Mineralisation and fate of soil sulphur and nitrogen in hill pastures

Abstract

A field experiment was conducted for 7 months using mini-lysimeters with ion exchange resin traps to study the amounts of sulphur (S) and nitrogen (N) mineralised from soil organic matter, taken up by pasture and lost by leaching. Four sites were selected on the basis of contrasting fertiliser history and land slope. The fertiliser histories since 1981 for the sites were 125 (LF) or 375 (HF) kg/ha per year single superphosphate (SSP) and the two slope positions were low (LS, 0-12°) and medium (MS, 13-26°). 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, respectively) than the LF site (12 ± 2.1 kg S/ha and 119 ± 8.0 kg N/ha, respectively). Despite the fact that approximately 10 times more N (275 and 134 kg N/ha for HF and LF sites, respectively) than S (26 and 11.7 kg S/ha for HF and LF sites, respectively) was taken up by pasture, sulphate (SO 42−) and not nitrate (NO3−) was the dominant anion in the drainage water. An average of 10 times more S (15.3 and 2.1 kg S/ha for HF 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 experimental 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 relationship between N mineralised and pasture uptake. The amounts of S and N mineralised in the field were compared with those mineralised under laboratory conditions. In these hill pastures, N cycling is more efficient than S cycling and a major proportion of annual S leaching loss is derived from the mineralisation of soil organic matter that occurs during cooler periods when plant growth is slow. © 1993 Taylor & Francis Group, LLC.