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N Losses in Drainage Water
Mean nitrate losses up to the end of March 2004 (until fertiliser N was applied) were
greatest (P0.05) on nitrate leaching losses up to the end of March 2004. Losses from
the autumn and winter applications were equivalent to 2% and 4% of the total N
applied, respectively. The low nitrate leaching losses from the grassland system were
probably a reflection of the recently established grass sward accumulating N within
organic reserves and greater crop uptake of slurry N (c. 20 kg/ha N compared with
less than 5 kg/ha N on the arable plots) in the period between application and the
start of drainage.
60
50
NO 3 -N
NH 4 -N
SON
40
N (kg/ha)
30
20
10
0
Autumn Winter S pring Autumn Winter S pring
Arable
Grass
Figure 3: Nitrogen leaching losses Brimstone Farm (2003/04)
Ammonium-N losses were less than 0.2 kg/ha N from all the treatments. Soluble
organic N losses ranged from 2.5 to 3.6 kg/ha N on the arable plots and 0.6 to
0.8 kg/ha N on the grassland plots.
CONCLUSIONS
The data from the first year of this study indicate that drained arable clay soils may
not be as retentive of slurry N as had previously been thought, although the arable
reversion grassland plots were very nitrate retentive. Moving the timing of the slurry
application to arable land from autumn to spring reduced nitrate leaching losses.
However, applying slurry to ‘wet’ drained soils, where rainfall follows soon afterwards,
can result in elevated drainflow ammonium-N concentrations (so called ‘pollution
swapping’). This is particularly likely where there is good connectivity between the
soil surface and drainage system via cracks/mole channels.
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