Wheel traffic may increase surface stratification of soil organic carbon in older apple plantations

Soil organic carbon (SOC) is key to maintaining soil health and supporting ecosystem services. However, the impacts of wheel traffic-induced compaction on SOC stocks in perennial horticultural systems, particularly across contrasting soil types, plantation ages, and deep soil profiles, remain unclear. This study was conducted in two commercial apple orchards in New Zealand with contrasting plantation ages: Canterbury (3, 12, and 40 years) and Tasman (12, 17, and 28 years). At each site, soil cores were collected from 0 to 90 cm across tree rows, wheel tracks, and inter-track areas. Stocks of SOC and hot water extractable carbon (HWEC) were calculated using an equivalent soil mass approach to account for compaction-related differences in soil mass. We then quantified position-related differences in SOC and HWEC and tested whether these effects were modified by plantation age (sampling position × plantation age interactions). In the 0–30 cm depth, across all plantation ages, wheel-track areas had the highest SOC and HWEC stocks, with SOC stocks increasing by 3–20% higher than in other positions and HWEC stocks 7–41% higher. The compaction effect was more pronounced in older plantations (e.g., ≥17 years). However, when considering the deeper (0–90 cm) soil profile, compaction effects were generally insignificant. This study highlights that wheel traffic-induced compaction is associated with surface enrichment of SOC and HWEC, particularly in older plantations (e.g., ≥17 years). Additionally, SOC and HWEC stocks may initially increase with plantation age and then decrease, reaching a peak at a certain plantation age (12 and 28 years for SOC in Canterbury and Tasman, respectively, and 12 years for HWEC at both sites). For future research, the accuracy of SOC stock estimations could be improved by explicitly accounting for spatial variability across soil depths and sampling positions, and by considering how this variability changes with management practices and plantation age.