Impact of rice field emissions on climate greater than believed
doi:10.1038/nindia.2018.138 Published online 28 October 2018
When rice farms are intermittently irrigated, they emit as much as 45 times more nitrous oxide (N2O) than farms that are continuously flooded – a figure missing from most estimates of greenhouse gas (GHG) emissions, according to a new study1.
These emissions of N2O, a long-lived GHG, from intermittently-irrigated farms – a common practice in South-Asia, Africa and South America – are not taken into account while calculating global climate impact. The researchers caution that due this lacuna, the actual impact of rice farming may have been ‘significantly underestimated’ till now.
The study by Environmental Defense Fund in the US, with partners in India, analysed GHG emissions from five rice farms across three agro-ecological regions in the Indian subcontinent. Results from the study indicate that N2O emissions per hectare can be three times higher (at 33 kg N2O per hectare per season) than the maximum previously reported. Across the Indian subcontinent, N2O emissions might be 30–45 times higher under intensified use of intermittent flooding than under continuous flooding, the researchers say.
Global rice cultivation is estimated to account for 2.5% of current anthropogenic warming because of emissions of methane (CH4), a short-lived GHG. This estimate assumes a widespread prevalence of continuous flooding of most rice fields and hence does not include emissions of N2O. The researchers say, together with CH4 emission estimates, N2O emissions from rice farms across the world "under intense forms of intermittent flooding" could be equivalent to annual carbon-dioxide emissions from about 200 coal power plants.
Based on the belief that minimising CH4 from rice cultivation is always climate beneficial, current mitigation policies promote increased use of intermittent flooding. Managing these fields with water, inorganic nitrogen and/or organic matter can decrease climate impacts caused by GHG emissions up to 90%, the researchers say, though nitrogen management might not be central to N2O reduction.
To create the most climate-friendly water management systems, an understanding of climate benefits and drawbacks of different flooding patterns over time is needed, the researchers point out. For this, region-specific studies of rice farming practices that map flooding regimes and measure effects of multiple variables on N2O and CH4 emissions are necessary, they say.
“While the experimental work was done in India, the global implications of the results are tremendous," says Tapan Adhya, Director of South Asia Nitrogen Centre, New Delhi, and one of the authors of the study.
"The good news,” he says, “is that 60% of net GHG emissions from irrigated rice farms can be decreased just through shallow (mild-intermittent) flooding."
1. Kritee, K. et al. High nitrous oxide fluxes from rice indicate the need to manage water for both long- and short-term climate impacts. PNAS (2018) doi: 10.1073/pnas.1809276115