Carbon cycling in rice field ecosystems in the context of input, decomposition and translocation of organic materials and the fates of their end products (CO 2 and CH 4)

Abstract

Rice fields are intensively managed, unique agroecosystems, where soil flooding is general performance for rice cultivation. Flooding the field results in reductive soil conditions, under which decomposition of organic materials proceeds during the period of rice cultivation. A large variety of organic materials are incorporated into rice soils according to field management. In this review, the kind and abundance of organic materials entering carbon cycling in the rice field ecosystem are evaluated first. Then, decomposition of plant residues and soil organic matter in rice fields is reviewed quantitatively. Decomposition of plant residues is shown to be the active process in carbon cycling in rice fields. Rice releases photosynthates into the rhizosphere (rhizodeposition), and they follow a different avenue of decomposition in soil from that of plant residues. Incorporation of rhizodeposition into microbial biomass and soil organic matter during the period of rice cultivation, and their fates after harvesting are evaluated quantitatively from 13C pulse labeled experiments. Percolating water transports inorganic and organic carbon from the plow layer to the subsoil layer. The amounts of their transport and accumulation in the subsoil layer are evaluated in relation to the amounts of soil organic C in the plow layer. Not only CO 2 but also CH 4 are produced in the decomposition process of organic materials in flooded rice fields. CH 4 evolution from rice fields is of global concern from the viewpoint of global warming. Origins of CH 4 evolved from rice fields are estimated first, followed by the fates of CH 4 in rice field ecosystems. Rhizodeposition is shown to be the main origin of CH 4 evolved from rice fields. Evolution to the atmosphere is not the sole pathway of CH 4 produced in rice fields. The amounts of CH 4 retained in soil, percolated to the subsoil layer and decomposed in soil are evaluated in the context of the amounts of CH 4 efflux. Thus, this review focuses on carbon cycling in the rice field ecosystem from the viewpoints of input, decomposition, and translocation of organic materials and the fates of their end products (CO 2 and CH 4).