Interactive Effects of Copper on Alfalfa Growth, Soil Copper, and Soil Bacteria

Copper sulfate (CuSO4) foot baths are a management practice used by dairy farms in an effort to control hoof infections. As an unintended consequence, agricultural soils experience Cu accumulation when spent foot baths are disposed of in wastewater lagoons utilized for irrigation purposes. We investigated the effect of Cu applications (up to 1000 mg kg-1) to a Xeric Haplocalcid (Declo series) and a Typic Calciaquoll (Logan series) on alfalfa (Medicago sativa) growth and Cu concentration, soil total and diethylenetriaminepentaacetic acid (DTPA)-extractable Cu, and the soil bacterial community diversity using ribosomal intergenic spacer analysis (RISA). Copper application up to 250 mg kg-1 did not affect alfalfa growth; above 500 mg Cu kg-1 alfalfa did not grow. The 100 and 250 mg kg-1 Cu application rates increased alfalfa Cu content grown in Declo soil, while the 250 mg kg-1 Cu application rate increased alfalfa Cu content when grown in the Logan soil. Regardless of initial application rate, 48 to 80% of the added Cu was still plant-available at the end of the study. Comparing DTPA-extractable Cu to alfalfa Cu concentrations, 63 or 95 mg kg-1 of DTPA-extractable soil Cu for the Declo and Logan soils, respectively, would be detrimental in terms of cattle dietary Cu intake. For Declo soils, bacterial diversity remained relatively stable across all Cu application rates; Logan soils saw a peak in bacterial diversity at the 50 mg kg-1 Cu application rate. Cluster analysis revealed differences in the bacterial RISA profiles between the lower and higher Cu application rates. To prevent excessive alfalfa Cu accumulation and negative impacts on the soil bacterial community, it is suggested available soil Cu not exceed 63 mg kg-1 in agroecosystems associated with these soil series.