School of Agriculture, Food and Ecosystem Sciences - Theses

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    Effects of adding nutrients on soil chemistry and tree growth in native Eucalyptus forests of south-eastern Australia
    Severino, Dean Christopher ( 2007)
    The decreasing area available for timber extraction in south-eastern Australia, due largely to social pressure to reserve greater areas of forest, has led to the consideration of fertiliser-application to increase wood output from the remaining available forest. Potentially deleterious effects of fertilising on water quality must be assessed before implementation on a wide scale. This is in accordance with relevant forest management policies. This study examined the effects of applying fertilisers containing nitrogen and phosphorus, on soil and soil-water chemistry in two pole-sized stands of mixed Eucalyptus spp in the Wombat Forest, in the Midlands Forest Management Area, Victoria, Australia. The findings are synthesised and discussed in relation to management of regenerating mixed-eucalypt forests in south-eastern Australia. Fertiliser treatments were none (R); 400 kg N ha-1 as ammonium-sulphate (N); or 400 kg ha-1 plus 202 kg P ha-1 as triple superphosphate coated with 10% sulphur (NP). It was calculated that incidental additions of S were 1371 kg ha -1 (N treatments), and 1696 kg ha-1 (NP treatments). It was expected that P would be principally adsorbed on soil surfaces; N immobilised in the soil organic pool and that metallic cations would enter the soil solution to varying degrees. Fertiliser-addition increased both plot-basal-area (BA) growth and the rate of stand self-thinning. In 3.8 years, BA in reference (R) plots at two sites increased by 7.3% and 23.4%. Where N alone was added, BA increased by 14.2% and 27.1%, while in NP plots BA increased by 17.1% and 42.7% respectively. Mortality was 9% in untreated plots compared to 14% in NP plots. Estimated increases in biomass growth equated to additional above-ground nutrient accumulation of 0.4 to 1.5 kg ha-1 of P, and 5.5 to 20.8 kg ha-1 of N. This represented only 0.2 to 0.7% of added P, and 1.4 to 5.2% of added N. Soil solution was extracted from 10 and 50 cm with porous-ceramic-cup tension-lysimeters (-0.6 kPa). Concentrations of P and N were low both before and after adding fertiliser. Across all treatments the maximum median PO43- concentration in soil-water at 50 cm was 0.12 ppm (mean 0.28 ppm). Typically PO43- concentrations were not higher than 0.03 ppm. The 400 kg ha-1 of added N was rapidly immobilised in the soil organic pool. The greatest mean NH4' concentration from a single sampling occasion was 1.1 ppm. The mean NO3 concentration at 50 cm was never higher than 0.26 ppm. After adding N in fertiliser the proportion of NO3- relative to NH4* in soil-water increased and was correlated with decreasing soil-water pH. Less than 1% of added P and N was recovered from soil solution at 50 cm. The largest pool of added P recovered was PO43- adsorbed to soil between 0 and 20 cm, due to the soil adsorption capacity being well in excess of the applied 202 kg P ha-1. Phosphate desorption using sequential extractions with a mild acid extractant (0.3M NH4F, 0.1M HCI) recovered between 25% and 116% of added P. Differences were attributed to both the amount of P added and the effect of time since treatment at different sites. Soil disturbance during sampler installation was found to be more likely to raise soil-water P concentrations at 50 cm than would adding up to 202 kg P ha-1. Among the ions in solution. SO42- and CI' were the dominant anions while Cat+ dominated the cation chemistry. In untreated forest 5042- in soil-water ranged from 7.7 to 16.0 ppm at 10 cm and 7.9 to 12.2 ppm at 50 cm. In fertilised plots up to 100.5 ppm SO42 was measured in soil-water at 50 cm depth. In the N treatment at 50 cm, SO42- in soil-water accounted for 9.4 % of applied S. compared to 14.0 % in NP. In untreated forest, soil-water Cl- and SO42- accounted for over 98% of the total soil-water anions, in roughly equal proportions at 10 cm, and CI- slightly higher at 50 cm. Following fertiliser-application soil-water pH at 10 cm fell from 6.3 in R to as low as 4.81 (N) and 4.45 (NP). At 50 cm pH never dropped below 6 and there were no visible departures from reference concentrations. Relative activities of K+ and Mg2+ in solution increased with decreasing pH, indicating increased leaching potential. Sulphate in soil-water increased total anion charge further in NP than in N. Total charge (cmolc L-1) for cations followed anions. A slight deficit in anion charge was likely due to the unquantified contribution of organic anions. These results confirm that despite the quantity of fertilisers added in this trial being double likely operational quantities, the forest and associated soils had the capacity to retain these nutrients through a variety of processes. The study validates the environmental sustainability of proposed intensive management practices including fertiliser-application in this forest type. It also emphasises the importance of understanding fundamental forest nutrient cycling processes when aiming to carry out intensive forest management practices in an environmentally sensitive manner.
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    Photosynthetic responses to light, nitrogen, phosphorus and pruning of Eucalyptus in south-eastern Australia
    Turnbull, Tarryn Louise ( 2005)
    Eucalypts frequently grow faster after additions of fertiliser, but more slowly in the shade or following `green pruning'. The coupling of rates of growth to environmental factors is at least partly due to acclimation of photosynthetic processes. Photosynthesis rarely proceeds at maximum rates in natural environments as photosynthetic processes and the supply of basic requirements of photosynthesis (CO2, H20, light, phosphorus and nitrogen) vary at both short (minutes to hours) and longer (days to months) time scales. Currently we lack mechanistic explanations for how these variables, alone and in combination underpin changed growth rates in Eucalyptus. This study examined growth and photosynthetic characteristics in glasshouse-grown seedlings and field-grown trees of Eucalyptus species that are commonly planted for forestry and revegetation purposes in central Victoria. Acclimation to light (among seedlings and within canopies), nutrient availability (phosphorus and nitrogen) and increased sink-strength for photosynthates were the primary foci of the study. In each instance I examined distribution of leaf nutrients within a canopy and allocation of N to Rubisco and chlorophyll to assess the degree to which nutrients limit photosynthesis in Eucalyptus. A novel technique was introduced to quantify the allocation of inorganic phosphorus within cells (cytoplasm versus vacuole), followed by an assessment of inorganic phosphorus allocation in response to a long-term reduction in phosphorus supply. In all circumstances, rates of growth were responsive to environmental conditions. Growth responses were underpinned by altered patterns of biomass partitioning and changed leaf morphology more than by rates of photosynthesis per se. There was little difference in adaptive strategies implemented by seedlings and trees: both were oriented towards the accumulation of nutrients rather than increasing rates of photosynthesis. Photosynthesis was reduced by shading (among different plants and within the canopy of a tree) and reduced phosphorus supply whereas N had little effect on photosynthesis. Analysis of pools of inorganic P revealed that adequate supplies were maintained for photosynthetic processes regardless of P supply, therefore reduced photosynthesis follows, rather than leads, a more general leaf-level response to reduced P. Similarly, changed partitioning of nitrogen between Rubisco and chlorophyll was unnecessary as leaf nitrogen concentrations were consistently maintained at well above published minimum levels. Hence, photosynthesis was not up-regulated following increased nitrogen or phosphorus supply; instead excess nutrients were accumulated and used to support increased biomass. One exception was after defoliation, when up-regulation of photosynthesis was observed, presumably to ensure the demand for photosynthates could be met by a reduced leaf area. Sensitivity analyses consistently revealed variation in photosynthetic rates owed more to altered biochemical activity (e.g. Jmax and Vcmax) rather than stomatal conductance regardless of growth condition (glasshouse versus field). Hence, whilst Eucalyptus has considerable photosynthetic potential, faster rates of carbon fixation are only exhibited in the short-term. In part, this is due to the multiplicity of factors involved in `optimisation' of photosynthesis and their individual and collective responses to environmental conditions. In the long term however, increased canopy photosynthetic capacity follows only an increased photosynthetic area.
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