Using High-Resolution Data to Assess Land Use Impact on Nitrate Dynamics in East African Tropical Montane Catchments

Land use change alters nitrate (NO₃-N) dynamics in stream water by changing nitrogen cycling, nutrient inputs, uptake and hydrological flow paths. There is little empirical evidence of these processes for East Africa. We collected a unique 2 year high-resolution data set to assess the effects of land use (i.e., natural forest, smallholder agriculture and commercial tea plantations) on NO₃-N dynamics in three subcatchments within a headwater catchment in the Mau Forest Complex, Kenya's largest tropical montane forest. The natural forest subcatchment had the lowest NO₃-N concentrations (0.44 ± 0.043 mg N L⁻¹) with no seasonal variation. NO₃-N concentrations in the smallholder agriculture (1.09 ± 0.11 mg N L⁻¹) and tea plantation (2.13 ± 0.19 mg N L⁻¹) subcatchments closely followed discharge patterns, indicating mobilization of NO₃-N during the rainy seasons. Hysteresis patterns of rainfall events indicate a shift from subsurface flow in the natural forest to surface runoff in agricultural subcatchments. Distinct peaks in NO₃-N concentrations were observed during rainfall events after a longer dry period in the forest and tea subcatchments. The high-resolution data set enabled us to identify differences in NO₃-N transport of catchments under different land use, such as enhanced NO₃-N inputs to the stream during the rainy season and higher annual export in agricultural subcatchments (4.9 ± 0.3 to 12.0 ± 0.8 kg N ha⁻¹ yr⁻¹) than in natural forest (2.6 ± 0.2 kg N ha⁻¹ yr⁻¹). This emphasizes the usefulness of our monitoring approach to improve the understanding of land use effects on riverine N exports in tropical landscapes, but also the need to apply such methods in other regions.