School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemStudies on soil carbon and nitrogen in plantation forests using mid-infrared spectroscopy( 2014)Australian forest plantations have significantly expanded in area since the mid 1990s to meet the increasing demands for wood based products and for environmental services. Establishment and management of plantations can affect soil organic matter (SOM) quantity and quality and thereby soil fertility and sustainable production. This thesis describes investigations of soil carbon (C) and nitrogen (N) in plantations, including effects of a) land-use change, b) management of harvest residues, and c) tree species in biodiverse environmental plantings. Mid-infrared (MIR) spectroscopy was used in tandem with conventional analyses to determine and characterise SOM. MIR spectra (4000-450 cm-1) and partial least squares regression (PLSR) predictions for soil properties were evaluated in land-use change soils from pasture to Eucalyptus globulus plantations (first rotation). Predictions were excellent for C, N and C:N ratio in calibration and in independent test validations, with strong relationships between measured and predicted values (R2 > 0.80) and minimum bias. Reasonable predictions were obtained for microbial biomass C and N, and total P (R2 > 0.70). While there was no significant difference in the concentrations of C and N between land-uses, the results demonstrated the potential of MIR spectroscopy-PLSR to rapidly and accurately assess soil C and N properties, and potential to improve the predictions through local calibrations (e.g., land-use, soil type, concentration range, climate). The effects of harvest residue management on C and N were investigated at two contrasting early-third-rotation Eucalyptus globulus plantations having different soil textures and fertility. Management treatments applied after the first rotation harvest included burning, removing, retaining or doubling the harvest residues; and except for the burning treatments were repeated after the second rotation harvest. Removing residues significantly reduced the mass, concentrations and stocks of organic matter, C and N in litter & residues (L&R) compared to retained or doubled treatments. The concentrations and stocks of soil C and N were not significantly affected by treatment, however there was a general trend of soil C and N increasing with the quantity of residue retained. A corresponding trend was more evident in soil respiration and potentially mineralisable N measurements. Characteristic absorption peaks in MIR spectra indicated that lignins, proteins and carbohydrates were dominant in L&R from residue retained treatments, but easily decomposable aliphatic and carbonyl groups were dominant in L&R from residue removed treatments. The peak area of the organic bands was well correlated with soil respiration. The results emphasised the importance of retaining harvest residues in plantation sites to maintain soil fertility and plant growth. The effect of tree species on soil and litter organic matter was investigated using MIR spectroscopy in five biodiverse environmental native tree plantings (Acacia implexa, A. mearnsii, Allocasuarina verticillata, Eucalyptus melliodora, E. polyanthemos). There were significant differences between litter from the species, with absorbances for proteins, carboxylic acids and carbohydrates dominant in Acacia, whereas those of lignins, cellulose and lower proteins dominant in eucalypts. Soil spectra indicated that only the aliphatic C functional group (3010-2810 cm-1) was significantly greater for A. mearnsii, and which increased with decreasing soil aggregate size. There was a strong correlation of aliphatic band intensity (2924 and 2854 cm-1) with total C indicating a higher degree of humification in soils below A. implexa and A. mearnsii. The results revealed that although there were little or no differences in soil C, there were differences in soil and litter chemical functional groups arising from different tree species. These differences may be useful in selection of species for environmental plantings and rehabilitation. The three studies improved the understanding on changes in soil C and N influenced by vegetation and management practices, and demonstrated the utility of MIR spectroscopy to characterise and interpret changes in SOM.