Northeast Thailand is one of the representative rainfed rice culture areas in Asia, where rice productivity is limited not only by unstable rainfall but also by poor soil. The objective of this study was to improve tainted rice yield at degraded fields in upper toposequence by incorporation of sediment soil accumulated at the lowest toposequence in mini-watersheds. We collected sediment soil from the pond when it was dry in the dry season in 2001 and placed it uniformly on the experimental field at a thickness of 5 cm, and incorporated it into the plow layer by double plowings and puddling. The clay content, soil organic carbon (SOC) content and cation exchange capacity (CEC) of the pond sediment soil were 0.42 kg kg(-1), 12.9 g kg(-1) and 13.8 cmol kg(-1), respectively, those of the field soil before the incorporation were 0.08, 4.6 and 5.0, respectively, and those after the incorporation were 0.19, 5.7 and 7.1, respectively. Rice cultivar KDML105 was grown under rainfed condition, incorporated with and without the pond sediment soil, each with and without fertilization (four treatments in total) for the seasons from 2001 to 2003. Incorporation of the pond sediments increased rice yield in the fertilized plot by 28% on the average over three seasons, but not in the unfertilized plots. The yield increase in the fertilized plot was associated with increased fertilizer-N recovery efficiency. Since incorporation of the pond sediment did not significantly improve field water holding capacity and mineralizable N in soil, its effect on the fertilizer-N recovery efficiency and rice yield could be ascribed to the increased CEC. As much clay sediments rich in SOC and clay are accumulated in the lower toposequence such as the bottom of ponds, the present study suggests that incorporation of pond sediment soil into fields is an effective technology to improve rainfed rice yield at upper toposequence in mini-watersheds in Northeast Thailand. (c) 2005 Elsevier B.V. All rights reserved.
