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, Pless than or equal to0.05) 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. pCu(2+) 8.64) than in the reference site (0.71 mg/kg, c. pCu(2+) 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, r(2)=0.68, Pless than or equal to0.01). 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 (qCO(2)) was also significantly greater.