1. INTRODUCTION AND BACKGROUND
3.2 Nutrient and Sediment Transfer Processes in the Environment
3.2.4 Nitrogen Migration
Many studies on nitrogen migration tend to be restricted to local catchments where the prevailing hydrological and climatic regimes are well known (Quinn, 2004). Although this may preclude the identification of geographically extensive mechanisms that control nitrogen transfer dynamics (Alvarez-Cobelas et al., 2008), it warrants the isolation and mapping of unique local catchment pulses that govern nitrogen migration (Pellerin et al., 2004). This enables the design and application of management strategies that are both relevant and specific to the catchment in question. The discussion contained in this and following sections will, therefore, be in reference to South African catchments and the prevailing local hydroclimatic conditions will be considered as a backdrop of the overall discussion.
The movement of nitrogen across the landscape is highly dependent on the availability of water (Figure 3.2). Additionally, temperature indirectly influences nitrogen migration by limiting the ability of bacteria to facilitate nitrogen transformation reactions (Carmago et al., 2005). In a study detailing the migration of nitrates across a watershed in Whatcom County, Washington, the highest concentrations of nitrates were primarily found in subcatchments with the highest number of water bodies. In such instances, nitrate concentrations were found to be as high as 39 and 19.7 mg/l in some of the water bodies studied (Almasri and Kaluarachchi, 2004). This study also showed that leaching and runoff are the main transport mechanisms by which nitrogen is transported to downstream areas and water bodies where it is deposited and may, in association with other nutrients and high enough concentrations, trigger eutrophication. Both leaching and runoff are functions of soil moisture content and precipitation and are highly influenced by local hydroclimatic regimes (UNEP, 2010).
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Pellerin et al. (2004) note that the ability of wetlands to increase water retention times influences the export of nitrogen by streams by promoting nitrogen retention and volatilisation through sedimentation and denitrification respectively. In the study by Almasri and Kaluarachchi (2004) the highest concentrations of nitrates were also found in watersheds with the highest average annual precipitation.
Of the forms of available inorganic nitrogen, nitrate-nitrogen (NO3--N) is considered the most mobile anion and migrates easily through terrestrial and aquatic ecosystems (Van Der Perk, 2006), as it is a water soluble anion that is not readily absorbed to soil particles (Felton et al., 2008). As noted above, the ability of NO3--Nto migrate so easily warrants high levels of concern for environmental pollution and human health. In addition to nitrate-nitrogen, the soluble component of inorganic nitrogen also includes ammonium-nitrogen (NH4+-N).
Similar to NO3- -N, NH4+-N is also considered to migrate by surface and subsurface leaching and by runoff. However, the environmental and human-related impacts of NH4+-N are considered inconsequential relative to those of NO3- -N. It is perhaps important to note that the migration of nitrogen through terrestrial and aquatic ecosystems is not limited only to its elemental soluble forms. Nitrogen is, in fact, considered to be transported in both dissolved and suspended forms. Particulate nitrogen species (e.g. HNO3) are known to migrate by adsorption onto sediment material generated from upstream erosion or from bank and bed erosion in the stream channel (Viney et al., 2000). Although particulate forms of nitrogen are not transported conservatively (i.e. do not remain constant over space and time) as opposed to their soluble counterparts, they still subscribe to processes of surface erosion, entrainment, runoff (baseflow plus quickflow discharge), settling and deposition. Therefore, by deduction, hydroclimatic and biophysical processes which influence these transport processes also influence the transport of particulate nitrogen species. Figure 3.3 is an extended diagram of the processes detailed in Figure 3.2 to include transport processes of particulate nitrogen species. Table 3.1 provides a summary of the various natural and anthropogenic sources of nitrates and their compositional and delivery characteristics.
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Table 3.1 Anthropogenic and natural sources of nitrate including general delivery characteristics, chemical compositions and conservation of mass (Adapted from Withers and Jarvie, 2008).
*Changes in concentration as nutrient is translocated across the landscape.
Although nitrogen is a critical element in the functioning of terrestrial and aquatic ecosystems, the incidence of this element in abnormally high concentrations can have deleterious impacts. However, these impacts only manifest when nitrogen exists in conjunction with other nutrients. For instance, eutrophication, an increasing water quality management problem in many global and local waterways, is commonly caused by both nitrogen and phosphorus. Since the focus of this section is limited to these two elements, the following section details the cycling and transport of phosphorus in terrestrial and aquatic ecosystems.
Source Delivery Chemical
Composition
Conservation of Mass*
Reference
Discharge Rainfall Dependency
Fertilizer Applications (Pastures, Feedlots etc.)
Episodic to semi- continuous
Low to
Medium
Variable Particulate
No Larsson et al.,
(2005);
Withers and Jarvie (2008) Fertilizer Applications
(Irrigation, Return flows etc.)
Episodic to semi- continuous
High Variable Dissolved
Yes Withers and
Jarvie (2008);
Pärn et al., (2011)
Industrial Continuous Low to
Medium
Concentrated Dissolved
No Edwards and
Withers (2008) Septic Tanks Episodic to semi-
continuous
Low to
Medium
Variable Particulate
No Carpenter et
al., (1998)
Landfill Sites Continuous High Variable
Particulate
No Honisch et al.,
(2002) Sewage and Wastewater
Treatment Works
Continuous Low Concentrated
Dissolved
Yes Novotny
(2003)
Rainfall Fallout Episodic High Variable
Dissolved
Yes Author
Residential Continuous Medium to
High
Variable Dissolved
No Novotny
(2003)
Dry Deposition Episodic Low Variable
Particulate
No Han et al.,
(2010)
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Figure 3.3 Structure of the nitrogen cycle including particulate nitrogen transport processes (Viney et al., 2000).