ISOTOPIC STUDIES OF THE NITROGEN-BALANCE IN A CRACKING CLAY .4. FATE OF THE 3 NITROGEN FERTILIZERS IN FALLOW SOIL IN THE FIELD

Urea, ammonium sulfate and calcium nitrate, labelled with 15N, were added to 30-cm diameter microplots at a cracking clay field site. Sorghum stubble was applied to one series of microplots, while another series was stubble-free. The plots were kept fallow for 14 weeks after which the loss, movement and immobilization of the 15N added in the fertilizers was measured. Losses totalled 9, 23 and 26% of the urea, ammonium sulfate and calcium nitrate respectively. Denitrification, which occurred during the sixth and seventh weeks of the experiment when the soil was waterlogged, was presumed to be the loss mechanism; the differences in rate of loss would then be related to the rate of nitrification of the urea and ammonium fertilizers. Adding sorghum stubble did not influence the rate of loss but caused net immobilization of an extra 5% of the added nitrogen; immobilization was 11, 11 and 7% in the microplots receiving urea, ammonium sulfate and calcium nitrate respectively. The 155 mm of rain received during the 14 weeks leached 20% of the added nitrate and only 10% of the other fertilizers below 30 cm. © 1979, CSIRO. All rights reserved.