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- Title
Land Use, Not Stream Order, Controls N<sub>2</sub>O Concentration and Flux in the Upper Mara River Basin, Kenya.
- Authors
Mwanake, R.M.; Gettel, G.M.; Aho, K.S.; Namwaya, D.W.; Masese, F.O.; Butterbach‐Bahl, K.; Raymond, P.A.
- Abstract
Anthropogenic activities have led to increases in nitrous oxide (N2O) emissions from river systems, but there are large uncertainties in estimates due to lack of data in tropical rivers and rapid increase in human activity. We assessed the effects of land use and river size on N2O flux and concentration in 46 stream sites in the Mara River, Kenya, during the transition from the wet (short rains) to dry season, November 2017 to January 2018. Flux estimates were similar to other studies in tropical and temperate systems, but in contrast to other studies, land use was more related to N2O concentration and flux than stream size. Agricultural stream sites had the highest fluxes (26.38 ± 5.37 N2O‐N μg·m–2·hr–1) compared to both forest and livestock sites (5.66 ± 1.38 N2O‐N μg·m–2·hr–1 and 6.95 ± 2.96 N2O‐N μg·m–2·hr–1, respectively). N2O concentrations in forest and agriculture streams were positively correlated to stream carbon dioxide (CO2‐C(aq)) but showed a negative correlation with dissolved organic carbon, and the dissolved organic carbon:dissolved inorganic nitrogen ratio. N2O concentration in the livestock sites had a negative relationship with CO2‐C(aq) and a higher number of negative fluxes. We concluded that in‐stream chemoautotrophic nitrification was likely the main biogeochemical process driving N2O production in agricultural and forest streams, whereas complete denitrification led to the consumption of N2O in the livestock stream sites. These results point to the need to better understand the relative importance of nitrification and denitrification in different habitats in producing N2O and for process‐based studies. Plain Language Summary: Humans affect the emission of nitrous oxide (N2O), a potent greenhouse gas, from river systems through land use change and increased nitrogen use in catchments. We know little about this in sub‐Saharan Africa, where human activity is increasing rapidly. Previous studies have found that N2O may be related to the size of the river, with smaller rivers having higher emissions than large ones, due to their close connection to the catchment. This study addressed whether land use (i.e., native forest, crop production, and livestock watering holes) affect N2O emission from the headwaters of the Mara River, Kenya. We found that N2O emissions were not related to the stream size but were more strongly affected by land use, with the highest emissions occurring in the agricultural areas. We also examined which processes in the river were responsible for the emissions by relating water quality parameters to N2O concentration. We found that different processes were most likely responsible in different land uses, with nitrification dominating in forested and agricultural areas, and denitrification dominating in livestock watering sites. These results illustrate the unique features of tropical rivers in montane ecosystems undergoing land use change. Further research should investigate processes and seasonal dynamics. Key Points: Land use, not stream size, was the strongest predictor of nitrous oxide (N2O) flux and concentration from the Mara River headwatersAgricultural land use showed the highest N2O flux, while livestock watering holes in the showed the lowest, similar to forest sitesNitrification appears to be an important source of N2O in agricultural and forest sites, while denitrification dominates in livestock sites
- Subjects
MARA River Watershed (Kenya &; Tanzania); LAND use; ANTHROPOGENIC effects on nature; NITROUS oxide &; the environment; NITRIFICATION
- Publication
Journal of Geophysical Research. Biogeosciences, 2019, Vol 124, Issue 11, p3491
- ISSN
2169-8953
- Publication type
Article
- DOI
10.1029/2019JG005063