GENERAL INTRODUCTION
2.7 Nitrogen (N)
2.7.1 I m portan ce of n itrogen and its role i n eutro p h i cation
N itrogen is present i n the environment i n many forms. The predominant form is nitrogen gas (N2) . Nevertheless, N2 is almost inert and cannot be used directly by most organisms since a substantial amount of energy is required to split the N2 triple bond (N = N). N itrate and ammonia are the two forms used by plants.
Organic N is mineralized by microorganisms to create these two forms that are found in our soil and water.
Eutrophication is a lso largely d riven by transportation of N from natural and anthropogenic sources (Co le et a l . , 2006). The N comes from a variety of sources, including runoff from agricultural fields, concentrated animal feeding operations, atmospheric deposition from fossil fuel combustion, and sewage and septic wastes (Howarth, 2005). In most estuaries, N limits primary productivity. However, if present in excess, it can lead to eutrophic conditions , which can adversely affect water and habitat quality. In freshwater systems, N along with P serves as the l imiting nutrient. The level of N determines the
biological productivity of the lake, pond , or stream. A sharp increase in the concentration of N can result in an algal bloom. Other effects of eutrophication i nclude loss of species diversity (loss of fishery), increases in water turbidity, and hypolimnetic loss of dissolved oxygen (anoxic conditions) for thermally stratified eutrophic reservoirs due to the oxidation of organic matter by microbes, which exerts oxygen demand ( Mason, 1 996).The increased vegetation may impede water flow and navigation and the decaying algae often causes taste and odour problems ( Mason, 1 996).
Soil environmental factors such as soil moisture and temperature control the release of N from organic N sources. The increase in soil moisture and temperature increase the net release of N depending on the organic source (Agehara & Warncke, 2005). Mineral forms of N do not bond as readily to soil minerals or organic matter as does P .
Chapter 2
The ratio of TN to TP is often used to define the limiting nutrient. P is considered the limiting nutrient when TN :TP is g reater than 1 7 whereas N is the limiting nutrient when TN :TP is less than or equal to 1 0 (Smith, 1 982). For TN :TP between 1 0 and 1 7, P and N are co-limiting nutrients. P is not needed for algal growth in such large amounts as carbon, oxygen, hydrogen, or nitrogen. N itrate (N03--N) and ammonium (NH/-N) are common inorganic N forms found in natural waters. The NH/-N is the preferred form of N for plant growth since the reduction of N03--N to the amino group (-N H2) requires additional energy. Nevertheless, the amount of N H4 + -N in the aquatic system is usually less than N03--N since unlike N H/-N , N03--N moves easily through soils.
2 . 7 . 2 Effect o f s heep g razing on n itrogen losses
Cattle (60 %), and sheep ( 1 2 %) produce the most animal manure N at a g lobal scale (Oenema & Tamminga, 2005). Faeces contain N predominately in the organic form, while 60-70 % of cow urine N and 70-80 % of sheep urine N is in the form of urea (Bellows, 2001 ) . U rine can pass through the soil macropores quickly and increase the N H/-N concentration in the soil solution due to extremely rapid hydrolysis of urine-urea (Haynes & Williams, 1 992). Cows deposit a higher rate of urine ( 1 0 L m-2) compared to sheep (5 L m-2) (Saggar et al. , 1 988b) . This indicates that under dairy conditions, macropore flow of urine, and the subsequent i ncrease in the N concentration of drainage, is more likely than with sheep grazing . Efficiency of N use by animals can be increased by diet manipulation. Grazing sheep and cattle on grasses h ig h in water soluble carbohydrate results in less N excretion in urine (Freney, 2005).
Elliott & Carlson (2004) reported that sheep grazing i n hill country increased concentrations of N in overland flow by factors of 33-76 and 5-7 for ammonium N and dissolved organic-N , respectively. Caution was advised for winter g razing under New Zealand conditions. Powell et aI. , ( 1 998) determined the effect of urine on soil chemical properties and reported that an average voiding of sheep urine to a sandy, siliceous soil i ncreased soil pH a nd ammonium levels d ramatically in the upper 1 0 -1 5 cm of soil, especially during the first week following application.
I n hill country, some areas of pasture (e.g. beneath trees, around gateways, and on ridges and hill crests) can have g reater N loadings and potential for leaching , or losses via runoff, due to strong camping behaviour of the sheep. Saggar et a l . , (1 988a) reported that 60 % of the d ung and 55 % of the urine are deposited on campsite areas that occupy 1 5-31 % of the land area.
N itrate leaching losses are generally lower i n sheep-grazed pastures than for cattle because sheep have a smaller bladder and urinate more often in smaller volumes (Morton et al., 1 993). Magesan et al. , (1 996) reported that periodically mob-grazing by sheep on flat land had no measurable effect on the nitrate concentrations in the dra inage collected during or immediately after g razing .
2.7.3 Effect of n itrogen ferti l iser application on N losses
Human activities, especially agricultural activities, have approximately doubled the rate of N input into the terrestrial N cycle and this rate is still increasing (Smith et al. , 1 999; Vitousek et al., 1 997) . There is widespread concern that N and P originating from agricultural land is causing contami nation of g round and surface waters leading to nutrient enrichment of New Zealand lake systems (Cullen et aI. , 2006; Dooley et aI. , 2005) of g reat tourism and cultural values (Carr, 2005; Hall & Stoffels, 2005) . The detection of the Cylindrospermmosis raciborskii planktonic freshwater cyanobacterium in lake Waahi (Waikato) has raised concern in recreational, stock drinking and potable water supplies in New Zealand (Wood & Stirling, 2003). N itrate leaching is the major cause of N loss from agricultural systems and the amount of N in surface runoff is strongly influenced by a combination of land use and management practices (Hamilton, 2004), soil types and climatic conditions.
In winter, up to 30 % of the N applied in fertiliser may be leached . This will result i n a relatively low pasture g rowth response to the added N . However, application of N fertiliser in autumn resu lts in larger pasture responses to N , which can be carried through for winter g razing, and minimises direct leaching of fertiliser N (Ledgard et aI. , 1 988).
Chapter 2
I n New Zealand , relatively low amounts of fertiliser N are used on sheep farms (e.g. 20 kg N ha-1 y(1 ) (Morton et a l . , 1 993) . Magesan et a l . , ( 1 996) measu red nitrate-N in the drainage from two hydrologically similar mole-and pipe drained paddocks over three years near Palmerston North, New Zealand, and reported that the application of urea (50 Kg ha-1 ) in early spring had no significant increase on the N03--N concentrations in the drainage water.
In general terms, as the application of N to pasture increases, N losses increase. Whilst the soil can act as an N sink by immobilising or absorbing N , increasing amounts are lost a s N03--N in drainage. Monaghan et a l . , (2005a) conducted research on the effect of increasi ng N on nitrate-N losses near Edendale township in eastern Southland, New Zealand. Treatments were established on hydrologically-isolated replicate plots (900 m2) where pastures received annual fertiliser N inputs of 0, 1 00, 200 or 400 kg ha-1 and were grazed throughout spring , summer, and autumn of each year by non-lactating dairy cattle. Significantly increased losses of nitrate-N in drainage were reported as the application rate increased . Considered from the perspective of promoting nuisance weed and a lgal growth in surface waters" losses of N in drainage water u nder the greater application rates exceeded accepted guidelines for N . It was suggested that annual fertiliser N inputs should not exceed approximately
2.8