Doctoral (PhD) Theses

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Theses submitted by Lincoln University doctoral students.

Students wishing to submit theses should see the Depositing theses and dissertations guide.

Recent Submissions

Now showing 1 - 5 of 1043
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    Understanding the growth and development of maize (Zea mays L.) : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University
    (Lincoln University, 2024) Mwayawa, Annette
    A main constraint to maize production in New Zealand is yield variability due to the low rainfall supply and also the erratic distribution of rain in the summer which consequently affects crop N uptake and utilization. The aim of this study was to understand the influence of N and water on canopy development, crop growth, phenological development and yield formation. Crops with different yield potentials were created using different levels of N and water availability. Two experiments were carried out, marking the growing season, Experiment 1 in 2015/16 and Experiment 2 in 2016/17. The two experiments were carried out at two different locations at Lincoln University, Canterbury, New Zealand. Experiment 1 was arranged in a split-plot design with four nitrogen (N) levels under different water regimes. The N levels were N1- nil, N2 -75 kg N/ha, N3 – 150 kg N/ha and N4 – 300 kg N/ha, and water levels at Irr1- Irr4 as defined by the accumulated potential soil moisture deficit as 443 (rainfed), 367, 301 and 226 mm, respectively. These created grain yields that were not different in all the treatments which averaged 12.4 t/ha and only varied in total dry matter (DM) accumulated. To create more distinct differences in grain yield, this experiment was repeated at another location with a higher dose of N. Experiment 2 used two levels of water and N in a randomised block design. The treatments were for N1 –nil N and N2 - 500 kg N/ha, rainfed and irrigation (accumulated potential soil moisture deficit at 536 and 296 mm respectively). In Experiment 1, grain yield was not different across the crops and average 12.4 t/ha. The total DM was 19.1 t/ha for the rainfed crop (Irr1) and averaged 22.4 t/ha for the irrigated crops. Grain yield and total DM as explained by intercepted photosynthetic radiation (iPAR) accumulated to 570 MJ/m2 for Irr1 and was higher at 1082 MJ/m2 for Irr2. All crops GAI reached a maximum at 3.7 m2/m2 at a rate of 0.01 m2/m2/°Cd in the duration of 677 °Cd which justified the similarities in grain yield. After the linear phase, the intercepted light in Irr1 immediately reached an asymptote as the leaves withered quickly due to water stress. The radiation use efficiency (RUE) was 2.21 g/MJ for N1 and 2.49 g/MJ for N4 and mainly because of the specific leaf N (SLN). The SLN was highest at 2.1 g N/m2 for the irrigated crop with N and lower at 1.66 g N/m2 without N. The contribution of SLN to yield was reflected in total DM. In Experiment 2, grain yield increased progressively from 0.98 t/ha under rainfed to 9.0 t/ha when irrigated and further to 16.3 t/ha with N. Total DM followed a similar response with rainfed accumulating only 4.10 t/ha and 14.3 t/ha under irrigation and doubling with N, creating total DM of 28.9 t/ha. The difference in total DM was explained by the differences in the total amount of iPAR. Under rainfed the total iPAR was 448 MJ/m2 and increased to 551 MJ/m2 with N, and when irrigated was 816 MJ/m2 and further increased to 1005 MJ/m2 with N. The rate and duration changed, indicating the capacity of the crop to capture light depended on the changes in pigment protein complexes, directly linked to development of the GAI as a process of leaf development and expansion. The maximum GAI was affected by the main effects of water and N where GAI increased from 2.14 to 3.49 m2/m2 with water and from 2.48 to 3.14 m2/m2 with N application. In the contribution of RUE to grain yield, SLN was a key factor connecting leaf N concentration to DM production. The SLN was 1.23 g N/m2 at 905 °Cd in all the crops, however, dissecting the canopy into cohorts, SLN varied. The main section of the canopy that supplied assimilates directly to ear development was the mid-cohort. This cohort was affected by both water and N, increasing SLN from 1.43 to 2.39 g N/m2 with water and from 1.46 to 2.37 g N/m2 with N. The changes at cohort levels were explained by GAI as parameter relating to canopy development and the allocation of N within the leaf, and light penetration through the hierarchical canopy arrangement. The amount of water used to produce the given yield in Experiment 2 depended on crop water use which was converted to WUE. Only the crop under irrigation and N was efficiently converting water to DM at 47 kg DM/mm of water. The WUE for the rainfed crops was 17.9 kg DM/mm and did not differ from the irrigated crop without N which had a WUE of 25.7 kg DM/ha/mm. Leaf and canopy photosynthesis has led to improvements in crop DM and yield through enhanced DM partitioning. In depth understanding into elements of SLN is essential for estimation of the SLN throughout the cropping season. The future of crop improvement strategies is dependent on maximising the leaf and canopy photosynthesis and converting DM accumulation into yield benefits.
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    The formulation of pasture seed mixture from a diverse pool of six forage species : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University
    (Lincoln University, 2024) Shampasivam, Arulmageswaran
    Optimising pasture yield and quality is needed to meet the global food demand. Pasture sward ecosystems that create beneficial diversity and productivity effects will contribute to this. This research used a multi-species pasture mixture experiment to identify optimal seed mixture species combinations under irrigated conditions. This reseach underpins the machanism of linking species mixed pasture properties to the beneficial diversity attributed to community responses. A large-scale diversity experiment used 69 mixtures from six pasture species to investigate the impact of different functional gropus. A simple mixture of combinations of 2 to 3 species from a grass and legume functional group were the key components. On average, mixture communities produced 16% higher biomass yield and contained 64% lower weed biomass than the average performance of the monoculture swards evaluated from 2018 to 2021. Pasture sward responses differed depending on the associated component species, and over time. The model identified several binary and ternary mixture combinations for improved biomass production, weed suppression, and maximising quality; crude protein (CP) and metablizable energy (ME). The highest contributions for herbage biomass were from binary and tertiary mixes of perennial ryegrass (PR), cocksfoot (C), white clover (WC) and red clover (RC), and plantain namely; PR*WC, PR*RC, C*RC, P*WC*RC, PR*WC*RC, PR*C*P and PR*RC*SC. Among those mixture communities, PR*WC and PR*RC maintained the greatest productivity over three years, giving an annual average herbage biomass yield of 13.6 and 15.2 t/ha, when no nitrogen fertilizer was applied. The mixture components in the ternary mixture (PR*WC*RC) produced an annual biomass yield of 15 t/ha. The mixture effect on weed suppression was strong and existed in several mixture communities even though increasing species richness continuously reduced unsown species biomass. Diversity contribution to the nutritional composition of herbage material was not beneficial, but the species' relative abundance in the mixture improved the herbage quality. The optimal mixture combination was PR*RC at the ratio of 50:50 sown seeds equivalent to 12.6 : 13.4 kg/ha (26 kg/ha) viable seed rate. This gave a mean herbage biomass yield per regrowth cycle of 1.98 t/ha (16 t/ha/yr), with a mean weed biomass of 0.13 t/ha/yr, 10.8 MJ ME/kg DM and 20.7% protein content. Further, the ternary mixture PR*WC*RC at the proportion of 45:11:44 equivalent of 11.3 : 0.8 : 11.8 kg/ha (24 kg/ha) had a mean yield per regrowth cycle of 1.97 t/ha (16 t/ha) DM, 0.8 t/ha weed yield, 10.8 MJ ME/kg DM and 20.8% protein. This ternary mixture combination at the ratio of 50:15:35 was equivalent to 12.6 : 1 : 9.4 kg/ha (23 kg/ha) and produced a mean yield of 1.95 t/ha (16 t DM/ha), 0.48 t DM/ha weed biomass, 10.1 MJ ME/ha and CP 20.2%. These higher productive mixtures were from legume and non-legume functional groups, which improved the pasture herbage biomass yield and quality compared with their monocultures. The sown species proportion of swards differed over time based on the functional species composition of the mixtures. Among the monoculture swards, grasses maintained a higher sown species proportion than legumes or plantain. On average, mixture communities suppressed the weeds, and the suppressive effect was increased with the increasing number of species in the mixture. PR*RC binary mixture maintained a higher sown botanical composition proportion over the 3 years than PR*WC. Modelled Relative Growth Rate Differences (RGRD), with the sown proportion, revealed that the equal proportion of the component species in the identified ternary mixture (PR:WC:RC) balanced the competitive growth and maintained the sown proportion until the 3rd year after establishment. This deviated from the optimal seed proportion that maximised the sward yield responses. Further, this experiment identified that the reason the sward yields differed was investigated in the third year and showed this was due to the quantity of fraction canopy light interception. Overall, species mixed pasture swards intercepted a higher fraction of canopy light (PAR). Average pre-grazing fraction light interception value of swards (2020/21) across the monocultures and two species mixture commmunities was modelled to the initial sown species component proportion. This quantified the diversity contribution of addtitonal fraction of canopy light interception. This was higher for PR*WC (0.47) than for PR*RC (0.10). The higher fraction of light interception contributed to the diversity attributed to the higher herbage biomass in PR*RC than PR*WC. The highest pre-grazing fraction of light interception and intercepted PAR energy was in summer/spring when temperature and available light levels were highest. However, the positive contribution from RC over WC appeared in autumn when the fraction of light interception from white clover may have been compromised by its shallower roots. In the third year, radiation use efficiency (RUE) of swards differed based on the functional species in the mixture community. Grasses had a lower RUE than clovers. There was a positive relationship between RUE and temperature for monocultures and binary mixtures, especially PR*WC and PR*RC. This research suggests farmers in irrigated conditions should sow a mixed pasture of 2-3 species that comprise a grass and a clover. The PR:RC mixture (50:50 = 12.6 : 13.4 Kg/ha) and PR*WC*RC in both proportion settings (45:11:44 or 50:15:35 = 11.3 : 0.8 : 11.8 kg/ha or 12.6 : 1.0 : 9.4 kg/ha) improved the pasture herbage biomass yield and quality over time compared with their monocultures. These mixtures provided diversity response that persisted and optimised biomass yield with minimal weed content and maintained the botanical composition which inturn optimised herbage quality.
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    Soil carbon, erosion, and the stormflow mobilisation of sediment and nutrients in a high-country landscape : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University
    (Lincoln University, 2023) Provost, Shyam Michael
    This study was carried out at Mt. Grand Station, a high-country pastoral farm in the South Island of New Zealand. The landscape (400 - 1300 m altitude) supports a gradient and mosaic of native and endemic woody shrub and tussock grassland vegetation amongst more productive exotic pasture, the latter established through aerial seed top-dressing and fertilisation. In recent years several areas of the farm at higher altitudes have been converted to conservation management following Tenure Review, placing additional pressure on the remaining farmland to maximise productivity, a situation similarly faced by many other high-country farms. However, further intensification of pasture grassland would compromise existing less productive native vegetation. This research project investigated soil conservation and loss, and freshwater quality, aiming to advance existing knowledge relating to environmental sustainability of the high-country. Topsoil carbon stocks were quantified beneath various vegetation communities at different altitudes of the station to gain a better understanding of soil carbon and its dynamics. Two watershed catchments were targeted for high-frequency sampling during rainfall events, to investigate the likely significance of water flow on the mobilisation of sediment and nutrients, and to help improve the accuracy of existing run-off estimates. In addition, soil erosion was estimated from differences in residual soil 137Cs activity, which was generated from historic Pacific nuclear testing, between two of the dominant types of vegetation cover. The results revealed the potential for native vegetation to enhance soil carbon sequestration. At low - middle altitudes (450 - 850 m) of the farm, topsoil beneath a woody shrub (kānuka) vegetation cover had significantly higher carbon concentrations and carbon stocks than areas of adjacent pasture. At higher elevations (>1000 m) topsoil beneath dominant snow tussocks had significantly higher carbon, nitrogen and phosphorus concentrations, with higher carbon stocks than adjacent inter-tussock spaces. The total loads of suspended solids, nitrogen and phosphorus exported to catchment waterways were significantly larger during high-flow events in comparison to baseflow conditions, and large proportions of the high-flow loads were mobilised on the rising hydrograph following high rainfall. These findings draw attention to the significance of taking account of the early stages of rainfall events to improve accuracy when quantifying high-country catchment loads. Data for 137Cs were variable but these provisional results indicate that soil beneath kānuka is likely to have undergone lower rates of erosion over the previous 65 years in comparison to areas of adjacent pasture. The combined findings of the three parts of the experimental work in this study are interpreted as being indicative of the present and future potential for South Island high-country farming environments to make a significant contribution towards climate change mitigation through vegetation management, resultant soil building and prevention of soil erosion. It is argued that closer attention to ecological restoration is likely to have mutual benefits for conservation, the farming system and the environment. Maintenance and better-informed management of the mosaic of native and exotic vegetation can play a more important role in longer-term sustainability of this high-country land management system than is currently appreciated.
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    Novel numerical methods for stochastic ordinary and partial differential equations in modelling complex systems : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University
    (Lincoln University, 2023) Tiwari, Parul
    Many natural and engineered systems are complex due to inherent uncertainty. Stochastic Differential Equations (SDEs) and Stochastic Partial Differential equations (SPDEs) provide a rigorous mathematical foundation for modelling these systems. Understanding the dynamics of complex systems under stochastic influences is crucial for predicting system behaviour. Numerical techniques often struggle to handle the complexity and stochastic nature of these equations. This research focuses on adapting and enhancing numerical methods to provide efficient and reliable solutions. The numerical accuracy and stability of these methods are assessed through simulations and examples. This study introduces the synthesis of stochastic spectral methods to solve complex systems by representing random variables as a sum of orthogonal polynomials. We applied Polynomial Chaos Expansion (PCE) methods to contaminant transport problem and to differential equations with random forcing term. We compute the Wick exponentials and show that Wick product coincides with the ordinary product for deterministic functions. We use Malliavin calculus to find the derivatives of a stochastic quantity and are visualised through graphs. We discuss numerical challenges associated with the PCE methods and their solution strategies. In all examples, our chosen method does better and allows us to lead the way in developing robust and efficient strategies to deal with randomness, ultimately enhancing the reliability and resilience of complex systems across various scientific and engineering domains.
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    Fragmentações, Violações, and Resistência: Weaving struggle and dreams in the Carajás corridor of the Brazilian Amazon : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University
    (Lincoln University, 2023) Figueiredo de Almeida Silva Campos, Leila
    The Estrada de Ferro Carajás is an 892 km railroad in Northern Brazil built to transports iron ore extracted by Vale S. A. from Serra dos Carajás in Pará state to ports in São Luís, Maranhão. Over the years, the connection between the railroad to other infrastructure and extractive projects has resulted on a logistics-export corridor – sometimes dubbed an “iron serpent” or “project of death” by local communities. Criss-crossing two Amazonian states, the Carajás corridor cuts through hundreds of territories of traditional peoples and communities, including Indigenous peoples, peasants, and quilombolas. Using document data and in-depth interviews interpreted through reflexive thematic analysis, this thesis aims to understand how the relationship between Vale, the state, and quilombolas contributes to the formation, expansion, and maintenance if the Carajás corridor. This thesis identifies contradictions at the heart of the iron serpent: instead of a connecting entity promoting progress, development, and cohesion, this research positions the railroad as a place of ‘Fragmentação’, ‘Violações’, and ‘Resistência’. Processes of Fragmentação (fragmentation) separate a contiguous quilombola territory in more easily manageable entities to favour the corridor. Violações (violations) encompass the cumulative harm caused by Vale and the Brazilian state in the enactment of resource regulations. Meanwhile, processes of Resistência (resistance) challenge the formation, expansion, and maintenance of the corridor, and are performed by quilombola communities and allies. This thesis brings important contributions to studies on development and resource extraction. Previous studies have tended to position mining companies as the most powerful actors in extractive areas, coercing the state to disregard regulations. My research points to a more nuanced scenario, in which the state itself enables, enacts, and relies on socioenvironmental harm - thus co-producing harm in the corridor alongside Vale. This thesis also introduces the concept of ‘silencing zones’ to reflect the physical, symbolic, and normative strategies enacted by Vale and the state to harm communities in the corridor. Finally, this study contributes to the growing literature on environmental justice by presenting community-led initiatives to resist extractive pursuits within and beyond the state, weaving new threads for state-making and territorial autonomy.