Decomposition of Nitrogen-15 Labelled Hoop Pine Harvest Residues in Subtropical Australia

lines. While windrowed harvest residues have been shown to be effective barriers against erosion, little is Information on decomposition of harvest residues may assist in known about their decomposition kinetics and how this the maintenance of soil fertility in second rotation (2R) hoop pine plantations (Araucaria cunninghamii Aiton ex A. Cunn.) of subtropiaffects nutrient conditions in following rotations. Succal Australia. The experiment was undertaken to determine the dycessful plantation management strategies require denamics of residue decomposition and fate of residue-derived N. We tailed knowledge of the residue decomposition process used 15N-labeled hoop pine foliage, branch, and stem material in and the fate of the nutrients subsequently released (Vanmicroplots, over a 30-mo period following harvesting. We examined lauwe et al., 1997). In hoop pine plantations, litterfall the decomposition of each component both singly and combined, and occurs within a closed system and forms a reasonably used 13C cross-polarization and magic-angle spinning nuclear magnetic constant part of the renewal process with a soil faunal resonance (13C CPMAS NMR) to chart C transformations in decomand microbial community that is adapted to, and preposing foliage. Residue-derived 15N was immobilized in the 0to 5-cm pared for, litter utilization. Conversely, harvest residues soil layer, with approximately 40% 15N recovery in the soil from the form a massive influx to a highly disturbed system where 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 combinedthe temperature and moisture regimes have been radiresidue microplots, with 20 to 40% of the residue 15N apparently lost. cally altered (Attiwill and Adams, 1993; Carlyle, 1994), When residues were combined within microplots the rate of foliage and where the soil physical environment has also decomposition decreased by 30% while the rate of branch and stem changed, with some areas subjected to compaction and decomposition increased by 50 and 40% compared with rates for these soil disturbance (Greacen and Sands, 1980). components when decomposed separately. Residue decomposition Microplots have been successfully used for the study studies should include a combined-residue treatment. Based on 13C of the decomposition of 15N-labeled leucaena residues CPMAS NMR spectra for decomposing foliage, we obtained good (Xu et al., 1993a, 1993b), allowing the fate of the residuecorrelations for methoxyl C, aryl C, carbohydrate C and phenolic C derived 15N to be traced through the soil profile. A major with residue mass, 15N enrichment, and total N. The ratio of carbohyproblem associated with this technique is the potential drate C to methoxyl C may be useful as an indicator of harvest residue decomposition in hoop pine plantations. for exclusion of invertebrates by microplot walls, however, the technique has better soil and microbe contact than the more commonly used litterbag method (Xu et al., 1993a, 1993b). By using 15N-labeled harvest residues T effect of management practices on the susfor this experiment, we expected to be able to trace the tainability of forest plantations becomes increasmovement of residue-derived N through the soil profile ingly important as subsequent rotations are grown on and, by performing a mass balance of the 15N remaining the same land (Jones et al., 1999). In the southern hemiwithin the system, elucidate N losses and gains from sphere, following canopy closure, a plantation is essenresidues and soils within the microplots. The use of tially a closed system for nutrient cycling (Folster and single-component and combined-residue microplots was Khanna, 1997) therefore major plantation disruption expected to yield data on the synergistic effects of mixoccurs at harvesting, and in the period between planting ing residues, a more realistic setting than single-compoand canopy closure. It is during this period that forest nent studies that have been undertaken (Parfitt et al., practices, including the management of harvest residues, 2001). may critically affect future soil fertility and tree nutrient The use of 15N-labeled harvest materials to study status and are therefore important in sustaining the residue decomposition and subsequent release of resiplantation system. due-derived N has been limited by the availability of A major management change within hoop pine planadequately 15N-labeled and representative material. Agtations in Southeast Queensland, Australia, has been ricultural research has made use of 15N-labeled clover the retention of harvest residues on site, pushed into (Wivstad, 1999), maize, and wheat (Thomsen et al., 2001). continuous mounds, or windrows, along the contour Nitrogen-15 enriched foliar material has been produced by spraying the leaves of trees with a 15N-enriched nutriT.J. Blumfield, Z.H. Xu, and N.J. Mathers, Cooperative Research ent solution (Cotrufo et al., 2000; Zeller et al., 2001). Centre for Sustainable Production Forestry, Griffith Univ., Nathan, Qld. 4111, Australia and Faculty of Environmental Sciences, Griffith However, this method produces only 15N-labeled foUniv., Nathan, Qld. 4111, Australia; Z.H. Xu, Queensland Forestry liage, not the 15N-labeled stem and branch material reResearch Institute, P.O. Box 631, Indooroopilly, Qld. 4068, Australia; quired for a comprehensive study of harvest residues. P.G. Saffigna, School of Agriculture and Horticulture, Univ. of In a study of residue decomposition in Pinus radiata Queensland, Gatton, Qld. 4343, Australia. Received 27 Nov. 2003. *Corresponding author ([email protected]). Abbreviations: CC/MC, carbohydrate C to Methoxyl C ratio; CPMAS Published in Soil Sci. Soc. Am. J. 68:1751–1761 (2004).  Soil Science Society of America NMR, cross-polarization and magic-angle spinning nuclear magnetic resonance. 677 S. 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