SPATIOTEMPORAL DYNAMICS OF RIPARIAN GREENHOUSE GAS FLUXES AND SOIL CARBON STOCKS IN THE BURA AND WUNDANYI CATCHMENTS, KENYA

Abstract

Agropastoral riparian zones are globally linked to increased greenhouse gas (GHG) emissions and soil organic carbon (SOC) depletion, but the Kenyan cases are underrepresented in global datasets. The study investigated variations in GHG fluxes and

SOC stocks across three livestock production density levels in the Bura and Wundanyi sub- catchments. Eighteen (18) riparian sites (high livestock density (HLD n=7), low livestock

density (LLD n=4), and zero density (ZLD n=7) were sampled between 2021 and 2022 in 2 dry seasons and 2 wet seasons in three topographies [(Upland at >1000 masl), (Midland 700 - <1000 masl), and (Lowland <700 masl)]. Results from Wundanyi showed higher CH4 fluxes under HLD, particularly in the midland zones, where emissions reached 302.5 μg CH4 m−2 d−1. In contrast, ZLD sites acted as net CH4 sinks. The mean CH4 flux for Wundanyi was 30.21 mg CH4-C m−2 d−1. Consistently high CO2 fluxes were recorded in Wundanyi with a mean of 900.17 mg CO2-C m−2 d−1. The highest mean was observed at the ZLD sites, Upland (1146.75 mg CO2-C m−2 d−1) and Midland (1003.25 mg CO2-C m−2 d−1). SOC stocks in Wundanyi averaged 6.91 Mg C ha−1, with higher concentrations recorded in Upland areas (8.01 Mg C ha−1). These stocks were greater during wet seasons (8.47 Mg C ha−1) and showed an increasing trend with livestock density: 6.67, 7.03, and 7.40 Mg C ha−1 in ZLD, LLD, and HLD, respectively. In Bura, CH4 fluxes were predominantly negative with mean absorptions in Midland LLD and Upland HLD areas (- 169.27 and -256.63 mg CH4-C m−2 d−1), respectively. CO2 emissions were moderate, with a mean of 527.07 mg CO2-C m−2 d−1, remaining <700 mg CO2-C m−2 d−1 across sites and livestock systems. N2O fluxes in Bura were moderate, peaking at 155.93 μg N2O-N m−2 d−1 in Midland zones under HLD. SOC stocks were highest in the Midlands at 8.29 Mg C ha−1, with values increasing alongside livestock density (7.13, 7.78, and 9.16 Mg C ha−1 for ZLD, LLD, and HLD, respectively. Bura was a strong methane sink, while Wundanyi recorded CH4 emissions in grazed areas and net uptake in ZLD. The upland and ZLD areas were significant CO2 sources. This study confirms that increasing livestock densities depleted SOC stocks with appreciable human disturbance contributions. Livestock production densities are dictated by topography, and GHG emissions are often part of natural exchange processes. To realise sustainability in livestock systems, sustainable stocking densities, protecting riparian strips, and enhancing vegetative cover should be practised. Long-term catchment-level research to document seasonal nutrient input-output dynamics would improve GHG inventories for sub-Saharan Africa.