Nitrogen volatilisation from urea fertiliser in mid-rotation Pinus radiata plantations in south-eastern Australia

Summary

Because of its perceived cost advantage, most N fertiliser applied to mid-rotation Pinus radiata in Australia is in the form of urea. However, the potential for large N-volatilisation losses from urea may reduce its cost-effectiveness, and result in significant over-estimation of expected growth responses.

The volatilisation loss of N from urea was compared with that from ammonium sulphate at two sites in radiata pine plantations aged 17 and 27 y, using a mass balance method in which ¹⁵N enriched fertiliser (200 kg N ha⁻¹) was applied to microplots. Rainfall was excluded from the plots, but deionised water was added periodically, equivalent to a total of 12 mm over 28 days. Recovery of N from each fertiliser form was determined at 1, 7, 14 and 28 days. After 28 days, 41% of the N applied as urea (equivalent to 83 kg N ha⁻¹) had been volatilised, compared with no significant N loss from ammonium sulphate. Significantly, 76% of the total loss of N from urea occurred during the first week. There was a significant difference in N losses between the two sites which may have been related to differences in forest floor mass, moisture content and evaporation rate. After accounting for N volatilisation, urea is still more cost effective than ammonium-based forms of N fertiliser. However, applying urea during winter or in formulations which reduce the rate of hydrolysis could reduce N volatilisation losses and thus improve the cost effectiveness of N fertiliser use in mid-rotation pine plantations.

Keywords:

• forest plantations
• nitrogen
• urea
• ammonium sulfate
• volatilization
• Pinus radiata