Land Use, Not Stream Order, Controls N₂O Concentration and Flux in the Upper Mara River Basin, Kenya

Area of Science:

• Environmental Science
• Biogeochemistry
• Ecology

Background:

• Anthropogenic activities are increasing nitrous oxide (N2O) emissions from river systems globally.
• Data on N2O emissions from tropical rivers are scarce, leading to uncertainties in global estimates.
• Human activity is rapidly increasing in many tropical regions, potentially exacerbating N2O emissions.

Purpose of the Study:

• To assess the effects of land use and river size on N2O flux and concentration in the Mara River, Kenya.
• To investigate the biogeochemical drivers of N2O production and consumption in different riverine habitats.
• To compare N2O emissions across agricultural, forest, and livestock-dominated stream sites.

Main Methods:

• Studied 46 stream sites in the Mara River basin during the transition from wet to dry season (November 2017–January 2018).
• Measured N2O flux and concentration, alongside parameters like dissolved organic carbon and CO2.
• Analyzed the relationship between land use, river size, and N2O dynamics.

Main Results:

• Agricultural stream sites exhibited the highest N2O fluxes (26.38 ± 5.37 μg N2O-N·m⁻²·hr⁻¹), significantly exceeding forest and livestock sites.
• Land use was a stronger predictor of N2O concentration and flux than stream size.
• N2O concentrations correlated positively with CO2 in forest and agricultural streams but negatively with dissolved organic carbon.

Conclusions:

• In-stream chemoautotrophic nitrification is likely the primary driver of N2O production in agricultural and forest streams.
• Complete denitrification may lead to N2O consumption in livestock stream sites.
• Further research is needed to elucidate the relative importance of nitrification and denitrification in diverse riverine habitats for N2O production.