Characteristics of carbon sequestration and methane emission in rice-fish system

Rice-fish systems are unique rice farming systems that coculture rice with fish (in this paper, “fish” refers to a wide range of aquatic animals including carp, crayfish, shrimp, crabs, and softshell turtles, and others). Studies have shown that the interactions between rice and fish profoundly change the cycling of C, N, and other elements in paddy ecosystems. Whether and how rice-fish coculture affects C sequestration and methane (CH₄) emissions are matters of concern. Based on recently published data, we presented a review of the properties of soil organic C (SOC) and CH₄ emissions in rice-fish systems. Compared with that of the rice monoculture system, the SOC content (0–20 cm soil layer) in the rice-fish system tended to increase. An extra C input due to the feeding and excreta transformation of aquatic animals contributed to the increased SOC in the rice-fish system. CH₄ emissions from the rice-fish system differed greatly among different studies. Some studies have shown that CH₄ emissions from rice-fish systems (e.g., rice-frog, rice-crayfish, and rice-carp) are significantly lower than those of rice monoculture systems, whereas some studies have found that the CH₄ emissions of rice-fish systems (such as rice-carp) are significantly higher than those of rice monoculture. These differences in CH₄ emissions in the rice-fish system could be caused by the type of fish (e.g., carp, crayfish, shrimp, crabs, and softshell turtles), rice variety, paddy environment, and farming management. To improve our understanding of C sequestration and CH₄ emissions in the rice-fish system, more studies and efforts are required. These efforts include i) quantifying the potential of C sequestration and CH₄ emissions in rice-fish systems by establishing long-term studies, examining the variation in C sequestration and CH₄ emissions among different types of rice-fish systems (e.g., rice-carp, rice-crab, rice-turtle, rice-frog, and rice-crayfish), and outlining the general trends of C sequestration and CH₄ emissions in the rice-fish system; ii) understanding the mechanisms by which aquatic animals affect soil C pools and C cycling in paddy ecosystems and examining whether this changed C cycling would affect CH₄ emissions; and iii) development of a technology package for culturing rice and fish, including breeding or selecting rice varieties that can adapt well to rice-fish systems and can reduce CH₄ emissions, optimizing fertilization rates and fertilization methods for rice, optimizing feeding rates and methods for fish, and optimizing strategies of the straw return rate.