A comparison of the effects of subsoiling on plant uptake and leaching losses of sulphur and nitrogen from a simulated urine patch

Synthetic cow urine labelled with ³⁵S and ¹⁵N was applied to large, undisturbed, monolith lysimeters sampled from subsoiled and non-subsoiled areas of a grass/clover pasture. For one year following the urine application, the lysimeters were subjected to a combination of natural rainfall, simulated rainfall and simulated flood irrigations. Drainage from the lysimeters was sampled regularly and monthly (approx.) pasture cuts taken. At the end of the year, the lysimeters were destructively sampled in 50 mm depth increments for soil analysis. Leachates, plant samples and soil samples were analysed for ³⁵S and ¹⁵N.There were no significant differences in plant uptake of ³⁵S and ¹⁵N between the subsoiled and nonsubsoiled lysimeters. Initially grass showed a higher degree of labelling than clover. Total amounts of ³⁵S and ¹⁵N leached from the subsoiled lysimeters were approximately twice that leached from the nonsubsoiled ones. Leaching patterns differed substantially between the two nutrients.Total recoveries of ³⁵S (in plants, leachates and soil extracts) accounted for 82% of the applied ³⁵S for the subsoiled lysimeters and 72% for non-subsoiled ones. The unrecovered ³⁵S is considered to have been incorporated into soil organic matter. Total recoveries of ¹⁵N (in plants, soil and leachates) were similar to those for ³⁵S, but unrecovered ¹⁵N is attributed to loss by denitrification.