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The results of the column experiment are summarised in Figure 3. These show that
P removal by the ochre/straw was nearly complete (equivalent to 92 kg P/ha), and N
removal equivalent to 188 kg N/ha during addition of 450 kg N/ha occurred, relative
to the sand only column. This confirms that the ochre and straw treatments have the
potential to remove substantial quantities of P and N respectively, if the hydraulic
properties are suitable.
40
mg/L NH4-N or SRP in leachate
35
30
25
20
15
10
5
column conditioning period
( approx 6 days)
mineral N, ochre+straw column
mineral N, sand only column
S oluble P, ochre+straw column
S oluble P , s and only column
188 kg N/ha removed
92 kg P/ha removed
0
0 50 100 150 200 250 300 350 400 450 500
cumulative N input kg/ha equivalent
Figure 3: Mineral N and P in leachate of control column (sand) and ochre +
straw column, receiving dilute slurry, showing removal of nutrients
equivalent to 188 kg N/ha and 92 kg P/ha of slurry nutrients by the
ochre + straw layers
DISCUSSION
The results show that the subsoil and ochre sublayers are both effective ways of
attenuating pollutants (NH 4
-N, TOC, faecal indicator organisms, P) from the leachate
from corrals, at least in the short term. The mineralization and oxidation of N from
the slurry leads to a rise in the nitrate concentration, which could create problems
with Nitrate Vulnerable Zone compliance. However, the results also suggest that
nitrate removal through denitrification is occurring; this will be greater than would
occur where the faecal and urinary nitrogen was deposited at lower rates (e.g. during
overwintering in fields).
The results interface closely with work on full-scale woodchip corrals (Vinten et al.,
2006) which shows that freely drained sandy soil is working as an effective mitigator to
prevent groundwater pollution by ammonium, organic carbon and faecal indicators,
but that high nitrate concentrations below corrals occur. Further work to assess the
amount of denitrification under corrals is ongoing. It should be borne in mind that the
nitrate leaching per unit area from such an installation can be very high, although the
nitrate leaching per animal will be lower than where animals are more spread out in
fields over winter.
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