Title: Nutrient dynamics in streams: Evaluating the influence of agricultural practices on the functioning of streams in the Mpanga and Dura catchments

Author: Dianah Busingye

Supervising Institution: BOKU University

Year: 2025

 

Abstract:

Streams are important channel networks for nutrient transport and cycling, but their functionality is highly influenced by surrounding land use, especially agriculture. As agriculture expands, the adverse effects on the physical, chemical, and biological characteristics of streams increase, leading to cases of eutrophication and increased sedimentation. Agricultural practices such as crop cultivation, livestock keeping and commercial agriculture can impact stream structure and functioning by increasing nutrient loads into these systems, thus affecting nutrient uptake and metabolism. Although the impacts of agricultural expansion are globally documented, little is known about the impacts in tropical streams such as Mpanga and Dura, whose catchments have experienced rampant conversion of forests to agricultural land use. This study, therefore, aims to evaluate the influence of agricultural practices and intensity on in-stream processing of nutrients in the Mpanga and Dura rivers. It involved assessing the impacts of different agricultural practices and intensity on the streams’ phosphate, nitrate and ammonium concentrations, metabolic rates of aquatic communities and retention/export rates of the nutrients in streams. Field experiments were conducted in sixteen sites, taking measurements of nutrient concentrations, stream metabolism, and nutrient additions experiments using the pulse addition method. Nutrient uptake parameters of focus were uptake length (Sw), uptake rate (U), and uptake velocity (Vf). Streams draining the croplands and animal farms had the highest nutrient concentrations compared to tea plantations, contrary to expectations. Stream metabolism was highest in tea plantations, and uptake parameters showed minimal significant difference among the different practices. Uptake efficiency of ammonia and SRP was however highest in tea plantations. The study shows that nutrient concentrations and stream functioning are not only attributed to land use or agricultural practices, but also to several factors in the catchment. In this study, the bareness of the land and slope played a crucial role in the transport of nutrient-bound sediments from agricultural farms to streams. Management practices such as contour farming, planting cover crops, controlled fertilizer application, and planting trees on the high slopes and buffer zones are recommended to reduce runoff of nutrients and sediments into the streams.

 

Keywords: Agricultural practices, Nutrient uptake, stream metabolism, Bare land, Slope