Transformations of nitrate 15N under different forest harvest residue regimes in a hoop pine plantation in Australia

Abstract

A study was conducted to quantify the effects of harvest residue management on denitrification, leaching and immobilization of 15N-labelled nitrate applied at 20 kg N ha–1 to 1-year-old hoop pine (Araucaria cunninghamii) in subtropical Australia. The experiment was undertaken in PVC microplots of 235 mm diameter and 300 mm long, driven into the soil (Lithosol) to a depth of 250 mm. Three replications were undertaken for each of the seven treatments: control without any residue, ground and unground foliage at 20 Mg dry matter (DM) ha–1, ground foliage at 40 Mg DM ha–1, ground and unground branches at 40 Mg DM ha–1, and ground branches at 80 Mg DM ha–1. In 15 days after simulated daily rainfall of 100, 50 and 25 mm respectively in the first three days, 6–26% of applied 15N was lost via denitrification, 14–35% was immobilized and 32–53% was leached. The treatment incorporating foliage materials lost more 15N (21–26%) via denitrification than other treatments. Measurement of 15N gases (15N2+15N2O) showed higher 15N gas emission on day 1, followed by low gas emissions thereafter. This study showed that significant amounts of mineral N could be lost through leaching and denitrification during plantation establishment.