MINERALIZATION AND FATE OF SOIL SULFUR AND NITROGEN IN HILL PASTURES

A field experiment was conducted for 7 months using mini-lysimeters wi th ion exchange resin traps to study the amounts of sulphur (S) and ni trogen (N) mineralised from soil organic matter, taken up by pasture a nd lost by leaching. Four sites were selected on the basis of contrast ing fertiliser history and land slope. The fertiliser histories since 1981 for the sites were 125 (LF) or 375 (HF) kg/ha per year single sup erphosphate (SSP) and the two slope positions were low (LS, 0-12-degre es) and medium (MS, 13-26-degrees). Pasture production at the HF site (8360 +/- 250 kg DM/ha) was twice that at the LF site (4986 +/- 278 kg DM/ha) and S leaching losses were 7 times greater at the HF site (HF, 15.3 +/- 2.9 kg S/ha, LF, 2.1 +/- 0.25 kg S/ha). Over the 7 months of measurement, the amounts of S and N mineralised were also greater at the HF site (27.5 +/- 4.3 kg S/ha and 251 +/- 15.2 kg N/ha, respective ly) than the LF site (12 +/- 2.1 kg S/ha and 119 +/- 8.0 kg N/ha, resp ectively). Despite the fact that approximately 10 times more N (275 an d 134 kg N/ha for HF and LF sites, respectively) than S (26 and 11.7 k g S/ha for HF and LF sites, respectively) was taken up by pasture, sul phate (SO42-) and not nitrate (NO3-) was the dominant anion in the dra inage water. An average of 10 times more S (15.3 and 2.1 kg S/ha for H F and LF sites, respectively) than N (1.3 and 0.23 kg N/ha for HF and LF sites, respectively) was found in the drainage water below the 250 mm soil depth. At low mineralisation. rates (< 15kg S/ha for the exper imental period), the majority of S was taken up by pasture whereas at higher mineralisation rates (> 15kg S/ha for the experimental period), the majority was lost by leaching. In contrast, there was a linear re lationship between N mineralised and pasture uptake. The amounts of S and N mineralised in the field were compared with those mineralised un der laboratory conditions. In these hill pastures, N cycling is more e fficient than S cycling and a major proportion of annual S leaching lo ss is derived from the mineralisation of soil organic matter that occu rs during cooler periods when plant growth is slow.