Nitrogen and sulfur dynamics of contrasting grazed pastures

The experimental area was located at the Big Ridge 2 site, CSIRO, Chiswick (30 degrees 31'S, 151 degrees 39'E), 20 km south of Armidale, New South Wal es, Australia. The site was established in 1955. In March 1966, phalaris an d white clover were sown and pastures were fertilised annually with superph osphate 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 r epresentative areas 1 m by 0.5 m were selected in each stratum of each trea tment. The selected areas were labelled with S-34-enriched (90%) elemental sulfur and N-15-enriched (99%) NH4Cl solution. All plots were grazed contin uously by sheep. No effect of pasture type on N leaching was apparent in this experiment. Se asonal variation of total soil mineral N in different soil layers, low N-15 recovery down to 60 cm soil depth, and low nitrate-N concentrations in dra inage water obtained in this experiment suggest that synchronisation of pas ture growth with mineralisation and nitrification, together with ammonium d omination 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 dra inage 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 increa se in sulfate sorption capacity at lower soil depths, resulted in a large a mount of sulfate stored at greater depth. However, retention of the S-34 ap plied in 1995 in the top 10 cm soils suggests that sulfate-S movement down the soil profile is slow.