Effect of acetylene provided by wax-coated calcium carbide on transformations of urea nitrogen applied to an irrigated wheat crop

The effect of acetylene (provided by wax-coated calcium carbide, CaC₂) on N transformations in a red-brown earth was measured in a field experiment with irrigated wheat by determining the change in the concentration and ¹⁵N enrichment of the organic N and mineral N pools with time. The study was conducted in the Goulburn-Murray Irrigation region of south-eastern Australia using 0.3 m by 0.3 m microplots fertilized with ¹⁵N-labelled urea (10 g N m^-2; 5 atom% ¹⁵N). Acetylene was effective in slowing the nitrification of both unlabelled and labelled N. Nitrate derived from the added fertilizer reached a maximum 19 days after sowing in the treatment without CaC₂, whereas little nitrate accumulated in the 8 g CaC₂ m^-2 treatment. There was significant immobilization of the urea N by 19 days after sowing in all treatments, but the extent of immobilization was not affected by the acetylene. The addition of acetylene slowed net mineralization of labelled and unlabelled N from the organic N pool, and resulted in increased accumulation of both unlabelled and labelled N in wheat tops.