Increased water productivity is critical for global water supply as it reduces the need for additional water and land in irrigated and rainfed systems. It can also ensure a balance between food production and ecological protection for sustainable human existence. This study determined the effects of water and soil management on rice (Oryza sativa) productivity in the wet and dry seasons under tropical inland valley swamp conditions. The water management included alternate wetting and drying, continuous flooding irrigation and aerobic rice system. Then the soil amendment included biochar addition to soil. In alternate wetting and drying, water was added to the depth of 60 mm over the land surface and allowed to dry to the depth of 150 mm below the surface before replenishment. In continuous flooding irrigation, 60 mm depth of water was maintained on the land surface for the entire growing period. Then in aerobic rice system, water was added to saturation only after 50% of the soil moisture was depleted. The irrigation treatments were for the dry season and rainfed treatment in the wet season, both with and without biochar. Based on the study, yields were generally higher in the dry season than in the wet season. Also, biochar treatments outperformed non-biochar treatments across the seasons and water treatments. Not only water productivity (expressed as grain yield per unit water), but also biochar productivity (grain yield per unit biochar) was higher in the dry season than the wet season. Although 114% (411 mm) more water was used in continuous flooding irrigation without biochar (774 mm) than in aerobic rice system with biochar (362 mm) during the 2015/2016 dry-season cultivation period, rice yield under continuous flooding irrigation was only 15% (533 kg/ha) higher. Again, even though 97% (883 mm) more water was used in continuous flooding irrigation without biochar than in aerobic rice system with biochar during the 2016/2017 dry season, rice yield under continuous flooding irrigation was only 11% (400 kg/ha) higher. Then while the effect of either biochar or water on crop productivity was significant, that of their interaction was limited. Most importantly, average water use across the treatments in the wet season (1235 mm) exceeded by 26% that in the dry season (984 mm) cultivation period in the study area. This suggested that irrigation increases productivity during second cropping in the dry season. It also suggested that biochar addition reduces irrigation while increasing productivity under continuous flooding irrigation system in tropical inland valley swamp ecologies.
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