Effects of soil compaction on soil physical and chemical properties in apple orchards : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Traffic-induced soil compaction is a widespread and detrimental process that substantially affects soil quality globally. This compaction often results in increased bulk density and penetration resistance, along with alterations in soil pore function and reduced soil fertility, all of which collectively degrades overall soil quality. Although assessments of soil compaction have primarily focused on structural changes, such as increased bulk density and reduced pore volume, the relationship between these structural alterations and key hydraulic properties have not been thoroughly explored. Consequently, most research has concentrated on the structural impacts of compaction, with limited emphasis on how compaction influenced hydraulic properties or how intrinsic soil characteristics affected this response. Furthermore, there is a notable lack of comprehensive reviews or quantitative studies examining the effects of compaction on soil water retention and physical quality, as well as the role of soil properties in this context. Additionally, research on soil compaction in New Zealand apple orchards, particularly regarding traffic-induced compaction, is scarce. Therefore, this highlighted the need for studies investigating the effects of compaction on soil physical and chemical properties to address the existing knowledge gap regarding the causes and extent of compaction in orchard soils. To date, the evaluation of soil compaction has mainly focused on structural changes like bulk density, with limited research on its impact on hydraulic properties. In New Zealand apple orchards, the effects of traffic-induced compaction on physical and chemical properties remain underexplored. This PhD study aims to address these gaps by investigating how orchard age and traffic patterns influence soil compaction, providing insights for better orchard management and long-term soil health. Three specific objectives were established: (1) conduct a comprehensive meta-analysis to examine how soil water retention functions respond to compaction and the role of basic soil properties in this response; (2) assess the impact of soil compaction on physical properties (penetration resistance, bulk density, aggregate stability, soil gas diffusivity, soil water retention properties, saturated hydraulic conductivity, and soil porosities) across different planting ages and traffic positions; and (3) evaluate the effects of compaction on key soil chemical properties, including carbon, nitrogen, C/N ratio, mineralisable nitrogen, hot water extractable carbon and carbon stock. The meta-analysis results indicated that while traffic and soil basic properties did not significantly impact field capacity, they led to a substantial reduction in macro-porosity and an overall increase in bulk density, which were influenced by factors such as traffic treatment, clay content, soil organic carbon, and soil moisture content at compaction. Furthermore, soil organic carbon and clay content significantly affected how soil responded to compaction, with variations in bulk density, macro-porosity, and available water capacity depending on soil moisture levels during compaction. The investigation into the impact of compaction on soil physical and chemical properties revealed that traffic positions significantly influenced most soil physical and chemical properties, except mean weight diameter and the C/N ratio in both orchards. In contrast, only mineralizable nitrogen was not significantly affected by the planting age across all soil properties in both orchards. Additionally, a strong interaction between traffic positions and planting ages on soil physical and chemical properties was observed in both orchards.