Mesocosm studies on the impacts of conversion of wetlands into fish and rice farming on sediment and nutrient loads to surface water 4

Alteration of water and nutrient cycles threatens land, water, climate, biodiversity and human health and livelihoods. The main objective of this study was to assess the effects of conversion of natural valley-bottom wetlands in southern Rwanda to rice and fish farming on sediment and nutrient flows. To achieve this, small scale mesocosm experimental wetland and rice plots and fishponds were established to measure sediment, nitrogen (N) and phosphorus (P) flows Sediment, N, and P discharges (concentrations and loads) in outflows were lower in wetlands than in fishponds and rice plots, indicating that wetlands retained and did not release sediment and nutrients. While wetland sediment retention was linked to the dense vegetation cover, low sediment, N and P releases were also attributed to low wetland disturbance, greater N and P transfer to the underground, higher uptake by plants and settling in sediments. Ploughing and weeding during the first three months of rice farming, and water renewal and dredging in the middle and end period of fish farming generate and discharge large amounts of sediment, N and P in the outflow of these systems. This makes fishponds a temporal sediment and nutrient storage during early farming stages and a source towards the end of farming. In contrast, rice farming generates sediments and nutrients early during the farming period (ploughing, weeding, transplantation and fertilizer application) and traps them towards the end. Despite higher fertilizer input in rice farms, N and P storage in soil decreased in rice farms (by 4.7 and 1.4%, respectively), but increased in fishponds (by 3.3% and 4.4%) and wetlands (3.8% and 1%). The decrease in nutrient soil storage was attributed to higher N and P uptake in rice plots (on average 662 and 270 mg m⁻²d⁻¹ of N and P, respectively) than in wetlands (359 and 121 mg m⁻² d⁻¹ of N and P) and fishponds (7.4 and 4.4 mg m⁻² d⁻¹ of N and P). The increase in nutrient storage increase in wetlands was attributed to senescence and decomposition of vegetation and subsequent storage in wetland sediments. Efficient reuse and recycling of water, sediments and nutrients within catchments may be achieved through rotational rice-fish and wetland-rice farming to balance the sources and sinks of sediment and nutrients in the landscape.