Wheel traffic compaction intensified with orchard age while hydraulic responses were partially decoupled in the top 30 cm

Abstract

Soil compaction from frequent wheel traffic is a concern in orchards. The potential impacts of soil compaction, particularly over orchard age, on soil physical properties, remain unclear. This study aimed to investigate the impacts of wheel traffic on soil physical properties, both mechanical and hydraulic, at various depths (0–10, 10–20, and 20–30 cm) across different plantation ages in two commercial apple orchards located in Canterbury (3, 12, and 40 years) and Tasman (12, 17, and 28 years), New Zealand. Soil samples and field measurements were taken at three positions: tree rows, wheel tracks, and inter-track areas. Penetration resistance, a mechanical property, was measured in situ as an indicator of compaction, while hydraulic properties, including total porosity, macroporosity, available water capacity, saturated hydraulic conductivity, and relative field capacity were measured in the laboratory using intact soil cores to assess water- and aeration-related functions. Results showed that wheel traffic significantly increased soil compaction, reducing available water capacity and saturated hydraulic conductivity and impairing aeration. Notably, these declines were also observed beyond the visibly compacted wheel tracks, suggesting more widespread functional impairment across orchard soils. Older plantations generally exhibited higher penetration resistance, and significant interactions between plantation age and sampling position were observed for penetration resistance. However, older plantations did not necessarily exhibit worse hydraulic conditions (e.g., available water capacity, saturated hydraulic conductivity), and no interactions between plantation age and sampling position were detected for soil hydraulic properties. These findings suggest that while wheel traffic-induced compaction impaired soil hydraulic functions, compaction intensity and hydraulic responses became less consistent with increasing plantation age. This study highlights the importance and potential of mitigating soil compaction to improve soil physical properties and environmental sustainability through targeted management interventions. Future research should focus on understanding the broader impacts of soil compaction on soil functions (e.g., water and aeration storage and transport) and ecosystem services in orchards.