Effects of restoration on soil, plants and water quality in Nakyesanja wetland, Wakiso District, Uganda

Abstract

The ecological performance of NAkyesanja, a restored wetland with Cyperus papyrus, was evaluated by comparing soil quality and plants characteristics with Tugavure, a reference natural wetland nearby. Assessment was also made of water quality at the inlet and outlet of the restored wetland. A description of soil profile pits of a degraded site, the restored wetland and the natural wetland were made and the soil physical and chemical characteristics namely color, texture, pH, organic matter, organic carbon, nitrogen, available phosphorus, potassium and calcium were assessed. Plant species were identified in 1m2 plots along two transects in the restored and natural wetlands. Cyprus papyrus phytomass density was also determined. Below ground water level was also measured in ten plots each in the restored and natural wetland. The pH, electrical conductivity (EC), total dissolved solids (TDS) and temperature of water at the inlet and outlet of the restored wetland were determined in situ and total nitrogen (TN), nitrate-nitrogen (NO3-N), ammonium-nitrogen (NH4-N), total phosphorus (TP) and soluble reactive phosphorus (SRP) were analyzed in the laboratory. The efficiency of the restored wetland for nutrient removal was calculated using the formula: (Load Inlet – Load Outlet) / Load Inlet * 100%. Soil properties in the restored wetland exhibited similar characteristics to the natural wetland. This was attributed to the planting of papyrus and colonization of vegetation that contributed to litter and other organic plant inputs. The decreasing soil properties in the degraded wetland were attributed to crop cultivation and the dumping of murram in the area. Plant diversity, phytomass, species richness, and heights in the restored and natural wetland were not significantly different with their respective p values being 0.911, 0.202, p=0.921, p=0.562. Water level was significantly lower in the restored wetland (p=0.001) and showed significant relationship to plant diversity in the natural wetland (r=0.69), meaning waterlogging controlled the growth and distribution of species in the natural wetland. The phytomass density of papyrus recorded higher values in areas of high water level; 4.9 and 2.5 Kg DW/M2 at 15 and 30cm water depth in the natural wetland and 2.8 and 2.4 Kg DW/M2 at 45 and 50 cm water depth in the restored wetland respectively. Aquatic macrophytes were established in areas of high water level but areas of low water level like the landward or wetland edges were colonized by opportunistic plants that could survive better in areas of fluctuating water conditions. This implies that water level was the main determinant supporting the growth and distribution of macrophytic vegetation in the restored wetland. The comparison of wet and dry season’s pH, discharge, EC, temperature, TDS, TN and N03-N in the wetlands were significantly different (p<0.05). However, TP, SRP and NH4-N were not significantly different (p>0.05). The variation of water quality parameters between the inlet and outlet concentrations of the restored wetland demonstrated an insignificant difference for all seasons. The nutrient removal efficiency of the restored wetland was higher in the dry than the wet season for all parameters. The lower nutrient removal in the wet season was attributed to the channelization of the water through the system, the small size of the wetland and the low influent load. The better dry season restored wetland water quality performance demonstrated a positive influence of macrophytes on nutrient removal process especially in areas dominated by papyrus. In order to ensure soil development, successful colonization of aquatic macrophytes and efficient nutrient removal in a restored wetland, the hydrology of the system should be designed to distribute water to most sections of the wetland, and not channelized. The Nakyesanja wetland should serve as a model of wetland restoration for wider dissemination in Uganda (and maybe beyond) given the simple, inexpensive and yet seemingly highly effective methodology employed.