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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ItemPredicting the grain protein concentration of wheat from non-destructive measurements of the crop at anthesis( 2005)Grain protein concentration is an important specification for wheat, which determines the quality grade and price received by growers. It is difficult to achieve target grain protein concentration in semi-arid southern Australia, because of the low and variable rainfall. Growers may benefit from being able to predict grain protein concentration before harvest, especially where there is a threshold or `window' requirement for a particular grade. Grain outside specifications could be forward sold into other grades while prices were good. Spatial predictions of grain protein concentration would allow the pattern of harvest to be managed to optimise profit. This thesis proposed a method for predicting grain protein concentration from non-destructive measurements of the crop (spikes, spikelets) at or after anthesis. The theoretical propositions underlying the method were then evaluated using data from nitrogen fertiliser experiments, data from the literature, and a simulation exercise. The proposed method was to estimate grain number from spike or spikelet number. Variance in grain number, together with the diminishing returns response of grain number to nitrogen, would then be used. to estimate maximum grain number. Maximum grain number would be linked to a unique `critical' grain protein concentration, from which grain protein concentration at other grain numbers could be estimated. Spike and spikelet number were counted throughout grain-filling in nitrogen fertiliser experiments to determine the importance of time of counting. The time of counting was important for absolute, but not relative spike and spikelet numbers: Spike and spikelet number varied, throughout grain-filling, but interactions with nitrogen treatments were rare. Inclusion of spikelets in counting was based on glume length, which interacted with time of counting. Spike death was frequently observed and occurred in proportion to post-anthesis growth, at 0.187(±0.018) spikes/g. The rate with respect to grain yield was similar, at 0.190(±0.038) spikes/g. An analysis of mass/number relationships between grain, spike and spikelet number, and crop and spike biomass at anthesis, showed that grain number was better related to spike biomass, and that spike and particularly spikelet number, were better related to crop biomass. Spikelet number changed at .a rate of between 6.6 and 9.3 spikelets/g biomass across 'a range of experiments; spike number changed at a rate between 0.14 and 0.62 spikes/g. The interrelationships showed grain number should be related to spikelets/spike, and proportion of crop biomass in the spike. The relationships, however, only existed in some experiments and were not universal. An alternative suggested by the analysis was use of spike number as a direct proxy for grain number (ie. assuming constant grains per spike). Spike number was tested as a proxy for grain number initially by analysing the components of variance of grain number across nitrogen, rotation and plant density experiments. Spike number was the main component of variance in grain number (59.8- 71.0% of log(variance)) in nitrogen experiments, with no significant covariance between spike number and grains per spike. Grains per spike and covariance were much greater components of variance in plant density experiments, and grains per spike and spike number were equal sources of variance in rotation experiments, with small positive covariance. Spike number would be an unbiased, but not perfect proxy for grain number when nitrogen was the main factor varying, but not for factors related to rotation or plant density. Spike number and crop biomass at anthesis were compared as estimators of grain number in nitrogen experiments, in an analysis of the nature of the responses to nitrogen fertiliser. Grain number as an estimator of grain yield was included in the analysis to understand the likely effect of using grain number rather than yield as a predictor of grain protein concentration. Crop biomass at anthesis, spike number and grain number all reached maxima at similar nitrogen fertiliser rates, but crop biomass at anthesis was a more precise estimator for the maximum rate required for grain number (RMSE of nitrogen for maximum, 2.4 kg N/ha vs. 26.4 kg N/ha). Grain number had a maximum consistently higher (+32.6±8.0 kg N/ha) than the maximum for yield. Once nitrogen fertiliser rates were corrected for the different maxima, grain number and yield had identical relative response rates to nitrogen. The response rates of crop biomass at anthesis and spike number were both related to the response rate of grain number by a power relationship with exponent 0.6. The lack of methods for anticipating phase differences caused by late nitrogen application and pre-anthesis water deficit will prevent exploitation of these relationships in all environments. The estimation of maximum spike number from its variance was simulated across the width of an air-seeder, using consistent variations in nitrogen fertiliser rate between tynes to drive variance in spike number. Nitrogen fertiliser was normally distributed. It was possible to extrapolate the variance/spike number relationship to estimate the maximum only where the slope of the relationship was negative. Slopes close to zero caused errors. of fitting, where the `signal' from the relationship was indistinguishable from the `noise' in estimating variance. This coincided with low (below 0.8) relative spike numbers and led to over-estimation of low relative spike numbers. Low spike number because of sub- or supra-optimal nitrogen could be distinguished by the second derivative of the fitted function, which was positive for supra-optimal nitrogen. There was no unique `critical' grain protein concentration (for maximum yield or grain number) in southeastern Australia, but there was a consistent relationship between `critical' grain protein concentration and grain weight. The relationship in terms of grain nitrogen content was a linear function of grain weight. The parameters also varied with genotype, and signed relative grain number, calculated as GRS=1-G/GMax for supraoptimal nitrogen, and GRS=G/GMax-1 for sub-optimal nitrogen, where G is grain number. The best estimation of grain nitrogen across genotypes was: Grain N (mg N/grain) = 0.317 + 1.00 x GRS + (0.0115 -0.0181 x GRS) x W, where W is grain weight in mg/grain. The root mean squared error of grain protein concentration estimated from this function was 0.91%. Grain weight would need to be estimated to estimate grain protein concentration. Errors due to grain weight had more effect at higher GRS, and at lower grain weight. The conclusion was that grain protein concentration may be predicted using crop biomass or spike number as a proxy for grain number. Predictions would be best in the absence of pre-anthesis water deficit or nitrogen applied after Zadoks 32. The predictions would be best for relative grain number greater than 0.8 at sub-optimal nitrogen, and for any relative grain number at supra-optimal nitrogen. A confidence interval could still be provided for grain protein concentration at lower relative grain numbers with sub-optimal nitrogen. Predictions would be most accurate if grain weight was reliably above 35 mg/grain.
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ItemEffects of metal-contaminated sewage sludge on biological N2 fixation and mineralization of legume nitrogen( 1995)The effects of sewage sludges contaminated with heavy metals on soil biological processes have been studied overseas, but there is little similar data for informative recommendations on sludge reuse on Australian agricultural soils. The effects of metal-contaminated sewage sludge on the symbiotic effectiveness of Rhizobiurn leguminosarum biovar trifolii, legume 12 fixation and N mineralization of legume shoots and soil-sludge organic matter were investigated in soil from a 10 year-old, sludge reuse experiment at Glenfield (NSW); in six agricultural soils (Rutherglen Red podzolic, Goulburn Yellow podzolic, Robertson Krasnozem, Condobolin Red-brown earth, Wagga Wagga Sand, Darlington Point Grey clay) recently-amended with four rates (0, 60, 120, 240 dry sludge tonnes (dst) ha 1) of Malabar sludge; and the Goulburn soil recently-amended with four different sludges (Port Kembla, St Marys, Quakers Hill, Richmond) at rates of nil, 60 and 240 dst ha-1 . However, the recently-amended soils were rapidly-reacted with sludge in seven wet-dry cycles each lasting approximately 7 weeks (6 weeks wet, 1 week dry). The Glenfield treatments, initiated in 1982, involved application of sludge at rates of 0, 40 dst ha -1 y-1 or 120 dst ha' y -1, for five consecutive years, to unlimed (pH 4.4) or limed (2.5 t ha -1, pH 5.1) soil. Total heavy metal concentrations in the Glenfield soil treatments ranged from 120-580 mg kg -1 soil, with a range of individual heavy metal concentrations (mg kg-1 soil): 43-255, Zn; 21-197, Cu; 15-45, Cr; 9-16, Ni; 25-77, Pb; < 0.06-1.9, Cd. In the recently-amended soils, total heavy metal concentrations ranged from 40-1070 mg kg -1 when amended with Malabar sludge and 86-788 mg kg -1 in Goulburn soil amended with the different sludges. Individual metal concentration ranges in soils amended with Malabar were (mg kg -1): 13-481, Zn; 3-236, Cu; 12- 182, Cr; < 3-80, Ni; < 7-80, Pb; < 0.06-5.5, Cd, and in Goulburn soil amended with the different sludges: 31-411, Zn; 23-210, Cu; 11-65, Cr; < 3-36, Ni; 11-65, Pb; < 0.06-5.2, Cd. Symbiotic effectiveness (ISE) of R. tr?folii isolated from sludge treated and control soils, was quantified by the dry matter or N content of seedlings of white (Glenfield) or subterranean (recently-amended) clover produced with only Rhizobium N2 fixation as an N source. The contribution of fixed N to legumes grown in unamended and sludged soil was determined by the 15 N isotope dilution method using annual ryegrass as the reference plant. N mineralization was determined in short-duration (10 d) incubation experiments with soil incorporated with or without lucerne shoots (2 g 100 g-1 soil) at soil moisture contents of field capacity (FC) and full water-holding-capacity (FWH). Net mineral N was measured as the sum of NH4+ and N03- . The difference in net mineral N between + lucerne and - Iucerne treatments was taken as the net mineralization of lucerne N termed ? min N. Historic sludge amendment of the unlimed or limed Glenfield soil (maximum heavy metal concentration 500-600 mg kg -1 ) had no adverse effect on ISE compared to unamended soil. The effect of sludge on ISE in the recently-amended soils depended on the soil type and sludge rate. The ISE of R. trifolii surviving in most sludged soils was not reduced at total metal concentrations up to 1000 mg kg -1, however the ISE of these rhizobia was reduced in the Krasnozem and Red-brown earth at sludge rates giving total heavy metal concentrations of 550- 1070 (60-240 dst ha-1) and 668 mg kg -1 soil (240 dst ha-1), respectively. The growth and amount of N2 fixed by white clover in the Glenfield soil was increased by sludge addition, with N2 fixation contributing nearly all (84-98 %) of the clover total N. In the recently-amended experiment, subterranean clover dry weight and total N were increased significantly by sludge amendment of the acidic, podzolic soils and fixed N was increased significantly in the Yellow podzolic. A decrease in N2 fixation was observed in some soils amended with higher rates of sludge. Where a reduction in ISE occurred fixed N was also poor. In other soils where ISE was not reduced, decreased N2 fixation is presumed to be due to either mineral N inhibition, or perhaps to heavy metal effects on nodulation. Mineralization of neither incorporated lucerne nor soil-sludge N were adversely affected by sludge amendment at Glenfield. In the recently-amended soils at FC, mineralization of N proceeded as effectively in sludge-amended soils as in sludge-free soil except that ? min N was slightly reduced in the Red-brown earth and Grey clay at sludge rates of 240 dst ha-1. However, at FWH less mineral N was recovered from incorporated lucerne shoots in sludge-amended soils in the heavier-textured or limed soils, which reduced ? min N relative to the control soil. Thus, there is potential for metal-contaminated sewage sludge to reduce symbiotic effectiveness,N2 fixation and the net recovery of mineral N from incorporated lucerne depending on soil type and sludge rate, but also to benefit clover production and N fixed in other soils
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ItemNitrogen fixation by subterranean clover in relation to soil acidity and moisture availability( 1990)Two of the major constraints to nitrogen fixation and seed production by pasture legumes in the dryland agricultural regions of southern Australia are acid soil infertility and the availability of soil moisture during the growing season. The results presented in this thesis describe the physiological responses of intact plants of subterranean clover (Trifolium subterraneum L.) to the combination of these environmental constraints on clover growth. Measurements of nitrogenase (acetylene reduction) activity using a flow-through gas exchange system were performed initially on a range of pasture legumes important in southern Australian agriculture. This procedure established that a rapid decline in nitrogenase activity and nodulated root respiration occurred within minutes of the introduction of 10 kPa acetylene to the gas stream flowing past the nodules. Based on the respiration coupled to nitrogenase activity and the carbon cost of nitrogenase activity (mol CO2 respired / mol C2H4 produced), calculation of the pre-decline rate of acetylene reduction activity was considered to be the most accurate determination of nitrogenase activity. Using these values, the C2H4 /15N2 conversion factor for the T. subterraneum cv. Seaton Park-Rhizobium trifolii strain WU95 symbiosis was 4.08. However, both the pre-decline rate and magnitude of the acetylene-induced decline in nitrogenase activity decreased during ontogeny. Gaseous diffusion into and out of unstressed subterranean clover nodules was examined at sub- and supra-ambient pO2 and revealed that the permeability of nodules to gases varied over a range of pO2. An increase in the resistance to oxygen flux to the bacteroids at supra-ambient pO2 was postulated to protect nitrogenase from oxygen inactivation and was accompanied by a decline in nitrogenase-linked respiration and hence, nitrogenase activity. As the rhizosphere pO2 declined below 21 kPa, the calculated nodule gas permeability did not alter initially although nodule RQ progressively increased. The resultant decline in nitrogenase activity was consequently due to oxygen deprivation. However, nodule gas permeability increased after 24 h which enabled the maintenance of high rates of nitrogenase activity. In order to more thoroughly study the nitrogen economy of subterranean clover subjected to drought in an acid soil, a glasshouse-based study simulated effects of slow drying of the soil on established plants. This was achieved by basal irrigation of 12 week-old plants grown in cores of (i) undisturbed acid soil (pH 4.3), (ii) cultivated acid soil (pH 4.3) and (iii) cultivated acid soil through which lime had been incorporated to a depth of 12 cm (pH 5.5). In this manner, the soil surrounding the nodules on roots in the uppermost regions of the soil profiles commenced drying. Dehydrating nodules were sensitive to moisture loss and rates of nitrogenase activity declined substantially when nodule water potential (?nod) fell below -0.4 MPa. In contrast, leaf water potential ( ?1) remained relatively independent of ?nod and plant growth rate was maintained due to compensatory nodulation at depth in the moister, less acidic regions of the cultivated soil (with and without the incorporation of lime). Growth rate and accumulation of nitrogen ceased in the undisturbed acid soil as no compensatory nodulation in the extremely acidic subsoil occurred. The characteristics of gaseous diffusion within, and nitrogen fixation by, water-stressed subterranean clover nodules was examined in more detail by passing a drying airstream over the intact nodulated roots. As water and ion uptake by the roots was unimpeded,?nod declined independently of ?1. The decline in specific acetylene reduction activity was most pronounced at a ?nod of -0.4 to -0.5 MPa. This coincided with a large reduction in hydrogen evolution, a pronounced decrease in the respiration coupled to nitrogenase activity and an increase in the carbon cost of nitrogenase activity. Nitrogenase-linked respiration accounted for 50% of nodulated root respiration in unstressed plants but had decreased to 30% of total nodulated root respiration after 8 days of gradual nodule dehydration. Consequently, the calculated nodule diffusion resistance increased continuously, but was most pronounced at a ?nod of -0.4 to -0.5 MPa. Nevertheless, photosynthesis was maintained and soluble carbohydrates accumulated in the shoots, roots and drying nodules. The recovery of nodule respiratory activity following nodule rehydration was due to nodulated root growth and maintenance respiration, not to nitrogenase-linked respiration and the lack of any short-term recovery in acetylene reduction activity was presumed to reflect partial nitrogenase damage. In conclusion, moisture stress resulting in nodule drying readily and irreversibly inhibited nitrogen fixation. In soil, however, growth and nitrogen fixation by subterranean clover were maintained during moderate drought provided that compensatory nodulation could occur at depth in more moist and less acidic subsoil. Leaf water relations were also maintained because water uptake continued whilst the root system was in moist subsoil. In very acid soil, further nodulation was restricted and the mild drought caused premature cessation of nitrogen fixation and growth.