School of Agriculture, Food and Ecosystem Sciences - Theses

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    Effects of urea and superphosphate fertilizers and soil moisture and temperature on forest soil respiration in a radiata pine plantation
    Ba Than ( 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.