Nitrous Oxide Emissions from Rice Fields : Past, Present & Future

Atmospheric presence of nitrous oxide (N2O) is known for many years but its concentration has increased alarmingly by 46 ppbv (17%) since 1750 and by 0.0008 ppb yr-1 during 1990-1999. Nitrous oxide is a 296 times more potent greenhouse gas than CO2 on a 100-year time scale and causes enhanced greenhouse effect, a phenomenon first discovered in 1976. Nitrous oxide emission from 1750 to 2000 has caused an atmospheric radiative forcing of 0.15 W m-2. Moreover, N2O is indirectly involved in catalytic destruction of stratospheric ozone. Due to its atmospheric lifetime of about 120-150 years, atmospheric N2O burden will persist for many years even if its emission is stopped now. Various estimates differ widely on the contribution of agriculture in anthropogenic N2O emissions viz. 96%, 81%, 76%, 65% etc. Although several studies on N2O emissions from rice have been done in last two decades, no clear-cut contribution of rice or any other crop, for that matter, on atmospheric N2O loading has been worked out till date. Only less than 1% of applied N is lost through N2O from rice fields and its overall amount and radiative forcing are much lower than CH4, the major greenhouse gas emitted from rice fields. Possible sources of N for N2O emission in rice fields are fertilisers, manures and green manures, irrigation water, N fixed by floodwater algae, plant debris and as long as rice cultivation exists, N2O will be emitted. It has been suggested that N2O emission from agriculture might increase by 90% during 1986 to 2026. Monitoring of N2O from rice hasn''t been done as extensively as CH4, as emission of former is low due to reduction to N2 in flooded rice fields. Since, overall amount of fertiliser N usage and acreage of rice are increasing, N2O emission might increase appreciably globally and so, monitoring of N2O emission from different rice ecosystems and estimating realistic regional and global budgets form rice ecosystems assume high significance. Efforts to predict N2O emissions through simulation of soil N pathways are underway, but it is difficult since emission from rice soil is controlled by the real-time field conditions and fluctuations in cultural practices. Whatever be the emission, it creates environmental disturbance slowly but surely. Efforts should be focused on the mitigation of N2O emission without compromising crop production, degrading the environment and creating pressure on financial resources.