Decomposition of nitrogen-15 labeled hoop pine harvest residues in subtropical Australia

Abstract

Information on decomposition of harvest residues may assist in the maintenance of soil fertility in second rotation (2R) hoop pine plantations (Araucaria cunninghamii Aiton ex A. Cunn.) of subtropical Australia. The experiment was undertaken to determine the dynamics of residue decomposition and fate of residue-derived N. We used 15N-labeled hoop pine foliage, branch, and stem material in microplots, over a 30-mo period following harvesting. We examined the decomposition of each component both singly and combined, and used 13C cross-polarization and magic-angle spinning nuclear magnetic resonance (13C CPMAS NMR) to chart C transformations in decomposing foliage. Residue-derived 15N was immobilized in the 0- to 5-cm soil layer, with approximately 40% 15N recovery in the soil from the combined residues by the end of the 30-mo period. Total recovery of 15N in residues and soil varied between 60 and 80% for the combined-residue microplots, with 20 to 40% of the residue 15N apparently lost. When residues were combined within microplots the rate of foliage decomposition decreased by 30% while the rate of branch and stem decomposition increased by 50 and 40% compared with rates for these components when decomposed separately. Residue decomposition studies should include a combined-residue treatment. Based on 13C CPMAS NMR spectra for decomposing foliage, we obtained good correlations for methoxyl C, aryl C, carbohydrate C and phenolic C with residue mass, 15N enrichment, and total N. The ratio of carbohydrate C to methoxyl C may be useful as an indicator of harvest residue decomposition in hoop pine plantations.