Long-term field investigations on the use of biosolids are scarce in the Mediterranean region, especially on non-food high-profit crops. Thus we studied the effects of repeated sludge application for 4 yr on trace element (both essential and non-essential) availability to cotton, by applying sludge at four increasing rates up to 50 Mg ha(-1). Although sludge had low metal concentrations, sludge-added trace element availability (assessed with soil-to-plant transfer coefficient) was higher in the first year compared to those in the subsequent years of experiment, but it decreased with time to the value of the unamended control. This shows that trace element mobility can be reduced within a time-scale of a few years, provided soils have a relatively sufficient retention capacity (high CEC, clay, and non-acidic pH) and applied sludge has low heavy metal content. We also found that sludge-borne organic matter greatly affected metal availability, since metal transfer coefficients decreased with elevated organic matter content.
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