Kwesiga, Julius: Assessing and targeting management options for smallholder rice-based systems in Kilombero floodplain, Tanzania. - Bonn, 2021. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Julius Kwesiga}},
title = {Assessing and targeting management options for smallholder rice-based systems in Kilombero floodplain, Tanzania},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2021,
month = apr,

note = {In sub-Saharan Africa, agriculture contributes up to 50% of the GDP. The increase in agricultural output in the region is predominantly from area expansion instead of improvements in productivity per unit area. In the past, cultivation was, for the most part, done in the upland areas. However, with changing climate and rising population pressure, cultivation has extended into wetlands. Prolonged periods of water supply and relatively fertile soils provide wetlands with great potential for expansion and intensification of agriculture production, thus contributing to food security.
The Kilombero floodplain, one of the largest rice-producing areas, was the focus of the BMBF funded project “GlobE Wetlands”. The Wetlands project was poised to assess the potential of transforming lowland wetlands into a breadbasket of East Africa, and provide science-based guidelines, tools and policy advice to facilitate the process. The knowledge gains obtained by different project groups within the GlobE-Wetlands project shaped the design of the agronomic experiments and guided the choice of treatments and their application in this thesis.
In Kilombero, smallholder farmers produce rainfed lowland rice mainly in floodplain environments that are characterised by low soil nitrogen contents of the predominant Fluvisols and highly variable hydrological conditions, resulting in low yields and large yield variations. This thesis's studies were designed to compare farmers’ management practices, evaluate the effects of alternative management options on lowland rice performance, and define key contributing factors towards improved site-specific management. Field experiments were carried out near Ifakara, Tanzania, in three hydrological zones of Kilombero floodplain, namely the potentially drought-prone fringe, the favourable middle and the submergence-prone center positions over four years. Treatments on varying land, water and fertilizer management were implemented in researcher-managed plots, following hierarchical yield gap procedures.
Grain yields of rice (averaged over the four treatments) were higher in the fringe (6.5 t/ha) and the middle (5.7 t/ha) than in the center positions (4.6 t/ha). Farmers’ practice with no field bunding and land levelling and no fertilizer application resulted in the lowest yield (3.0 t/ha) and highest yield variability, with an adjusted coefficient of variation of up to 91% between years and positions. Simple soil and water management such as land levelling and the building of water-retaining field bunds significantly increased rice grain yields beyond farmers’ practice in the fringe and middle positions, where grain yields were generally higher than in the submergence-prone center position. Also, yield variability and hence the production risks were highest in the center and lowest in the fringe positions.
Depending on the position within the floodplain, organic treatments increased rice grain yields by >60%. Sole green or farmyard manure applications had similar effects on grain yield. In contrast, a combination of green and farmyard manure led to a significant increase in grain yield beyond both the control and sole application of organic amendments. Despite partial N balances being mostly negative, we observed positive residual effects on the non-amended rice in the fourth year of the study. Manure applications significantly increase soil C and N contents, hence enhancing soil fertility and increasing rice grain yields. On average across years and positions, the potential, attainable, and farmers’ actual yields were 11.5, 8.5, and 2.8 t/ha, respectively. Most management options tested contributed substantially to closing sizeable prevailing yield gaps. Thus, simple field bunds combined with land levelling closed up to 35% of the exploitable yield gap. Mineral N and organic amendments contributed up to 60% of the potential yield. Combinations of improved land, water management, and mineral N application closed up to 80% of the exploitable yield gap. Mineral N tended to be more effective in closing the yield gap than green or farmyard manure. While fertilizer strategies improved soil fertility and reduced yield gaps, their relative benefits showed a high site-and system-specificity. Thus, this thesis provides insights on the rice performance at different hydrological positions and in different years, highlighting the potential for a sustainable increase in rice yield in highly variable floodplain wetlands. Combined with recommendations from other groups of the Wetlands consortium, these findings contribute to guiding policy formulation and agronomic recommendations for Kilombero floodplain, and possibly beyond, to environments with similar climatic, edaphic and socio-economic conditions.},

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