Title: Denitrification potential in a natural wetland in an agricultural catchment receiving urban runoff and partially treated wastewater.

Author: Emmanuel Omonding

Supervising Institution: IHE Delft  - Institute for Water Education

Year: 2014



Denitrification in Namatala wetland in Eastern Uganda may be important to protect the Mpologoma River and may ameliorate the delivery of N to Lake Kyoga, from urban runoff and wastewater. The three months study analyzed the potential of a natural wetland receiving urban runoff and partially treated wastewater to denitrify reactive N in an agricultural catchment. The study was to identify which land covers in the wetland had high denitrification potential, to identify whether carbon or nitrate controled denitrification in different land use/land cover types, and to understand which soil properties affected potential denitrification rate. Potential denitrification of this wetland was compared with rates in other East African wetlands.

Potential denitrification rates (PDR) were measured using denitrification enzyme assay on soil samples (0-5 cm depth) from land use type of mixed vegetable, maize, rice, sugar cane and natural papyrus vegetation in the Namatala wetland, Uganda. These land uses were influenced by either urban runoff or wastewater. Soil samples were analyzed for NO₃, NH₄, total organic carbon, bulk density, moisture content, and pH. Data was analyzed using repeated measures ANOVA with pairwise t-test adjusted p (Holm) method and one way ANOVA with TukeyHSD post hoc for comparison of means. Simple regression models were used to see which physio-chemical factors were important to explaining the variation of PDR.

Potential denitrification rates ranged from 2.8 to 9.4 μg N₂O gˉ¹ DW hˉ¹ in papyrus and 0.21 to 15.04 μg N₂O gˉ¹ DW hˉ¹ to 15.04 agricultural plots with the potential to remove 6.4 to 10.50 (mg N₂O kgˉ¹ DW hˉ¹ and 1.92 to 15.04 mg N₂O-N kgˉ¹ DW hˉ¹ respectively.

Carbon was limiting in agricultural plots with nitrogen showing limitation in water logged papyrus and rice. Soil moisure was the main factor controlling denitrification.