Title: Phosphorus Binding Fractions In Oxidized Migina Wetland Sediments, Rwanda

Author: Sheela Sharma

Supervising Institution: IHE Delft  - Institute for Water Education

Year: 2014



Phosphorus (P) is an essential element for plants and animals. Though it is one of the limiting macro-nutrients, in sediments it can be corrected by applying fertilizers. Excess P in sediments however reduces plants ability to uptake essential micronutrients (Fe and Zn). Several soil test phosphorus (STP) methods are in use to provide index measurement of available P in soil. These methods are also the basis for recommendation of fertilizers. The aim of this study was to assess three different STP methods to determine the available or extractable-P in oxidized and rewetted sediments from the Migina wetland in Rwanda. Sediments from the Gahenerezo rice farm (RM), Rwasave maize farm (RM), Tonga clay mining (TC), Gihindamuyaga forest (DF) and Ngorore natural wetland (NW) of Migina catchment were analyzed using STP methods: Water extractable-P, Olsen-P and modified Morgan-P methods were used with - Milli-Q water, buffered sodium bicarbonate and buffered sodium-acetic-acetate were used as single extractant respectively. As these methods uses completely different extractant, extractable-P value from each method cannot be compared with other one rather can be considered high or low "extraction power".

Water extractable-P extracted 0.23-0.4 mg P/kg and modified Morgan-P extracted 0.3-0.5 mg P/kg which is < 1 mg P/ kg of sediment for all land uses. Olsen method however extracted significantly higher quantity compared the other two methods (4.2 - 9.4 mg P/kg). Higher Olsen-P was found for GR (9.4 mg P/kg) while lower was for TC (4.2 mg P/kg). In follow-up experiments 16-81% lower Olsen-P value were determined.

Keywords: STP methods, extractable phosphorus, Olsen, modified Morgan, rewetted sediment, Migina wetland.