Copper and zinc soil accumulation and plant concentration in irrigated maize fertilized with liquid swine manure

Fertilization of crops with liquid swine (Susscrofa domesticus) manure (LSM) is a common practice throughout the world. In the Ebro Valley (northeast Spain) intensive swine production is very important and generates high quantities of LSM. Fertilizing maize (Zea mays L.) with LSM is a common waste disposal option. Nevertheless, continuous LSM application could have negative effects as heavy metal soil contamination could lead to plant toxicity. We assessed the effects of applying 29 and 51 m(3) LSM ha(-1) yr(-1) to a field of maize during 6 yr. We measured the accumulation of total and extractable (EDTA) Cu and Zn in the soil and the concentration of these nutrients in maize plants and grain. During the 6 yr of the experiment a total of 6.6 to 11.9 kg Cu ha(-1) and 12.8 to 22.5 kg Zn ha(-1) (29 and 51 m(3) LSM ha(-1) yr(-1), respectively) were applied to the soil. Total Cu and Zn soil concentrations increased by 32 and 11%, respectively, after 6 yr of LSM application. Extractable Cu and Zn soil concentrations increased more than 60% after 6 yr of consecutive LSM applications. It would take at least two to three centuries of regular LSM application to reach phytotoxic soil concentrations for Cu and Zn. Maize grain yields were about 13 to 14 Mg ha(-1) over the 6 yr period, which also seems to confirm the absence of phytotoxicity. Copper and Zn concentrations in whole maize plants and grain during the last 2 yr of the experiment were lower than threshold values for animal and human ingestion (30 mg Cu kg-(1) and 500-1300 mg Zn kg(-1)).