Nitrogen in interstitial waters in the Sahel; Natural Baseline, Pollutant or Resource?

Abstract

Nitrate in the unsaturated zone between the soil surface and the water table was studied in agroforestry Parklands in north western Senegal by examination of samples obtained by hand auger. Depending on location, water tables existed at depths between 10 and 35 m below ground. Previous studies of groundwater in this region had found that large concentrations of nitrate were unconnected with anthropogenic activity. The objective of this study was to determine whether nitrogen fixing vegetation had a role in groundwater nitrate accumulation and whether roots of trees were located deeply enough to access the nitrate. Accordingly, sample profiles were augered close to stems of nitrogen fixing trees, non-nitrogen fixing trees and also in adjacent areas that were unaffected by tree presence. These adjacent areas were typically open pasture or cultivated fields. Tree fine roots were quantified in the samples and examined for the presence of mycorrhizas. Similarly, sand/soil samples were examined and tested for the presence of nitrogen fixing rhizobia that were capable of forming functional nodules on appropriate host plants. Concentrations of nitrate were greatest in soils beneath nitrogen fixing trees and nitrate was more plentiful in profiles augered beneath nitrogen fixing crops than it was elsewhere suggesting that N-fixation was the source of the nitrate. The concentrrations of nitrate that were found in the unsaturated zone were greatly in excess of the WHO recommended limit for nitrate in drinking water. High NO 3 -N/Cl ratios confirm in situ production of nitrate, and indicate that this is a natural baseline occurrence related to N-fixation. The nitrate is moving down the profile and impacts the groundwater unless it can be intercepted by plant roots. NO 3 -N amounts in solution in the soil profile varied between 75 and 1000 kg ha −1 beneath trees and between 120 and 400 kg ha −1 in areas outwith tree crowns. Although these quantities of N occupy the lower end of the range of N values obtained in north American deserts, they comprise a considerable dryland resource where amounts of organic fertilizer are limited and where cost prohibits the use of commercial fertilizers. Roots of both nitrogen fixing and non-nitrogen fixing trees were deep enough to access the nitrate but the small amounts of available water at intermediate depths suggest that large scale uptake of nitrate will only be possible in the wetter zones located close to the water table. Shallow roots tended to be more heavily colonized by mycorrhizas than deeper roots but mycorrhizas were recovered from roots located 22 m below ground. Tree roots and rhizobia had similar patterns of distribution. They were commonest close to the soil surface, less frequent at intermediate depths and tended to increase in frequency close to the water table. * FAX No: +44 131 445 3943.

Geochemistry of sediment moisture in the Badain Jaran desert: Implications of recent environmental changes and water-rock interaction

“…High NO 3 concentrations in the USZ are found widely in other arid and semi-arid areas including western United States (Walvoord et al, 2003), the Sahel in northern Africa (Edmunds and Gaye, 1997;Deans et al, 2005), the Kalahari desert in southern Africa (Heaton et al, 1983;Stone and Edmunds, 2014), Australia (Barnes et al, 1992) as well as China (Gates et al, 2008c). In these areas, as in the present study, aerobic conditions permit the inert behaviour and preservation of biogenically produced NO 3 .…”

Groundwater recharge in natural dune systems and agricultural ecosystems in the Thar Desert region, Rajasthan, India

“…In contrast, some regions in the southwestern USA have very low NO 3 inventories under native vegetation, e.g., 24-76 kg N/ha, Southern High Plains, USA (McMahon et al 2006;Scanlon et al 2008). Large NO 3 reservoirs have also been found in semiarid regions in Senegal (≤ 1,000 kg N/ha), which are attributed to N fixation by native trees (Deans et al 2005). The lack of accumulation of NO 3 in soils in the Thar Desert from atmospheric deposition may be attributed to downward movement of NO 3 caused by recharge.…”

“…The hydrostratigraphic records trace past changes in recharge in response to variations in forcing from land-use change and climate variability (Scanlon et al , 2007aGates et al 2008;Favreau et al 2009). Unsaturated zone pore water NO 3 concentrations provide information on whether there is natural NO 3 in the profile from atmospheric deposition, as found in parts of the southwestern USA (Walvoord et al 2003), from nitrogen fixation by native vegetation, as found in Senegal (Deans et al 2005), or from mineralization and nitrification of soil organic nitrogen and over application and leaching of applied fertilizers in cropland areas (Scanlon et al 2008). Large NO 3 reservoirs in unsaturated media or in groundwater can be considered a resource for crop production, minimizing the need for fertilizer application (Deans et al 2005).…”

“…Unsaturated zone pore water NO 3 concentrations provide information on whether there is natural NO 3 in the profile from atmospheric deposition, as found in parts of the southwestern USA (Walvoord et al 2003), from nitrogen fixation by native vegetation, as found in Senegal (Deans et al 2005), or from mineralization and nitrification of soil organic nitrogen and over application and leaching of applied fertilizers in cropland areas (Scanlon et al 2008). Large NO 3 reservoirs in unsaturated media or in groundwater can be considered a resource for crop production, minimizing the need for fertilizer application (Deans et al 2005). Unsaturated zone soil cores and associated soil water Cl, NO 3 , and PO 4 concentrations can be used to assess relationships between land use/land cover and climate, and subsurface water and nutrient cycles.…”

Impact of recharge variations on water quality as indicated by excess air in groundwater of the Kalahari, Botswana

“…The investigated groundwater is affected by high nitrate concentrations of non-anthropogenic origin (Stadler et al, 2008). Similar findings are known from other groundwater resources in semi-arid regions (Heaton et al, 1983;Barnes et al, 1992;Edmunds and Gaye, 1997;Deans et al, 2005). The aim of this study was to relate groundwater quality with indicators of recharge processes and groundwater residence time to gain insight into the time scale, conditions, and processes that control solute entry into groundwater.…”