The potential impact of long-term copper fungicide usage on soil microbial biomass and microbial activity in an avocado orchard

The impact of copper-based fungicides on soil microbial function in an avocado orchard was assessed. Copper (Cu) residues (280 and 340 mg/kg, respectively) in surface soils (0-2 cm) of an established avocado orchard were shown to be significantly (n = 6, P ≤ 0.05) greater than a nearby reference site under natural vegetation (13 mg/kg). The bioavailable fraction of Cu in these soils was also shown to be significantly greater (2.15 and 1.29 mg/kg, c. pCu²⁺ 8.64) than in the reference site (0.71 mg/kg, c. pCu²⁺ 9.2), as measured by ion-selective electrode in CaCl₂ extraction. Similar trends were observed for the 2-10 cm soil profile. Data suggest that the Cu residues are responsible for significant reductions in biomass carbon (C_mic) even though the orchard soils had similar or elevated levels of total organic carbon (C_org). The C_mic:C_org ratio was significantly lower in all of the Cu contaminated soils, and a significant correlation was observed between CaCl₂-extractable Cu in the surface soils and C_mic (n = 16, r² = 0.68, P ≤ 0.01). Soil respiration in surface soils from the orchard were elevated (6.04 and 5.57 mg CO₂-C/kg.day) compared with the reference soil (3.04 mg CO₂-C/kg.day), and the metabolic quotient (qCO₂) was also significantly greater.