CLORANFENICOL EN TEJIDO ANIMAL, HUEVOS Y OVOPRODUCTOS POR HPLC-MS n

Season and climate

The greatest potential for nitrate leaching losses occurs with a build-up of nitrate within the soil profile and an excess of soil water. Consequently, the greatest nitrate leaching losses usually occur during the late autumn, winter and early spring months when plant growth is slow and uptake of nitrate is low, and soil drainage is occurring due to a surplus of rainfall over plant evapotranspirative demand. Timing of rainfall and plant N demand, therefore, is critical to the size of losses from N fertiliser and/or urine application. For instance, Di et al. (1999) compared N leaching losses from

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autumn-applied N fertiliser (NH4Cl) to that applied in spring and found that these ranged between

15-19% in autumn but 8-11% in spring. Similarly, Decau et al. (2003) found that timing of urine application to pastures between autumn and spring had a significant effect on urinary-15_{N recovery}

in drainage, with an order of magnitude greater loss in leachate collected after the autumn application (0.7% vs 16.7%, respectively).

Higher residual soil nitrate left after a crop has been harvested or from the ploughing-in of pasture prior to sowing of a crop in autumn can lead to significant nitrate leaching losses (Cameron & Wild 1984; Adams & Pattinson 1985; McLenaghen et al. 1996). Urine deposited over a summer-dry period may also lead to increased nitrate leaching losses due to low pasture growth and a lack of N uptake. This N can leach over the succeeding winter (Scholefield et al. 1993; Shepherd et al. 2011; Snow et al. 2011) and indeed losses can be considerably higher than from an irrigated system on the same soil-type (Burgess 2003). Similarly, optimally-applied N fertiliser and irrigation can reduce N leaching losses by more efficient plant uptake of N (Hahne et al. 1977). Excess irrigation, however, has also been implicated in increased nitrate loss (Haynes 1986e).

Seasonal effects on soil temperatures can also influence nitrification as when ammonium is not limiting, and nitrite is not accumulating, rates can be shown to be largely zero-order and increase linearly with temperature (Figure 2.4-6) (Flowers & O'Callaghan 1983; Macduff & White 1985). This is more likely in grassland topsoils where the Nitrosomonas and Nitrobacter populations are near maximal and consequently the nitrification rate for urea-N in urine, for example, can be largely predicted based on prevailing soil temperatures under moist conditions (Macduff & White 1985). If soil temperatures remain low (~5°C) the nitrification rate in a urine patch is likely to be slowed, and the nitrate leaching potential less, than in warmer conditions where nitrate will tend to accumulate until the pool of ammonium is largely exhausted.

Generally, there is a relationship between the timing and size of rainfall events and nitrate leached where the latter increases in direct response to increased drainage, especially under fallow conditions after cropping or pasture renewal (Scholefield et al. 1993; Francis 1995). The source of N in many cases is from residual nitrate after harvesting or from grazing and/or mineralisation of organic-N and low N uptake over the winter period (Francis 1995; Drury et al. 2014). Nitrogen fertiliser can also incur large nitrate losses if rainfall occurs soon after application (Haynes 1986e).

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Many factors therefore can influence nitrate leaching, depending on soil condition, the climate pattern and prevailing land management.

Figure 2.4-6. Nitrification rates vs. soil temperature for a Dunkeswicke soil after addition of either 50 or 250 mg N kg-1 _{soil as ammonium sulphate or pig slurry. Adapted from McDuff and} White (1985).

Soil properties

Soil texture and structure have an important influence on nitrate leaching, with sandy, coarse textured soils generally having higher hydraulic conductivities than finer-textured silt or clay soils (McLaren & Cameron 1990b). Slower water movement in clay soils can create conditions that promote higher rates of denitrification (Saggar et al. 2013) and hence, fewer nitrate ions are available for leaching. Di et al. (2009a) showed that nitrate leaching from urine application to three soils of different textures under the similar leaching regimes was about 25% less for the finer, silt- dominated soil compared with the coarser-textured soils, with the difference attributed to denitrification in the silt-dominated soil.

Differences in leaching rates can be substantial due to hydraulic conductivities varying by several orders of magnitude between sandy and clay-textured soils (Lin et al. 2001). However, macropores, cracks and fissures can affect actual nitrate loss through the by-pass and preferential leaching mechanisms described in section 2.4.4. For example, Silva et al. (2000) showed

increased N leaching from macropore flow of urine under field-saturated flow conditions in a NZ pastoral soil. Similarly, agricultural drainage systems can increase nitrate leaching in heavy soils through a combination of shortened flow paths and improved aeration that decreases denitrification and increases N mineralisation (Scholefield et al. 1993).

A considerable quantity of nitrogen emanating from pastoral and cropping soils can be attributed to mineralisation and nitrification of SOM rather than from fertiliser application. Cropping soils in particular can leach large quantities of nitrate if cultivated and/or left fallow over winter (Dowdell & Webster 1984; Adams & Pattinson 1985; Francis et al. 1998) with N fertiliser potentially only a small part of total N losses (Dowdell & Webster 1984; Adams & Pattinson 1985). However, research has

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also shown that N fertiliser can also act as a “primer” for increased SOM mineralisation and not just as part of normal immobilization-mineralisation turnover (Haynes 1986d).