Nitrate beneath the surface in drylands

High levels of nitrate (NO3-) in groundwater, above the WHO guideline value of 11.3 mg as NO3--N/l, is recognised to be a global environmental issue of considerable concern (e.g. Spalding and Exner, 1993) because of the deleterious implications of consuming high NO3- water for both human and animal health (e.g. Canter, 1997; Sindelar and Milkowski, 2012). Elevated NO3- levels in the subsoil in drylands as the result of long-term natural processes have recently been discovered and discussed as an unexpected and unrecognised global phenomenon (Walvoord et al., 2003). In southern Africa, a compilation of 50,000 groundwater borehole measurements indicates high NO3- is a problem across a wide part of the subcontinent (Tredoux et al., 2009). This is particularly problematic in the Stampriet Artesian Basin (SAB) in the southwest Kalahari (Heaton, 1984; Heaton et al., 1983), where groundwater is the primary source of freshwater and poor water quality has led to the south-eastern part being termed the ‘saltblock’ (Christelis and Struckmeir, 2001). Sources of elevated NO3- range from N-fixing vegetation, bacteria associated with root nodules, soil crusts and termites to animal excreta and human pollution. Source attribution is essential to guide sensible land-use and groundwater management. However it is not always straightforward to identify source(s) by only observing the landscape and land-use practices, and NO3- concentrations alone do not give this information. Stable isotopic (δ15N and δ18O) analysis of nitrate is a key tool that can provide an insight (Kendall, 1998).

In the Kalahari δ15N analysis has been applied to groundwater (Heaton et al., 1984; Heaton, 1983) and to surface soils to help identify the contributions from atmospheric N2 fixing, the role of N-fixing vegetation and cyanobacteria, termites and contributions from animal excreta (e.g. Aranibar et al., 2003; 2004; 2008; Stadler et al., 2010). However, there remains an important gap in analysis - that of the unsaturated zone (USZ) between the surface soil and groundwater. The USZ is the pathway for water containing nitrate from the land surface to groundwater. Stone and Edmunds (2014) have discovered spatially heterogeneous zones of elevated subsurface NO3- (100-250 and 250-525 mg as NO3-N/l, per litre of elutriated water) across parts of the USZ of the SAB. Water flux rates through the USZ for the same vertical profiles range from 4 to 27 mm/y (using chloride mass balance methods) indicating a vertical movement of these nitrate pulses toward the water table over multi-decadal timescales. Detailed analysis of δ15N paired with δ18O will for the first time help connect understanding of the nitrogen cycling processes in dryland soils with the presence of high NO3- in groundwater in regions such as the SAB where groundwater is the vital primary source of freshwater. We outline results of a pilot study undertaken with Dr Sarah Bennett before setting out the context, location and nature of the samples we apply to NIGFSC for analysis.

Methodology

The combined methodology of the project overall is to:
1) ascertain USZ pore moisture NO3- concentrations using elutriated moisture (30 g sample to 15 g ultrapure deionised H2O) (e.g. Ma and Edmunds, 2006) (completed by the PI)
2) select samples containing elevated NO3- for stable isotope analysis at the NGISF facility (to be prepared via elutriation, using greater mass to provide more NO3- if necessary).
3) develop the research further with involvement of soil and plant science collaborators with expertise in the nitrogen cycle.