Integrated rice–fish production and methane emission under greenhouse conditions

A greenhouse experiment was carried out including the treatments: (1) rice combined with common carp, Cyprinus carpio, (2) rice combined with a mixed culture of common carp and Nile tilapia, Oreochromis niloticus, and (3) rice-only. Methane emission and some related water and soil characteristics were determined at regular interval. Generally, methane emissions were quite high and reached a maximum of >20 mg m−2 h−1 in all treatments, 34–40 days after flooding. Presence of fish tended to boost methane emission. Average methane emission was significantly higher in the carp treatment than in the rice-only treatment with the carp/tilapia group ranging in between. Floodwater-dissolved methane concentration was highest in the carp treatment because of methane dissipation due to fish activity. Fish perturbation of the soil entailed water turbidity due to resuspension of sediment granules and a higher level of particulate inorganic matter. Moreover, the presence of fish resulted in a decrease of the average pH and dissolved oxygen level, especially with tilapia. This can be explained with reduced photosynthesis due to fish herbivory. Oxygen level was negatively correlated with methane emission and positively correlated with the soil redox potential. These results suggest that fish stocking tends to increase methane emission due to: (1) increased release of soil-entrapped methane, and (2) a decline of floodwater oxygen, which fosters the anaerobic character of the soil environment.