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

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.

An intense precipitation event causes a temperate forested drainage network to shift from N2O source to sink

“…On the other hand, this study reports higher N 2 O concentration than a river in New Zealand (114% saturation, Clough et al 2011) and African rivers in general (0.26 μ g‐N L −1 , Borges et al 2015), and higher N 2 O fluxes than a coastal watershed in North Carolina (0.31 mg‐N m −2 d −1 , Stow et al 2005), rivers on the Tibetan Plateau (0.18 mg‐N m −2 d −1 , Qu et al 2017), and boreal rivers (0.05 mg‐N m −2 d −1 , Soued et al 2016). However, our average N 2 O concentration is lower than nearby first‐order streams in Connecticut (0.63 μ atm, Aho and Raymond 2019), forested (1.6 ± 2.1 μ g‐N L −1 ) and agriculture (1.3 ± 1.8 μ g‐N L −1 ) streams in Sweden (Audet et al 2020), and agriculture‐influenced rivers in Kenya (0.51 μ g‐N L −1 , Mwanake et al 2019), Belgium (1406%, Borges et al 2018), and the Upper Mississippi River (250%, Turner et al 2016). In summary, the forested watershed in this study had moderate N 2 O concentration and fluxes compared to global rivers and streams of various land uses and land covers, with values higher than boreal and alpine watersheds but lower than many headwater streams and agriculture‐influenced systems.…”

“…Spatiotemporal controls on lotic N 2 O dynamics are poorly understood, which limits modeling efforts. Spatially, urban (McMahon and Dennehy 1999; Beaulieu et al 2010; Beaulieu et al 2011) and agriculture (Beaulieu et al 2011; Audet et al 2017; Mwanake et al 2019) landcover types have been associated with high terrestrial nitrogen loading and subsequent high lotic N 2 O emissions. However, a recent study showed that forested rivers have similar N 2 O concentration as agriculture rivers (Audet et al 2020).…”

Late Pleistocene and Holocene Afromontane vegetation and headwater wetland dynamics within the Eastern Mau Forest, Kenya

“…However, areas under current agricultural production when converted to agroforestry lead to a decrease in runoff while increasing groundwater uptakethis is often because of the choice of tree species sich as Eucalyptus that has a massive water demand (Hubbard et al, 2020). Land use, predominantly deforestation and transition to agriculture, was found to influence nitrous oxide levels in the lower elevations of the Mara River network (Mwanake et al, 2019). The effect of forest cover loss leading to reduced capacity to act as a catchment during heavy rainfall and mediate streamflow was identified as a major disruptor to economic activities to the south-west of Mau (Otuoma et al, 2012).…”

“…Recent politicking has led to increased illegal settlement, logging and charcoal burning in the forest as a source of income (Nkako et al, 2005;Were et al, 2013). These increased human population pressures on the forests have further fragmented wildlife populations, with subsequent erosion and water distribution issues having consequences for downstream ecosystems and populations (Gichana et al, 2015;Mwangi et al, 2017;Dutton et al, 2018;Mwanake et al, 2019). Neighbouring Mau forest blocks have undergone varying degrees of anthropogenic modifications impacting vegetation biodiversity, soil geochemistry and topsoil seed bank; however, ecological restoration potential remains (Kinjanjui et al, 2013).…”

Greenhouse Gases Emissions in Agricultural Systems and Climate Change Effects in Sub- Saharan Africa

“…It is also anticipated that the GHG emissions will increase soon due to modern land-use changes that the sub-Saharan Africa region has adopted (Popp et al 2010). Although the fluxes are anticipated to be high, there are various uncertainties in the estimates provided due to large insufficient temporal and spatial representation of emissions from agricultural soils (Mwanake et al 2019). In SSA, these changes have significantly impacted the climatic patterns leading to more extended droughts and excessive rainfall that does not support agricultural productivity at the field level.…”