Transformations and plant uptake of urine N and S in long and short-term pastures

A field trial was carried out to compare the transformations and plant upta ke of urine N and S in a short-term pasture from within an arable/pasture l ey rotation and a long-term pasture. Animal urine labelled with N-15 and S- 35 was applied to microplots at both sites. These microplots were destructi vely sampled at various time intervals over 12 months and analysed for N-15 and S-35. It is known that soil organic matter accumulates under short-ter m pastures compared with a long-term pasture in which accumulation and degr adation are in balance. Consequently, it was hypothesised that immobilizati on of urine N and S is more intense in the short-term. However, in this stu dy there was considerably less immobilization of N-15 and S-35 into soil or ganic forms under short-term pasture than long-term pasture. This was attri butable to a greater pasture dry matter response to urine application under the short-term pasture (due to its inherently low N fertility) resulting i n a greater plant uptake of N-15 and S-35 with less N-15 and S-35 consequen tly being available for immobilization. At both sites, all of the applied S -35 was accounted for through plant uptake and recovery in the soil, but 21 -48% of the N-15 was unaccounted for and presumed to have been lost through gaseous emissions. It was concluded that accumulation of soil organic N an d S under short-term pastures is likely to be attributable to turnover of p lant residues (particularly root material) and does not appear to be relate d to immobilization in urine patches.