Zn in the soil solution, which may have occurred because part of the
Zn could have been adsorbed not only to functional groups of organic
particles but also to the soil minerals (Croué et al., 2003; Brunetto
et al., 2014). Moreover, in the first crop, the slight increase of Zn in
soil solution in the treatment supplied with 30 mg Zn kg−1 may have
contributed to reduce the Cu activity in the soil solution, thus reducing
Cu toxicity to plants, which resulted in higher shoot dry matter production
than in the vineyard soil with 0 mg Zn kg−1 (Fig. 1a). In the higher
doses (N60 mg kg−1), however, Zn toxicity decreased shoot dry matter
production. Such reduction likely occurred because the Cu and Zn
adsorption processesmay be affected by the presence of other elements
in the soil solution, which has been shown in studies in which the soils
were simultaneously treated with Cu and Zn (Arias et al., 2006). Conversely,
the concentration of Zn in the soil solution may have been
very high and caused toxicity to the plants at the highest doses applied
to the soil. A decrease in the root and shoot dry matter production in the
first crop was only observed with additions higher than 90 mg of Zn
kg−1, although in the second crop, such decrease occurred whenever
Zn was applied, regardless the dose.