Proposal of coexistent ecosystems with paddy and water-covered plant for promoting an efficient production of biomass resources

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In an ordinary paddy field whose cultivated dominant crop is a paddy, we tried to propose a new cropping systems based on coexistence with the plural crops for expecting the mitigation of environmental impact and escape of monoculture systems. In this study, the effect of plant density in coexistent systems on growth rate, structure of community and yield of the two crops were investigated in the 2002-2003 experiments with three repetitions under the same condition of application rate of fertilizer. We used a water hyacinth as the water-covered model crop. Plant densities were 22.2 hills/m^2 for paddy, and 22.2, 16.7, 11.1, 5.6 and 0 hills/m^2 for water hyacinth, respectively. As a result, an introduction of water-covered crop contributed the increment of carbon fixation rate per unit area although some increased plant densities affected the competition of resources utilization and paddy yield negatively. Finally the optimum plant density of water hyacinth was estimated at the neighbor of 11.1 hills/m^2 from the viewpoint of carbon fixation.