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types. This paper reports results from the first year of a 3-year study to quantify the
effects of different timings of cattle slurry application on nitrogen losses in water from
a drained clay soil under arable and grassland management.
MATERIALS AND METHODS
The experiment was carried out on a heavy clay textured (60% clay) soil of
the Denchworth Association, at the Brimstone Farm experimental facility, near
Faringdon in Oxfordshire. The site consists of 18 hydrologically isolated plots (each
40 m x 48 m), which had been in arable production for over 20 years until autumn
2001 when grassland was established on nine plots. The soils are drained with pipe
drains at 1 m depth and 48 m spacing, and have gravel backfill to within 30 m of the
surface, with secondary mole drains at 2 m spacing and 50 cm depth at right angles
to the pipe drains. Cattle slurry (c. 40 m 3 /ha, 120 kg/ha total N) was applied to the
arable and grassland plots in September and December 2003, and March 2004,
using an 11-m 3 Joskin tanker fitted with a 12-m trailing hose boom. There were
three replicates of each application timing. Inorganic nitrogen fertiliser was applied
all plots in early and late April 2004, using standard recommended rates, to ensure
that subsequent crop growth was representative of commercial practice.
Drainage and surface run-off volumes were measured continuously using V-notch
weirs. Drainage water samples were collected on a flow proportional basis using
automatic water samplers and analysed for nitrate-N, ammonium-N and soluble
organic N (SON).
RESULTS AND DISCUSSION
Drainage Volumes
Low rainfall volumes during autumn 2003 (70% of long-term average between 1st
September and 30th November 2003) meant that drainage did not begin until the end
of November 2003 on the arable plots, and mid-December on the grassland plots.
A wet April, when rainfall was 60% greater than the long-term average, meant that
drainage continued until mid-May 2004. Mean drainflow volumes during the whole
drainage period (up to mid-May 2004) were 77 mm from the grassland compared with
130 mm from the arable plots (long-term arable mean drainage at Brimstone = 204
mm). The lower drainage volumes and later return to field capacity on the grassland
plots reflected the greater soil moisture deficit that had developed as the grass grew
during the summer and early autumn. Surface run-off volumes were less than 7 mm
on all plots.
N Concentrations in Drainage Water
On the arable plots, nitrate concentrations in drainage water up to late March 2004
(until inorganic fertiliser N was applied) were greatest (P0.05) on nitrate concentrations in drainage waters,
probably as a result of the cold and wet soil conditions delaying the nitrification of
slurry ammonium-N to nitrate-N. Nitrate concentrations declined on all the treatments
to between 30–50 mg/L NO 3
-N after c. 30 mm of drainage (Figure 1a), but remained
above the EC limit of 11.3 mg/L NO 3
-N throughout the drainage period.
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