Proceedings of The 7 th ACSA Conference 2011 29 Toposequnential Variation in Soil Fertility and Limiting Nutrient for Rice Growth in the White Volta Floodplain of Northern Ghana Yasuhiro Tsujimoto a* , Yukiyo Yamamoto a , Keiichi Hayashi a , Alhassan I. Zakaria b , Yahaya Inusah b , Tamao Hatta a , Mathias Fosu b , Jun-Ichi Sakagami a a Japan International Research Center for Agricultural Sciences, Japan b Savanna Agricultural Research Institute, Council for Scientific and Industrial Research, Ghana *Corresponding author: Japan International Research Center for Agricultural Sciences, 1-1 Ohwashi Tsukuba, Ibaraki 305-8686, Japan Tel.: +81 29 838 6368; fax: +81 29 838 6355. tsjmt@affrc.go.jp Abstract Integrated floodplain resource management for rice cultivation is imperative to satisfy the growing demand of rice in West Africa. Irrigated pot experiments were conducted with different fertilizer treatments to identify toposequential variation in soil fertility and limiting nutrient for rice growth within the White Volta floodplains in Ghana. Eighteen experimental soils were examined across a riverside to upland gradient, ranging from 898 to 4200 m in distance from the main riverside, from 73 to 106 m in elevation, and from 0.7 to 2338.1 m in distance from water sources. The soil analysis revealed close correlation between N-mineralization rates and carbon contents of the soils, which were exponentially decayed with distance from water sources. In the non-fertilized treatments, plant N uptakes at maturity also decreased along the same transect from the water sources. However, the dry matter production was little relevant to this toposequential factor. Various fertilizer treatments identified remarkable effect of sulfur on rice growth, which was more significant on soils closer to waster sources. NPK application without S increased only N concentration and N: S ratio in plant tissues but not biomass production. The results indicated that sulfur is the primary limiting element for rice growth, and its supplementation would be more beneficial as closer to water sources so as to effectively utilize greater N- supplying capacity of this agro-environmental soils. Keywords: rice, sulfur deficiency, floodplain, soil carbon, water source Introduction River floodplains, consisting of wide and flat plain of alluvium bordering rivers, are expected to support a large expansion in rice cultivation area and production in Africa, of which the major share is currently unexploited. Major constraints to expand agricultural activities into river floodplains include difficulty in water control, risk of complete submergence of plants, occurrence of water-borne diseases, and disadvantage in access to the road and market (Balasubramanian et al. 2007). On the other hand, this geographical environment can provide water resources as well as relatively fertile alluvial soils compared to uplands (Buri et al. 1999). Moreover, the use of river floodplains for rice cultivation should not cause spatial competition with the other crops due to the risk of periodic flooding. In the traditional lowlands in Asia, large variance in cultivation conditions and rice productivity are commonly recognized within relatively small areas, and farmers‘ management practices are adapted to this variance according to the sequential changes in soil fertility and water availability (Fukai et al. 1998). However, either quantitative data or farmers‘ practical adoption on the toposeqential distribution of environmental resources are scarce for rice cultivation within river floodplains in West Africa. Primary objectives of this study were, therefore (1) to quantify the rice productivity of soils in relation to small-scale toposequence and (2) to identify deficient nutrient for rice growth, which