USE OF NITRIFICATION INHIBITORS TO INCREASE FERTILIZER NITROGEN RECOVERY AND LINT YIELD IN IRRIGATED COTTON

This paper describes field experiments designed to evaluate the effectiveness of several nitrification inhibitors to prevent loss of fertilizer nitrogen (N) applied to cotton. The usefulness of nitrapyrin, acetylene (provided by wax-coated calcium carbide), phenylacetylene and 2-ethynylpyridine to prevent denitrification was evaluated by determining the recovery of N applied as N-15 labelled urea to a heavy clay soil in 1 m X 0.5 m microplots in north western N.S.W., Australia. In a second experiment, the effect of wax-coated calcium carbide on lint yield of cotton supplied with five N levels was determined on 12.5 m X 8 m plots at the same site. The N-15 balance study showed that in the absence of nitrification inhibitors only 57% of the applied N was recovered in the plants and soil at crop maturity. The recovery was increased (p < 0.05) to 70% by addition of phenylacetylene, to 74% by nitrapyrin, to 78% by coated calcium carbide and to 92% by 2-ethynylpyridine. In the larger scale field experiment, addition of the wax-coated calcium carbide significantly slowed the rate of NH4+ oxidation in the grey clay for approximately 8 weeks. Lint yield was increased (p < 0.05) by the addition of the inhibitor at all except the highest level of N addition. The inhibitor helped to conserve the indigenous N as well as the applied N. The research shows that the effectiveness of urea fertilizer for cotton grown on the heavy clay soils of N.S.W. can be markedly improved by using acetylenic compounds as nitrification inhibitors.