The experimental area was located at the Big Ridge 2 site, CSIRO, Chiswick
(30°31′S, 151°39′E), 20 km south of Armidale, New South
Wales, Australia. The site was established in 1955. In March 1966, phalaris
and white clover were sown and pastures were fertilised annually with
superphosphate until 1993. There were 3 pasture treatments, each with 2
replicates: degraded pasture (low phalaris content), phalaris dominant, and
phalaris–white clover. Each of 6 experimental plots was divided into 3
strata. Two representative areas 1 m by 0.5 m were selected in each stratum of
each treatment. The selected areas were labelled with 34S-enriched (90%) elemental sulfur and 15N-enriched (99%) NH4Cl solution. All plots were
grazed continuously by sheep.
No effect of pasture type on N leaching was apparent in this experiment.
Seasonal variation of total soil mineral N in different soil layers, low
15N recovery down to 60 cm soil depth, and low
nitrate-N concentrations in drainage water obtained in this experiment suggest
that synchronisation of pasture growth with mineralisation and nitrification,
together with ammonium domination of the soil N system, is the key ecological
feature in preventing N leaching in this environment.
Unlike N, potential S leaching was found with evidence of a large amount of
sulfate stored deeper in the soil profile and high S concentrations in
drainage water. High KCl-40 extractable S concentration in the top 20 cm soil
layers was associated with the long history of superphosphate application.
Long-term applications of superphosphate (1967–93), together with an
increase in sulfate sorption capacity at lower soil depths, resulted in a
large amount of sulfate stored at greater depth. However, retention of the
34S applied in 1995 in the top 10 cm soils suggests
that sulfate-S movement down the soil profile is slow.
Nitrogen Balance of Effluent Irrigated Silage Cropping Systems in Southern Australia