Free Cu2+ Ions, Cu Fractionation and Microbial Parameters in Soils from Apple Orchards Following Long-Term Application of Copper Fungicides

Soil samples were collected from apple orchards 5, 15, 20, 30, and 45 years old, and one adjacent forest soil was used as reference to investigate the free Cu2+ ion activity in soil solution and the soil Cu fractionation in the solid phase following long-term application of copper fungicide, Bordeaux mixture, in apple orchards and to investigate the relationships among soil free Cu2+ ions, Cu fractionation and soil microbial parameters. The total Cu concentration in the orchard soils varied from 21.8 to 141 mg kg−1, increasing with the orchard age, and the value for the reference soil was 12.5 mg kg−1. The free Cu2+ ion concentrations in the soil solutions extracted by 0.01 mol L−1 KNO3 ranged from 3.13 × 10−8 (reference) to 4.08 × 10−6 mol L−1 (45 years-old orchard). The concentration of Cu complexed in the fulvic fraction increased with orchard age from 5.16 to 52.5 mg kg−1. This was also the case for other soil Cu fractions except the residual one. The residual soil Cu remained practically constant, ranging from 4.28 to 5.66 mg kg−1, suggesting that anthropogenic soil Cu mainly existed in the more labile active fractions. Regression analyses revealed that both the free Cu2+ ions in the soil solution and the humic acid-complexed Cu fraction in the solid phase were strongly related with soil microbial parameters.