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.