A COMPARISON OF THE EFFECTS OF SUBSOILING ON PLANT UPTAKE AND LEACHING LOSSES OF SULFUR AND NITROGEN FROM A SIMULATED URINE PATCH

Synthetic cow urine labelled with S-35 and N-15 was applied to large, undisturbed, monolith lysimeters sampled from subsoiled and non-subsoi led 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. Drainag e from the lysimeters was sampled regularly and monthly (approx.) past ure cuts taken. At the end of the year, the lysimeters were destructiv ely sampled in 50 mm depth increments for soil analysis. Leachates, pl ant samples and soil samples were analysed for S-35 and N-15 There wer e no significant differences in plant uptake of S-35 and N-15 between the subsoiled and non-subsoiled lysimeters. Initially grass showed a h igher degree of labelling than clover. Total amounts of S-35 and N-15 leached from the subsoiled lysimeters were approximately twice that le ached from the non-subsoiled ones. Leaching patterns differed substant ially between the two nutrients. Total recoveries of S-35 (in plants, leachates and soil extracts) accounted for 82% of the applied S-35 for the subsoiled lysimeters and 72% for non-subsoiled ones. The unrecove red S-35 is considered to have been incorporated into soil organic mat ter. Total recoveries of N-15 (in plants, soil and leachates) were sim ilar to those for S-35, but unrecovered N-15 is attributed to loss by denitrification.