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

Abstract

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 (Cmic) even though the orchard soils had similar or elevated levels of total organic carbon (Corg). The Cmic : Corg 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 Cmic (n = 16, r2 = 0.68, P b 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.