School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEffects of mulching logging residue on soil water relations, soil temperature and early growth of Pinus radiata on sandy soils in south-west Victoria( 1990)The timber industry in Australia is becoming increasingly dependent on plantations of exotic softwoods, with Pinus radiata D. Don (Monterey pine) the most important species planted in Victoria and South Australia. In general, these plantations have been highly productive in the first rotation. However, in the second rotation there is evidence of a decline in productivity on relatively infertile sandy soils, particularly where the litter and logging residue remaining after clearfelling is burnt. Research has indicated that this decline can be arrested by careful control of cultural inputs, such as chemical weed control, fertiliser and planting of legumes, or by mulching the litter and logging residue. This thesis reports on a study that examined the interacting effects of mulch, weed control and fertiliser on soil and plant water relations, weed biomass, soil temperature, root development, tree growth, and biomass and nutrient accumulation during the first five years following re-establishment of a P. radiata plantation. Mulch, weed control and fertiliser were applied (present, absent) in a 2 x 2 x 2 split-plot factorial design, resulting in eight treatment combinations. Mulch was applied prior to planting; weed control prior to planting and at regular intervals thereafter to age two years; fertiliser at 1.5, 10 and 14 months after planting, a total of 58.1 kg ha-1 of nitrogen and 19.1 kg ha-1 of phosphorus (as part of a complete mineral mix, including zinc). During the first two years after planting, soil and plant moisture levels, soil temperature and tree growth were monitored at four-weekly intervals, and weed biomass three-monthly. Annual growth was also monitored to age five years. Root development, tree biomass and nutrient accumulation were examined in detail at age two years. In the absence of mulch and weed control, soil moisture was significantly reduced during the first two summers after planting due to evapotranspiration losses, with a consequent reduction in tree growth. Both mulch alone and weed control alone were similarly effective in conserving soil moisture in the surface 10 cm of soil, through a reduction in evaporation and transpiration losses respectively. There was evidence that transpiration losses from weeds in mulch had a greater impact than evaporation losses from bare soils at depths from 10 to 40 cm. Maximum soil moisture levels during the summer months occurred where the mulch and weed control treatments were applied together. Mulch reduced weed biomass on average by half for the first two years after planting compared to that on bare soils with no weed control, though weed biomass by mid- to late- summer was similar for both treatments. Weeds which establish in the mulch treatment benefit from improved soil moisture availability as do the P. radiata trees. Mulch significantly reduced soil temperatures compared to those under bare soils during the spring and summer months, particularly in the surface 10 cm of soil. This is likely to have led to less favourable conditions for root growth and mineralisation of soil nitrogen under mulch during spring, when soil moisture availability is not limited by any of the treatments. During summer however, conditions were more favourable under mulch due to higher soil moisture availability. Where mulch and weed control were both applied, root length at age two years was almost twice compared to that for weed control alone. This difference occurred in the surface 10 cm of soil, and is attributed to the combined effect of higher soil moisture availability and stable temperatures within the optimum range for root growth under mulch during the summer months. Roots were also found in the mulch, which indicates that P. radiata roots can either extract nutrients, water or both from the mulch itself. All growth parameters show that in the absence of fertiliser, the responses to mulch alone and weed control alone were similar up to age five years, and that growth was significantly increased when both treatments were applied. These responses were however confounded, as severe zinc deficiency was induced on the weed control alone treatment. In the presence of fertiliser, the responses to mulch alone, weed control alone and both treatments combined were similar by age four years. The largest response to fertiliser was for the weed control alone treatment, probably due to the correction of zinc deficiency. A fertiliser response was initially found where both the mulch and weed control treatments were applied, but by age four years this response was no longer detectable. Growth at age five years was a maximum on, and not significantly different between, the mulch and weed control, mulch and fertiliser, weed control and fertiliser, and mulch, weed control and fertiliser treatment combinations. The nitrogen concentration in needles less than one year-old was at or below the critical level of 14 g kg-1 for all treatment combinations at age two years, except for the weed control alone treatment which was affected by severe zinc deficiency. Despite this, foliage on the treatments associated with good growth did not show obvious nitrogen deficiency symptoms. It is likely that there was net immobilisation of nitrogen under the mulch treatment during the first two years, but subsequent to this it is considered that the mulch would have been a source of nitrogen to the P. radiata trees. Foliar concentrations of both phosphorus and potassium were adequate for all treatment combinations. Foliar concentrations of zinc were at or below the critical level of 10-11 mg kg-1 for those treatments showing maximum growth, which indicates that demands were just being met. They were well below the critical level for the weed control alone treatment. There is clear evidence that the mulched litter and logging residue is a source of zinc to the replanted P. radiata. Relationships between concentration of nutrients in biomass components and tree diameter were generally weak and not significant, and thus nutrient accumulation closely mirrored the pattern of biomass production. The results show that mulching of the litter and logging residue in situ has the beneficial effects of conserving soil moisture, which in combination with more favourable soil temperature conditions during summer, resulted in satisfactory early growth. The results also indicate that mulch is a source of nutrients to the re-established crop. Although care must be taken in extrapolating these results, forest managers should consider mulching as an alternative site preparation technique to burning, particularly on infertile sandy soils.
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ItemEffects of urea and superphosphate fertilizers and soil moisture and temperature on forest soil respiration in a radiata pine plantation( 1987)The effect of urea and superphosphate fertilizers and the influence of temperature and soil moisture on forest soil respiration were investigated over a 9- month period. The investigation was conducted in an 18- year-old Pinus radiata D.Don. plantation in southeastern Australia. The fertilizers were applied to the forest floor at two rates (200 and 400 kg N ha-1, and 100 and 200 kg P ha-1) in a 3 x 3 factorial design. Soil respiration was measured as CO2 evolution using a soda lime absorption technique. Measurements were conducted at 14-day intervals from the 18th November, 1985 to the 6th August, 1986. The influence of temperature and soil moisture on soil respiration was more marked than fertilizer application, rates being highest when soil moisture was non-limiting. When soil moisture was under 12.5% (oven-dry weight basis), CO2 evolution was strongly moisture-dependent and 31% to 90% of the variability in CO2 evolution was explained by soil moisture. At soil moisture levels above 12.5%, soil maximum temperature was the best predictor of soil respiration, showing a significant positive correlation. This correlation explained 35% to 88% of the variation in CO2 evolution. The influence of moisture and temperature could be accurately predicted by a model which incorporated a moisture dependent Q10. The same model and model parameters successfully estimated soil respiration in the majority of fertilizer treatments. The addition of superphosphate fertilizer resulted in a significant stimulation of respiration rates, while the effect of urea addition was less marked. In treatments which received urea alone, CO2 evolution was depressed, although not significantly. However, in the presence of superphosphate, urea addition increased CO2 evolution with rates being highest in treatments which received the highest urea application rate. This result was attributed to an increased availability of phosphorus since microbial and faunal activity in this ecosystem appeared to be limited by P availability. An attempt was made to estimate the contribution of the forest floor, roots and soil to total forest soil respiration. Measurements were taken on two sampling occasions. On the first occasion, the forest floor plus mineral soil and roots contributed 85.0% and 15% to total soil respiration, respectively. On the second contribution was 68.7% and 31.3% respectively. In the field experiment, inorganic nitrogen concentrations in the soil and forest floor were measured on 60, 90, 240, 270 and 300 days after fertilization. Ammonium-N was the dominant N form. Inorganic-N concentrations were consistently highest in the treatments which received urea alone. Nitrate concentrations were highest in the treatments which received the highest urea application rate. A 30-day incubation experiment was conducted in the laboratory using the forest floor and soil collected from the experimental site one year after fertilization. Initial water-soluble phosphate (W-S P) concentrations were considerably higher in superphosphate treatments in the forest floor, while no appreciable concentrations were detected in any of the soil samples or in the forest floor of the control and treatments which received urea alone. During incubation, W-S P concentrations increased in all treatments with the highest rates being found in the treatments which had received phosphorus. Nitrate production and net N mineralization rates were highest in treatments which received urea alone, both in the soil and the forest floor samples. Superphosphate addition increased net N mineralization in soil samples. However, it only increased net N mineralization in the forest floor in the presence of added N. Urea elevated pH while superphosphate depressed it. No clear influence of pH changes on CO2 evolution were evident.
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