The Potential Impact of Long-Term Copper Fungicide Sprays on Soil Health in Avocado Orchards

The long-term use of copper (Cu)-based fungicide sprays in orchards is associated with changes in soil Cu levels. However, there is a gap in knowledge regarding the potential accumulation of Cu in orchards and the associated impacts on the soil microbial structure. This study assessed the possibility of Cu accumulation in different avocado orchard farms and further evaluated the potential effect on soil microbial activities. Soil Cu levels were quantified in Tauranga and Northland, and three avocado orchards were analysed in each experimental location. All avocado farms in both sites received Cu-based fungicide sprays for over eight years. Soil samples were collected at a 0–20 cm depth from all six orchards. The soil total and bioavailable Cu, changes in soil chemical properties, microbial biomass, dehydrogenase activity, alkaline phosphatase activity, and acid phosphatase activity were measured. The results revealed that the total Cu and bioavailable Cu concentrations in Tauranga orchards were 81.3 and 0.32, 196.7 and 0.82, and 33.6 and 0.31 mg Cu kg−1 in Farms 1, 2, and 3, respectively. In Northland orchards, the total Cu and bioavailable Cu were 54.5 and 0.06, 18.4 and 0.77, and 46 and 0.34 mg Cu kg−1 in Farm 1, 2, and 3, respectively. Five out of six of the avocado orchard farms assessed in this study had total Cu concentrations greater than 30 mg Cu kg−1 reported in New Zealand native land. The magnitude of Cu accumulation was linked with soil pH and C content. No clear trend was observed between soil Cu concentrations and the soil microbial activity. Our study results demonstrated that the long-term use of Cu-based fungicide sprays can elevate Cu concentrations in orchard soils. Mitigation strategies need to be explored to abate the accumulation of Cu in orchard soils.