School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemNitrogen fixation by Casuarina oligodon agroforestry in the Papua New Guinea central highlands( 2006)Casuarina oligodon L. Johnson is a multipurpose tree species grown in the highlands of Papua New Guinea (PNG). The integration of C. oligodon into agricultural systems is seen by villagers as means of restoring soil fertility, controlling soil erosion, providing shade for crops and producing fuel wood and building materials. Biological nitrogen fixation by C. oligodon through symbiotic relationships with Frankia (micro-organism) under field conditions in short (5-10 years) and long (11-15 years+) fallows in the PNG central highlands was investigated using the 15N natural abundance technique. Results from the study showed that as much as 70% of N in C. oligodon was derived from the atmosphere. The rate of N2 fixation was relatively low in short fallows of casuarina and increased as the trees aged in the long fallows. A rate of N2 fixation up to a maximum of 36 kg N ha -1 year -1 was estimated based on commonly practiced tree stocking rates and field conditions in the PNG highland areas. Although casuarina fallows tend to accumulate higher total N and C compared with equivalent period of grass fallows, the amounts of N and C in the surface soils of all systems under the study showed no significant difference. The amounts of total N and C under long fallows of casuarina (11-15 years+) were generally greater than short fallows of casuarina (5- 10 years). A significant proportion of the total N was stored in the above ground biomass of trees that were more than 10 years of age. Management of the standing biomass, particularly when the fallow is converted back to the cropping phase, is therefore critical in ensuring that the farmers are able to gain maximum benefit from the fixed N. Whilst the removal of stem wood for use as fuel or building material may be an important product of the agroforestry system, retaining the foliage, small branches and bark on the site is vital in sustaining agricultural productivity.
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ItemSoil physical and chemical properties under Casuarina cunninghamiana Miq. (river sheoak) shade trees in northern Victoria( 1994)Casuarina cunninghamiana Miq. (river sheoak) is an Australian native tree with a natural distribution throughout eastern Australia. It is one of the three Australian casuarinas being widely grown in many countries as an exotic multipurpose tree. This thesis reports on a study of soil physical and chemical properties under C. cunninghamiana shade trees in northern Victoria. The study aimed to assess whether the presence of the trees had any effect on soil properties (bulk density, organic matter and chemical properties) by comparing soil under the canopy with soil under the adjacent open pasture. The study was conducted at the Department of Agriculture Rutherglen research station which has a mean annual rainfall of 590 mm and moderately to strongly acid Rutherglen loam soil. The study involves two main treatments: under the tree canopy and under open pasture. The treatments were replicated in each quarter of the sample plot containing nine sample trees. Soil samples were collected only once for soil bulk density, organic matter and chemical properties. The results showed that the presence of C. cunninghamiana shade trees did not improve soil bulk density, organic matter content and chemical properties of the surface soil (0-15 cm depth) under the canopy. Soil bulk density and organic matter content under the canopy were not significantly different (1.12 g cm-3 and 4.44% respectively) compared with open pasture (1.19 g cm-3 and 4.07% respectively). Soil pH and total nitrogen were significantly less (pH 4.20 and 0.08% respectively) under the canopy compared with under the open pasture (pH 4.48 and 0.01% respectively). Available phosphorus and exchangeable potassium levels under the canopy (2.09 ?g/g and 4.22 cmol kg-1 respectively) were not significantly different compared with open pasture (0.773 .Lglg and 4.88 cmol kg- I respectively). The lack of improvement in soil bulk density and nutrients under the canopy is probably due to the inherent acidity of the soil. This may make conditions unsuitable for litter breakdown and nitrogen fixation. Low rainfall and seasonal flooding of the study site may also be influencing these soil properties. The results demonstrate that the potential of C. cunninghamiana to improve soil properties may be variable depending on the site conditions.