Effects of Land Use on Nutrient Concentrations, Carbon Dynamics and Stream Metabolism in River Rupingazi, Kenya

Abstract

Rivers and streams store, process, and transport carbon and nutrients between terrestrial and marine ecosystems. Concentrations of these elements vary from one part of the stream to another depending on the land use change, which alters infiltration, surface runoff and evapotranspiration, thus influencing nutrient input into rivers and streams. This study was conducted in Embu, Kenya between November 2020 and January 2021 to determine how land use affects nitrogen (N), phosphorus (P) concentrations, carbon dynamics and stream respiration in River Rupingazi, since significant land use change has been observed over the years yet little has been documented about the effects. Water samples for nutrients and Dissolved Organic Carbon (DOC) analysis and sediment samples for respiration measurements were collected from ten selected sites in upstream, mid-stream and downstream of the river identified according to land use. In the laboratory, ammonium, nitrites, nitrates, Total Nitrogen, Soluble Reactive Phosphorus, Total Phosphorus and DOC were determined. Stream respiration was investigated through incubating sediment samples and measuring organic matter content afterwards. Questionnaires were administered to locals to find out the dominant land use type and main agricultural activities. Data from this study was analysed using R software version 4.0.0 and SPSS software version 25. One way ANOVA was used to check for differences in nutrient, DOC concentrations and respiration in the three land use types. Pearson correlation was used to determine the relationship between the variables. Statistical tests were done at 0.05 significance levels. Most nutrients and physicochemical variables showed significant differences (ANOVA, p<0.05) among the sampled sites except pH and TP. The agricultural land use had significantly higher nutrient concentrations compared to urban and forest land uses. Concentrations of DOC increased along the longitudinal continuum and ranged between 0.44 ± 0.14 mg/L in the forested section and 0.67 ± 0.30 mg/L in the agricultural section. Stream respiration rates were highest in site R5 (1.8 mg O2/h) in the agricultural section and lowest in R10 (0.9 mg O2/h), also in the agricultural area and did not show significant variations among the land use types. Therefore, results from this study bridge the gap between the influence of land use changes on N, P, DOC and stream metabolism and form baseline information for sound catchment management to improve the health of River Rupingazi. This study recommends that nutrients, DOC dynamics and stream respiration be studied during both the wet season and dry season to get the dynamics well