Land use affects total dissolved nitrogen and nitrate concentrations in tropical montane streams in Kenya

Abstract

African tropical montane forests are facing fast and dynamic changes in land use. However, the impacts of these changes on stream water quality are understudied. This paper aims at assessing the effect of land use and physical catchment characteristics on stream water concentrations of dissolved organic carbon (DOC), total dissolved nitrogen (TDN), nitrate (NO-N) and dissolved organic nitrogen (DON) in the Mau Forest, the largest tropical montane forest in Kenya. We conducted five synoptic stream water sampling campaigns at the outlets of 13-16 catchments dominated by either natural forest, smallholder agriculture or commercial tea and tree plantations. Our data show a strong effect of land use on TDN and NO-N, with highest concentrations in stream water of catchments dominated by tea plantations (1.80±0.50 and 1.62±0.60mgNl, respectively), and lowest values in forested catchments (0.55±0.15 and 0.30±0.08mgNl, respectively). NO-N concentration increased with stream temperature and specific discharge, but decreased with increasing catchment area. DOC concentrations increased with catchment area and precipitation and decreased with specific discharge, drainage density and topographic wetness index. Precipitation and specific discharge were also strong predictors for DON concentrations, with an additional small positive effect of tree cover. In summary, land use affects TDN and NO-N concentrations in stream water in the Mau Forest region in Kenya, while DOC and DON were more related to hydrologic regimes and catchment properties. The importance of land use for NO-N and TDN concentrations emphasizes the risk of increased nitrogen export along hydrological pathways caused by intensified land use and conversion of land to agricultural uses, which might result in deterioration of drinking water quality and eutrophication in surface water in tropical Africa.

Ecosystem structure and function of afrotropical streams with contrasting land use

“…Surprisingly, our two focal farm streams had lower water column nutrient concentrations than forested streams (TN = 942–977 μg/L at farm sites vs. 372–381 μg/L at forest sites and TP = 89–183 μg/L at farm sites vs. 30–67 μg/L at forest sites). This is opposite to what is usually reported (Allan, ; Bernot et al., ; Jacobs, Breuer, Butterbach‐Bahl, Pelster, & Rufino, ; Masese et al., ). Additional sampling would be required to explain this pattern, but we speculate that one important factor could be the high biomass of emergent macrophytes at farm sites potentially increasing nutrient uptake rates (Levi et al., ; higher algal biomass at farm sites could also alter uptake rates).…”

“…is opposite to what is usually reported (Allan, 2004;Bernot et al, 2010;Jacobs, Breuer, Butterbach-Bahl, Pelster, & Rufino, 2017;Masese et al, 2017). Additional sampling would be required to explain this pattern, but we speculate that one important factor could be the high biomass of emergent macrophytes at farm sites potentially increasing nutrient uptake rates (Levi et al, 2015; higher algal biomass at farm sites could also alter uptake rates).…”

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

“…In agricultural sites, nitrification was likely not N limited due to the lack of correlation between N 2 O and NH 4 -N, and this is consistent with the higher dissolved inorganic N concentrations in agricultural systems compared to forest streams, likely due to the use of the N-based fertilizer (in this case, calcium ammonium nitrate), which enters streams as runoff or leachate. Higher dissolved N values were also reported from other studies in agricultural systems, including some from the same tributaries (e.g., TDN 6.6 ± 2.60 mg/L, NO 3 -N 6.1 ± 6.1 mg/L, NH 4 -N 0.04 ± 0.03 mg/L; Masese et al, 2017), although lower values (TDN 1.80 ± 1.50 mg/L, NO 3 -N 1.62 ± 0.60 mg/L) have been reported in similar streams in the Sondu River located in the South west part of the Mau Forest Complex (Jacobs et al, 2017). These differences could be in part due to the sampling season as the latter sampling was conducted in the dry season (February-March), while both Mara studies were conducted after the short rains (November-January).…”

Land use alters dominant water sources and flow paths in tropical montane catchments in East Africa

“…Riparian forests of up to 30 m width are well-maintained and contain native tree species, such as Macaranga kilimandscharica, Polyscias kikuyuensis, Olea hochstetteri and Casearia battiscombei (Ekirapa and Shitakha, 1996). A more detailed description of land use in the study area can be found in Jacobs et al (2017). 10…”

“…January to March are the driest months. Long-term annual precipitation at 2 100 m elevation is 1 988±328 mm yr −1 (Jacobs et al, 2017). 15…”