S ISTEMAS DE CALEFACCIÓN ELÉCTRICA DIRECTA

A wide range of factors affect NO3-N leaching losses from fertilisers and soils. These have been reviewed in detail by Cameron and Haynes (1986) and Juergens-Gschwind (1989) and can be summarised as follows:

Crops: The amount of leaching loss from fertiliser depends on the rate of application relative to the plant requirement. The ‘safe rate’ to apply depends on the crop, soil and climatic conditions. Research in Europe has indicated that for arable crops application above 160 kg N/ha/year can result in increased leaching losses, however it is not known if this applies to New Zealand conditions. Cereals generally use fertiliser N efficiently and leaching losses are therefore small at optimum fertiliser rates (Prins et al., 1988). In New Zealand, Mohammad et al. (1986) recorded a short term increase in NO3-N concentration in tile drain effluent when urea was applied to barley. Adams and Pattinson (1985) concluded that fertiliser applied at between 25 and 50 kg N/ha had little effect on NO3-N leaching from wheat grown on Wakanui soil in Canterbury. Fertiliser N recovery by field vegetables is generally low (e.g. lettuce 11%; spinach 31%; red beet 52%; potatoes 50%) due to their shallow rooting depth and the high rates of irrigation often used (Prins et al., 1988).

Pasture: Extensive pasture systems and pasture cut for hay or silage generally have low leaching losses (Cameron and Haynes, 1986). Relatively large leaching losses can however occur under intensively managed pasture, particularly when high stocking rates

are combined with high fertiliser inputs. Animal urine patches contain large amounts of N (the equivalent of up to 500 kg N/ha for sheep and 1000 kg N/ha for cattle - Steel, 1982). These amounts are greater than the plant can assimilate and therefore significant leaching losses can occur. Lysimeter studies in New Zealand have found that between 8 and 20% of urine-N may be leached (Field et al., 1985b; Frase et al., 1994). There are very few measurements of fertiliser leaching losses from grazed pasture in New Zealand. Sharpley and Syers (1979) reported that the application of urea at 60 kg N/ha/year resulted in a temporary increase in the concentration of NO3-N in drainage water from a Tokomaru silt loam at Palmerston North. Field et al. (1985a) reported large leaching losses from sheep grazed pasture in the Manawatu region.

Soil properties: Sandy soils usually retain less water, and leaching is therefore more rapid and extensive on lighter soils. Split applications of fertiliser on light soils reduce leaching loss. Denitrification, immobilisation and ammonium fixation can reduce the loss whilst mineralisation can increase the loss. Nitrate-N leaching following mineralisation of ploughed pasture and clover residues can be a major source of NO3-N contamination of groundwater (Cameron and Wild, 1984; Francis et al., 1992).

The rate of water and solute moving through the soil-water matrix depends on many factors e.g. soil type, climatic conditions, soil conditions, and amounts and types of agricultural chemicals and nutrients applied. Macropores (i.e. cracks and biologically created channels in the soil-water matrix) can induce preferential flow paths into the soils systems. These macropores only make up a small proportion of the total pore space. However, these macropores can allow the bulk of water and solute movement through soil when the soils’ water flow conditions are near saturation (Clothier and White, 1981).

Plant uptake: As it is well known that plant uptake is a primary factor affecting the amount of NO3-N leached. The greater the uptake, the lower the concentration of NO3- N in the soil solution, and thus the lower the potential for leaching.

Irrigation: Irrigation applied at optimum rates for plant growth can enhance N uptake and reduce NO3-N leaching; however excessive irrigation rates can increase the loss of NO3-N. Irrigation of pasture permits an increased stocking rate and thus there is a potential for a higher leaching loss due to greater urine returns. Irrigation may also increase denitrification losses and thus reduce the concentration of NO3-N in solution. The influence of irrigation on groundwater contamination in New Zealand is unclear with conflicting opinions (e.g. Quin and Burden, 1979; Burden, 1980; Close, 1987) due to different irrigation and soil conditions.

Organic wastes: Application of organic wastes (e.g. piggery waste) at rates that can be utilised by a pasture or crop (say 200 kg N/ha/year) appears to pose no greater threat to groundwater quality than many other agricultural practices (e.g. ploughing pasture) (Cameron and Rate, 1992). Application at higher rates results in larger leaching losses and higher NO3-N concentrations in drainage water (Cameron and Rate, 1992).

Weather and season: In New Zealand, most N leaching occurs from late autumn to early spring when plant N uptake is low and rainfall exceeds evaporation. However, leaching can also occur at other times of the year if the soil is wet and heavy rainfall or irrigation is received. The pattern of the rainfall over autumn/winter has a major impact on the extent of leaching losses, especially from cultivated soils. Thus, for about the same total amount of winter drainage, leaching losses can be twice as great when drainage occurs late in winter rather than earlier in winter (Francis et al., 1998). This difference is due to the greater accumulation of mineral N in the soil profile later in the winter, following a longer period for net N mineralisation of soil organic N and plant residues.

The amount of drainage in the spring can also be important, and it can determine the amount of leaching from recently applied fertilisers, animal returns and mineralised NO3-N (Roberts et al., 1996). Rainfall intensity and amount are both important in determining leaching losses. Heavy rainfall may induce macropore flow and a proportion of NO3-N in the soil may be rapidly leached to depth (Cameron et al., 1995).

However, studies have also shown that intermittent rainfall can be more effective than continuous rainfall at leaching NO3-N from soil due to ‘holdback’ of NO3-N in soil macropores in response to slow diffusion rates (e.g. Francis et al., 1988). A dry summer can cause an additional accumulation of NO3-N in the soil profile due to poor crop uptake and this may result in enhanced leaching losses over the following winter.