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 uptake of urine N and S in a short-term pasture from within an arable/pasture ley rotation and a long-term pasture. Animal urine labelled with ¹⁵N and ³⁵S was applied to microplots at both sites. These microplots were destructively sampled at various time intervals over 12 months and analysed for ¹⁵N and ³⁵S. It is known that soil organic matter accumulates under short-term pastures compared with a long-term pasture in which accumulation and degradation are in balance. Consequently, it was hypothesised that immobilization of urine N and S is more intense in the short-term. However, in this study there was considerably less immobilization of ¹⁵N and ³⁵S into soil organic forms under short-term pasture than long-term pasture. This was attributable to a greater pasture dry matter response to urine application under the short-term pasture (due to its inherently low N fertility) resulting in a greater plant uptake of ¹⁵N and ³⁵S with less ¹⁵N and ³⁵S consequently being available for immobilization. At both sites, all of the applied ³⁵S was accounted for through plant uptake and recovery in the soil, but 21–48% of the ¹⁵N was unaccounted for and presumed to have been lost through gaseous emissions. It was concluded that accumulation of soil organic N and S under short-term pastures is likely to be attributable to turnover of plant residues (particularly root material) and does not appear to be related to immobilization in urine patches.