Denitrification, leaching and immobilisation of 15N-labelled nitrate in winter under windrowed harvesting residues in hoop pine plantations of 1-3 years old in subtropical Australia

Abstract

A field study was carried out to investigate the impacts of windrowed harvesting residues on denitrification, immobilisation and leaching of 15N-labelled nitrate applied at 20 kg N ha-1 to microplots in second-rotation hoop pine (Araucaria cunninghamii) plantations of 1-3 years old in southeast Queensland, Australia. The PVC microplots were 235 mm in diameter and 150 mm long, and driven into the 100 mm soil. There were three replications of such microplots for each of the six treatments which were areas just under and between 1-, 2- and 3-year-old windrows of harvesting residues. Based on gaseous N losses estimated by the difference between the recoveries of bromide (Br) applied at 100 kg Br ha-1 and 15N-labelled nitrate, denitrification was highest (23% based on 15N loss) in the areas just under the 1-year-old windrows 25 days after a simulated 75 mm rainfall and following several natural rainfall events. There was no significant difference in 15N losses (14-17%) among the other treatments. The 15N immobilisation rate was highest for microplots in the areas between the 1-year-old windrows and generally higher for microplots in the areas just under the windrows (30-39%) than that (26-30%) between the windrows. Direct measurement of 15N gas emissions (15N2+15N2O) confirmed that the highest denitrification rate occurred in the microplots under the 1-year-old windrows although the gaseous 15N loss calculated by gas emission was only about one-quarter that estimated by the 15N mass balance method. A significant, positive linear relationship (P<0.05) existed between the gaseous 15N losses measured by the two methods used. The research indicates that considerable mineral N could be lost via denitrification during the critical inter-rotation period and early phase of the second rotation. However, the impacts of windrowed harvesting residues on N losses via denitrification might only last for a period of about 2 years.