Methane and nitrous oxide emissions from an integrated rainfed rice–fish farming system of Eastern India

Integration of fish stocking with rice (Oryza sativa L.) cultivation promises an ecologically sound and environmentally viable management of flooded ecosystem. Rice agriculture contributes to the emission of greenhouse gases CH4 and N2O, but little is known on the effect of fish rearing in fields planted to rice on the emission of these two greenhouse gases. In a field study, CH4 and N2O fluxes were measured from a sub-humid tropical rice field of Cuttack, eastern India, as affected by integrated rice–fish farming under rainfed lowland conditions. Three Indian major carps, Catla catla H., Labeo rohita H. and Cirrhinus mrigala H., and Puntius gonionotus B. were stocked in rice fields planted to two rice cultivars in a split-plot design with no fish and fish as the main treatments and two rice varieties as sub-treatments with three replicates each. Fish rearing increased CH4 emission from field plots planted to both the rice cultivars with 112% increase in CH4 emission in cv. Varshadhan and 74% in case of cv. Durga. On the contrary, fish stocking reduced N2O emission from field plots planted to both the rice varieties. Movement of fish and associated bioturbation coupled with higher dissolved organic-C and CH4 contents, and lower dissolved oxygen could be the reasons for release of larger quantities of CH4 from rice + fish plots, while higher dissolved oxygen content might have influenced release of more N2O from the rice alone treatment. The total greenhouse gas emission, expressed as CO2 equivalent global warming potential (GWP), was considerably higher from rice + fish plots with CH4 contributing a larger share (91%) as compared to rice alone plots (78–81%). On the contrary, N2O had a comparatively lesser contribution with 19–22% share in rice alone plots that was further reduced to 9% in rice + fish plots. However, considering the profit-loss analysis based on the market price of the produce, rice–fish system provided a net profit of $453.36 ha−1 over rice alone system in spite of higher carbon credit compliance of a rice–fish ecosystem due to larger cumulative GWP.