By applying climate smart farming practices, small-holder farming communities in India can become more climate resilient, as well as improve yield and profit. But as recently revealed by Environmental Defense Fund, rice farmers using the well intentioned current prescribed irrigation practice – intermittent flooding – to improve mitigation and adaptation could be contributing to elevated emissions of nitrous oxide (N2O), a powerful, long-lived greenhouse gas.
In collaboration with the Fair Climate Network and a coalition of grassroots NGOs across India, we worked directly with small-holder farmers across 16,000 acres in five states between 2012 and 2016 to perform high frequency monitoring of methane (CH4) & N2O emissions for both business-as-usual and potential climate smart farming practices.
Photo by Tamil Selvi. A farmer learning to measure water levels in the field using a field water tube
Our resulting study, just published in Proceedings of the National Academy of Sciences, and our accompanying global analysis describe the under-appreciated role of N2O emissions from rice farms across the world. We specifically show that intermittently flooded rice paddies can emit as much as ~33 kg-N2O ha-1 season-1, three times more than ever reported before in global literature. We also show that emission factors (percentage of added fertilizer-N converted to N2O) currently used for rice in national inventories submitted to Unites Nations are too low (i.e., 0 to 0.7%) as compared to our results (up to 30%). In fact, all the world’s major rice producers, including India and China, provide estimates of methane emissions from rice farms but do not report rice-N2O or related emission factors to the UN.
It is not surprising then that our accompanying geospatial extrapolation suggests that N2O emissions from rice farms might be greatly underestimated. Under intense forms of intermittent flooding, we show that Indian rice farms may be emitting 0.5 to 0.8 million metric tons of N2O annually, which is 30-40 times more than the current estimates that assume continuous flooding at most farms and use low emission factors.
The good news is that it is possible to reduce climate impacts of rice cultivation over both the short (e.g., 20 years) from CH4 emissions and long term (e.g., 100 years) from N2O emissions if organic matter and nitrogen additions are co-managed along with shallow (mild-intermittent) flooding. We have found that over the long term (100 years), integrated management can decrease N2O and CH4 emissions by 0-6 tCO2e100 ha-1 season-1 and 0-2.5 tCO2e100 ha-1 season-1, respectively. In contrast, alternate wetting and drying (or intermittent flooding) without properly co-managing nitrogen or organic matter inputs, can significantly increase N2O emissions. We encourage the integration of our recommendations into existing guidelines issued by Indian research institutes and other organizations promoting intermittent flooding because they would maintain or increase soil carbon even under intermittently flooded conditions, resulting in higher nitrogen and water use efficiencies, and improved farm profitability.
Nitrogen fertilizer and organic matter added to rice farms have a positive and negative influence on N2O emissions, respectively. However, we found that flooding regime is the most important factor influencing the extent of N2O emissions during rice cultivation. In intermittently flooded rice farms, soil oxygen levels are more favorable for high N2O but less so for high CH4 emissions. Our paper classifies flooding regime by water index (cumulative extent of flooding as determined by field water tube levels during the season) and the number of long flooding events (where water stays above soil level for over three days).
Mild-intermittent flooding can minimize net climate impacts of rice. Alternate wetting and drying usually advocated to reduce CH4 emissions allows water to drop down to 15 cm below soil level multiple times in a season and roughly corresponds to our medium-intermittent flooding regime which increases N2O emissions in comparison with continuously flooded farms. Mid-season drainage (-20 cm for 7-8 days) would imply a water index of 100-450 cm.
Based on our in-depth analysis of climate impacts of both business-as-usual and alternate framing practices from many farms in India, we can offer the following recommendations to farmers.
- Water index should stay between -250 and 250 cm for shallow (i.e., mild intermittent) flooding.
- The number of times water stays above soil level for >3 days should be limited to control CH4 emissions.
- The amount of inorganic N fertilizer additions should be limited and optimized to manage crop yields. For regions that remain intermittently flooded, inorganic N should be added in split doses right before a flooding event.
- Farms should not be drained too much and the water levels should stay above -5 to -7 cm during the growing season (except close to harvest).
- For farms where water likely does not percolate down quickly (or water index is high), organic matter use should be reduced to limit CH4 emissions, especially when soil organic carbon content is high.
- For farms where water percolates quickly (or water index is low), higher amount of organic carbon can be added to reduce N2O emissions.
These recommendations are general, and much more region-specific research is necessary to determine 1) the actual extent of flooding at irrigated rice farms; 2) how methane and nitrous oxide emissions correlate with soil and management parameters in areas outside our study regions; and 3) how to maximize rice yields while reducing climate impacts.
Improved rice seed varieties (with tolerance for flooding, drought or salinity) can help farmers adapt to changing climate. However, the higher costs associated with hybrid seeds, and lower suitability for subsistence farmers and rain-fed systems have kept their adoption rates very low in Asia, except for China. Unless these new rice varieties are managed to optimize resource use efficiency and sustainable yield, rice farming will not be able to actualize its large potential to mitigate climate change while providing long-term food security. Thus, it is crucial that all climate smart farming initiatives choose the right flooding regime for that region to mitigate both methane and nitrous oxide emissions from rice farms.
For additional resources, including a global risk map of rice nitrous oxide emissions, visit edf.org/riceN2O
This blog was co-authored by Richie Ahuja, Tapan Adhya & Kritee
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