Using calcium carbide with the acetylene inhibition technique to measure denitrification from a sprinkler irrigated vegetable crop

Abstract

The use of calcium carbide (CaC2) pellets as a source of acetylene (C2H2) for the C2H2 inhibition technique to measure denitrification in the field was examined in a sandy loam soil cropped to lettuce. CaC2 was used in combination with vented closed chambers. Three grams of 2–4 mm CaC2 pellets were inserted at 7 and 20 cm for raised beds, and 6 and 15 cm for furrows, in four holes located at right angles, 2.5 cm outside a 20 cm dia. chamber pushed 4 cm into the soil. Acetylene concentrations were determined in the centre of the chamber 1.0 and 4.0 hours after applying CaC2, in beds (two dates) and furrows (one date) at different depths to 20 and 15 cm, respectively, for beds and furrows. At soil matric potentials of ca. 0.01 and ca. 0.06 MPa, soil C2H2 concentrations were >1.0% (v/v) for all determinations. Weekly measurements of denitrification were made from a sprinkler irrigated lettuce crop for 8 weeks. Measurements were made 24 hours after irrigation and on one occasion 48 hours after. Measured mean rates of denitrification were generally <100 g N ha−1 d−1, much less than published data following furrow irrigation of vegetable crops in a nearby region. In another sandy loam soil at ca. 0.08 MPa, soil C2H2 concentrations were ≥2.0 % at 5 and 15 cm, 1.0 and 6.0 hours after adding CaC2. Using CaC2 to generate C2H2 in soil is a convenient approach for field measurements where the C2H2 inhibition technique is used with chambers to determine the nitrous oxide flux. In comparison with C2H2 supply via probe systems, CaC2 overcomes the requirements for cylinders of pressurised C2H2 gas and complex on-site plumbing.