Title: Effects of livestock on carbon and nitrogen fluxes and storage in watering pans of semi-arid areas: TaitaTaveta county (Kenya)
Author: Michael Waiswa
Supervising Institution: IHE Delft - Institute for Water Education
Watering pans are relatively large excavated artificial ponds in which surface runoff collects. They are used in water scarce areas to especially in semi-arid areas for both livestock watering and domestic use. Through surface runoff, the watering pans accumulate elevated amounts of terrestrial carbon, manure and urea from livestock and nutrients from the rangelands whose biogeochemical processing results in greenhouse gas (methane CH4, carbon dioxide CO2 and nitrous oxide N2O) emissions. Therefore, to understand their role as “new” aquatic ecosystems in rangelands for carbon and nitrogen emissions and storage, we analyzed gas and water samples together with sediment cores from 24 watering pans in Taita-Taveta County (Kenya) between November and December 2019. For a fair representation of the herding themes in Taita-Taveta County, the watering pans are classified into six livestock systems ranging from those in large cattle ranches, encroachments on wildlife conservation areas, and small herds within settlement areas along mixed crop farming. We hypothesize that different livestock systems will influence how much nutrients, manure, and or terrestrial organic matter reaches the watering pans. Gas samples for flux and concentration quantification taken using the static gas chamber and headspace equilibrium technique respectively while the water sample analysis was used to relate fluxes and concentrations to biogeochemical controls. The livestock systems appeared not to be important for understanding the variation of gas fluxes; however the typical fluxes could have been obscured by the dilution effect of the systems from the heavy rains. CH4 concentrations ranged from 0 to 2.8 μmol L-1 having been produced through anaerobic respiration using DOC as an energy source. CO2 concentrations on the other hand ranged from 0.5 to 130 μmol L-1, possibly being formed through oxidation of CH4 by methanotrophs and mineralization of labile DOC in heterotrophic respiration. Lastly, N2O concentrations correlated positively with NO3- N which could suggest production through denitrification. Diel gas fluxes and concentrations measurements at Gdma watering pan showed no significant variations during the day and night. Our comparisons of CH4, CO2 and N2O emissions from watering pans in similar studies showed fluxes of up to 3 orders of magnitude higher than those we measured (Mean ± SE; CH437.5 ± 10; CO2 37 ± 93 and N2O; 6 ± 15) which further supports the idea of dilution by rainfall. The comparing gas fluxes from watering pans with the landscape, showed that watering pans emit CH4 and N2O of 2 orders of magnitude higher than grazing areas for instance. These elevated fluxes from watering pans could be influenced by presence of water logging in the landscape (as watering pans) that induces biogeochemical processing for GHG production. CO2 fluxes were comparable to other rangeland landforms possibly due to soil respiration. Overall, CH4 had the highest global warming potential of 76 % compared to other gas fluxes for CO2 equivalents. Therefore, inclusion of these fluxes from watering pans gives a more holistic GHG footprint of the landscape. Studying these systems over long time scales to also include dry seasons could be helpful to represent the fluxes in a semi-arid context. To study the carbon and nitrogen stocks in the sediment, a total of 62 cores were taken using a piston corer along an assumed depositional gradient from the main inlet of surface run off, to the middle section and lastly the far end section of the watering pans. These were used for sediment organic carbon (SOC) and total nitrogen (TN) stock analysis per 3 cm depth interval on each core. We observed no significant effect of livestock systems or depth profiles on the carbon and nitrogen stocks. The carbon and nitrogen stocks were highest in the middle section of the watering pans. In general, the watering pans store about 1 kg C m-2 and 480 g N m-2. In comparison with the rangeland landscape, the watering pans stored the least amount of carbon and nitrogen. The lesser carbon and nitrogen stocks in watering pans could relate to the short timescale over which they have been accumulating sediment as compared to the terrestrial landscape which has always been accumulating organic matter through plant die off for instance. Management practices such as desilting could also be in play however these are beyond the scope of this report. For a finer comparison, these higher distal controls may be included in subsequent studies thus accounting for elements such as surface area to catchment area ratio of each watering pan.