Fate of 15N-labelled nitrate in a wet summer under different residue management regimes in young hoop pine plantations

Abstract

A field study was conducted to investigate the fate of 15N-labelled nitrate applied at 20 kg N ha−1 in a wet summer to microplots installed in areas under different residue management regimes in second-rotation hoop pine (Araucaria cunninghamii) plantations aged 1–3 years in south-east Queensland, Australia. PVC microplots of 235 mm diameter and 300 mm long were driven into 250 mm soil. There were three replications of each of eight treatments. These were areas just under and between 1-year-old windrows (ca. 2–3 m in width) of harvesting residues spaced 15 m apart, and with and without incorporated foliage residues (20 t DM ha−1); the areas just under and between 2- or 3-year-old windrows spaced 10 m apart. Only 7–29% of the added 15N was recovered from the top 750 mm of the soil profile with the leaching loss estimated to be 70–86% over the 34-day period. The 15N loss via denitrification was 3.7–6.3% by directly measuring the 15N gases emitted. The microplots with the incorporated residues at the 1-year-old site had the highest 15N loss (6.3%) as compared with the other treatments. The 15N mass balance method together with the use of bromide (Br) tracer applied at 100 kg Br ha−1 failed to obtain a reliable estimate of the denitrification loss. The microplots at the 1-year-old site had higher 15N immobilisation rate (7.5–24.7%) compared with those at 2- and 3-year-old sites (2.1–3.6%). Incorporating the residues resulted in an increase in 15N immobilisation rate (24.5–24.7%) compared with the control without the incorporated residues (8.4–14.3%). These findings suggest that climatic conditions played important roles in controlling the 15N transformations in the wet summer season and that the residue management regimes could also significantly influence the 15N transformations. Most of the 15N loss occurred through leaching, but a considerable amount of the 15N was lost through denitrification. Bromide proved to be an unsuitable tracer for monitoring the 15N leaching and movement under the wet summer conditions.