tive in regions with reliable irrigation infrastructure, in-cluding large parts of Asia.
Maintaining moderate amounts of straw and other crop residue in the field can help boost soil fertility, but too much can increase meth-ane emissions and accelerate the loss of carbon from the soil. Another option is to convert part of the residue into biochar- burning it un-der low-oxygen conditions before mixing it into flooded soils. Biochar can help sta-bilize soil carbon and reduce methane emissions.
Improving water man-agement can be a powerful tool for reducing emissions. Periodically draining fields reduces methane production, though it may slightly raise nitrous oxide emissions. This strategy is particularly effec-tive in regions with reliable irrigation infrastructure, in-cluding large parts of Asia.
Managing fertilizer use is also an effective mitiga-tion strategy, particularly in highly fertilized systems, including parts of China and South Asia. Excess nitrogen increases nitrous oxide with-out a clear increase in crop yields and increases water pollution. Reducing overap-plication of nitrogen reduces emissions and water pollu-tion, and it saves farmers money in the process.
A climate ceiling for rice production
The bottom line is both hopeful and sobering: Target-ed sets of optimised practices can deliver meaningful emis-sion reductions without los-ing rice yields, but the total global possible reduction is modest.
